Getting Started with Ethereum Solidity Development by Sebastien Arbogast and Said Eloudrhiri

hello everyone and thanks for coming so early I mean come on it's 9:30 and it's minus 1 degrees outside so I understand how difficult it was today we are gonna talk about getting started with a theorem sorry TT development so I hope that's what you're here for because well otherwise just previously on the box so we're here last year to talk about guess what blockchain development any failure but mostly to talk about why you should get interested in the blockchain so we won't go over that again if you want to watch the detail three-hour presentation you can go to youtube and see the presentation for yourself but we still want to give a brief introduction for those who were not here last year and just as a way that everybody starts with the same points and understands what we're talking about one of the main feedbacks that we got last year is that we our presentation was very broad and lacked some code and after all we are in a developer conference so fear not this year there will be plenty of code to draw on but we dive right into it who was here last year at our presentation ok but 10 I'd say who is here because they want to get into this world and get ripped fast with icos and stuff still a few ok ok just a word of warning we're not going to talk about cryptocurrencies here I mean not for the sake of it but really focus on development who's already written some solidity code and can understand what this does couple three four ok and who is one of our udemy students nobody that's good because if you were you would be able to enjoy this amazing course that we have ok whatever so who we are very briefly we are all time freelance developers we've on all sorts of projects from back-end to mobile to front-end web smartwatches Smart TVs fridges whatnot went to together we own a fortune of about five yrs so we're pretty rich I mean come on I mean not now maybe next year and yeah two months ago we released a course on udemy we have more than a thousand students about film development I don't know what I mentioned anyway so what are we going to talk about today first of all I'm gonna give you a short introduction to blockchain architecture how it works what it includes and so on and then we are going to create adapt a decentralized application from scratch with truffle you will see what truffle is later on will work right a smart contract we'll see how to test small contracts and to interact with them once they are deployed how to create a front-end very briefly and then did I mention we have an online course on udemy sorry so very brief introduction to the blockchain there are a few things that block chains are awesome at the first thing it's not just for FinTech so I'm sure you've heard of the blockchain in the world of banks and insurance companies and so on it's clearly not the only vertical I would say where they're useful and I would even argue that it's not the best one because those industries are highly centralized and we're talking about a technology that was designed to be decentralized so be aware of this limitation what they're awesome at is robustness they were designed to work decentralized and distributed because well the same as the way the web was designed first if you remove a node it still works if you remove half of the node it still works so it's really made for those kinds of applications where you need to be sure that the system will run no matter what another thing is great at is censorship resistance I'm sure you've heard since the invention of Bitcoin well one day will disappear a nation-state will take it and shut it down not possible it would have liked to after all it is an alternate currency and by most nation-states laws you're not allowed to create it an alternate currency but still they can't take it down because it's censorship resistant by design another thing it's great at is being completely borderless there is no notion of a national border on the blockchain none whatsoever so it's really powerful for those kinds of applications where you need to transfer value when do stuff on a global scale another thing it's great at is being secure but that's why I chose this image secure in a very slow way ok you trade off security for speed basically at least that's the case for now but don't believe those who would say that's this that's the the way things are in the blockchain world I mean we are talking about a technology just very early very young and it's evolving very fast so most of these problems will be solved at some point so basically it's great for disintermediating mat markets that's the main thing so whenever you have an industry that's run by a category of actors of yeah type of companies whose main job is to serve as a trust 40 or an intermediary that's the thing where you can rethink the business model based on the blockchain and that's what scares the out of thanks basically because well again they are intermediaries and their main business is to take a cut of everything but that's not the only one notaries insurance companies I said it's not just for FinTech it's also for in the energy market and plenty of industries really and basically what it allows you to do is to create a network of value so you might already understand that the internet is designed as a network of information which means that when I send something I keep a copy of it so everything works by replication but on the blockchain it works by copy so when I send you something I don't have it anymore and that's absolutely key that's something that we couldn't do before and that we can do now thanks to the way the blockchain is designed and yes it's also very good at looking hype so this is just the progression of the price of a share of a company and the last part of the graph is the day they decided to add blockchain to the name so yeah of course there's that also but what I would like to you to remember after this talk is that it's also very important in terms of opportunities for you guys developers ok I know we're all excited by the last JavaScript framework and and the latest evolutions in spring and so on but here we are talking about the technology and I'm mark my words I'm talking about the technology that's going to be the foundation of businesses for the 20 years to come it's not ready for completely ready for prime time yet but this is it this is going to decentralize so many industries and it's going to go beyond businesses so I hope you all realize the importance of that moment the role that you may have to play in this transition and yeah the incredible world that's before us so how do blockchain achieve what we just talked about first by building a distributed ledger so Ledger's are a very efficient way to to manage value whether in a real world with paper Ledger's or in the existing world that's the way banks manage their backends basically and the difference with the block change that this ledger instead of being stored in one central database is distributed across the network so every single node in the network has a full copy of the entire database ok so that's the main key and what when you hear a blockchain that's what it is it's a chain of locks where each block contains a list of transactions so when you change locks together you indeed have a global ledger of all the transactions the way it does it is through decentralized consensus now this is something that we talked about last year but it's really really important to understand this is a problem that's been bugging the that had been bugging the computer science community for y'all since the 90s basically and it's called the Byzantine generals problem okay so again imagine a city that's surrounded by armies each armies led by general and they all have to decide independently whether they want to attack or if they want to go home if they all attack at the same time the city will be taken and the armies will be victorious if they all go home then nobody is harmed the cities remains intact and everybody is happy now if some of them decide to attack and the others go home then we have a problem it's gonna be a massacre not just for those who attack and get butchered but also for those who get home and will have to pay the price and this happens in context where the communication between generals is very unreliable okay let's say they are talking via horse messengers for example and messengers can get corrupted they can lie they can get intercepted replaced whatever so you can see the analogy here this is exactly a situation where we were in we have to agree on a common version of that distributed ledger okay - all work on the same version of the truth but we have to do it on a network that's highly unreliable it's called the Internet and and we all have to come to an agreement okay so that's decentralized consensus thing and again there is no super general that arbitrates the decision in the end another thing is that it does that over a peer-to-peer network which means that at any point knowing notes can join or leave the network without any words of warning okay and this is absolutely key because this kind of problems the Byzantine general problems we already had solved it in the 2000s before Bitcoin but in a very specific context where the network size was always the same and the nodes were unknown and so on and all of that happens in a trustless community of anonymous notes so you don't need to authenticate notes before they join the network there are mechanisms in place to make sure that all those notes behave in a consistent way and you don't need to trust the notes themselves Trust is built into the network that's a key difference and all those nodes are here to contribute time stamped tamper proof and immutable transactions and again and in the case of Bitcoin those transactions are money transfers but we'll see that on a more generic blockchain so those transactions can be much more powerful and include any kind of value and for any kind of network of value you need your transactions to be immutable you don't want people to be able to change history you need them to be tamper proof so whenever they're sent across the network we have a way to verify that nobody fiddled with them and there are time steps so everything keeps I mean remains in order and thanks to all that we can transfer any kind of value as I said not just crypto currency but really any kind of value that we can represent and again remember that the blockchain the Bitcoin blockchain was designed as a way to create a monetary system and to transfer currency that's already a pretty nice achievement but what we have now with the theorem is the ability to do much more than that so concretely how do you use a blockchain well first of all you need to install Network client okay there is no server to connect to or whatever you just install this application on your computer and you start it when you start it it connects to the network so it finds a few peers around you and it starts to download a copy of the distributed ledger and that's something that it needs in order to start secure transactions so this can take a little bit of time okay right now when you when you start a new node from scratch it takes about two to three hours to download the entire theorem blockchain the goal for the developers is to bring it down to under ten minutes but still that's that's something that you need I mean at least if you want to start a full note the next thing you have to do is to create an account which on blockchain is pretty simply a key pair so private key and a public key your public key helps you identify your accounts your private key you keep it to yourself because that's the way you sign transactions and if you lose your private key if you somebody gets a hold of it you're screwed so keep that very very securely of course creating such an account a key pair is completely free so you can do it as many times as you want and nobody will require you to send you a copy of your identity card then to use the network you will need some cryptocurrency in the case of a film that's ether and the case of Bitcoin that's Bitcoin of course and every open blockchain has its own intrinsic cryptocurrency and you can do that very simply by going to something like going BAE's any marketplace where you can buy some cryptocurrency with your credit card or sending a wire transfer it's very easy now of course you can also instead of buying some cryptocurrency you can also mine it but that requires a kind of a bigger investment and then again if you buy some cryptocurrency on those marketplaces don't forget to transfer it somewhere safe because those those marketplaces are not what we couldn't call safe they are a centralized database where they store your private key for your account so if somebody gets to hack these well it happened in the past and that's where all the thefts happen there was never a known bug in the blockchain itself it was all the in the market places that happens that's the weak link in the network really and once you have some cryptocurrency then you can start using it to deploy a smart contract and will actually show you how to do that and then you can call functions on smart contracts so you can see small contracts like I'm gonna get user buzzword like micro services okay that you deploy somewhere and then it there's an API and you can call functions on it when you deploy your contract to the blockchain doesn't do anything it's just sitting there as is as a service with a state so with a persistence database of sorts and then you can call functions on it to modify that data and to get data so what happens behind the scenes this is a typical example of a transaction so you send a transaction from an account you send it to another account which can be either another human account I would say if I want to transfer money in this case it's a contract account so the nonce is just a way to avoid replay attacks so it's a kind of a counter if you want the gas price and the gas limits this is very important because that's a way for that's a way to pay for the execution of the functions that you're calling so yes nothing is free on the blockchain now of course as we said before this price is very very little for a simple transfer thing now it's under 1 cents of a dollar and then of course it depends on the complexity of your of your network sorry of your functions and that's the way that network that the blockchain network prevents people from abusing and overusing the capacity of the network if you want then you can with every transaction you can associate value again not just for money transfers but also if you call a function on on a contract you can also associate value to that to that call and in that case it's important to understand that every contract has its own balance so you are effectively sending money to a contract and then inside the contract you can send when your way and the data field is to include for example the name of the function you want to call the parameters and so on and of course everything is is encoded there so once you've created your transaction you sign it with your private key and it spreads over the entire network starts with the appears and then your peer send it there appears and so on and very fast it spreads over the entire network using a what we call a gossip protocol it's called the deaf p2p protocol and I won't go into details here but you can find this information on the ethion wiki then miners who are just other users of them on the network with a little option enabled in that client they can pick up your transaction and try to mine it into a block again we won't go over the consensus algorithm the mining stuff in here but I strongly advise you to have a look at this address where they really show you how mining works and you can really see those transactions work and it's all very simple there is a video on YouTube and there's also the prototype that you can try online so - pick up your transaction and mine it into a block and then blocks get built and append into the chain actually at which point they can spread across the network again and this s stats tool is just a way for you to see the actual live blockchain work and see the blocks being headed by by I mean who are the miners who appended the block and so on when a node receives a new block you can easily check it and that's what the note software does it does behind-the-scenes without you even noticing and we can check a few cryptographic primitives and then we're sure that the block is authentic and it was really processed via mining and again you can use as a chain that organized or anything to actually inspect the blocks see what's inside when it was produced by whom and so on and you rinse and repeat every few minutes so on the blockchain on the Bitcoin blockchain is every ten minutes on the thing emits about every 15 seconds now I think and this process goes on and on before we dive into the real blockchain again we just want to emphasize a few things that block chains are not very good at so if you want to run a blockchain on a small device think again because the of course as I said every transaction is appended into the the distributed ledger that everybody has a copy of so it grow and grows and grows okay these are approximate figures but for Bitcoin for example right now the blockchain is about 140 gigs and grows by 4 gigs a month for a film it's about 70 gigs I say about because actually and the etherium case it's a little bit more complicated to calculate it all depends on the mode that you choose to synchronize and so on so I won't go into details with that but it's about that order of magnitude another thing that block chains are not very good at is throughput we said it before if you compare for example the Bitcoin and the a theorem block chains compared to traditional centralized networks Bitcoin is about 11 transactions per second now a theorem is about 20 but and that's a max that's a cap it cannot be go beyond for PayPal that's about 115 but that's an average it's not a cap these are about 2000 transactions per second but again they can go up to 40 50 60 thousand transactions per second especially when it's holidays and everybody is buying like crazy so that's also something to remember again we paid the decentralization aspect with a little bit of speed another thing real-time so when you send a transaction to a network it has to spread across it be mine into a block the block has to be spread again across the network so all of that takes time and when you send a typical TransAm call to a REST API for example it can take a few hundred milliseconds but when you send a transaction on Bitcoin you have to wait for at least six blocks to be sure that the transaction is an absolutely immutable and that's about an hour for a theorem it's much shorter as I said blocks are produced every 15 seconds or so and you have to wait well it's with 17 seconds last year and you have to wait 12 blocks to have absolute confirmation of your transaction so that's also something to remember another thing is that block chains were designed to work in big networks with a lot of noobs so when he tried to apply it in small networks well you get centralization again and you get plenty of issues okay so it works great in big networks a few thousand nodes or something but as soon as you try to apply it to smaller context well it's clearly not efficient enough and it has a problem with privacy that's something again that's gonna change in the near future but for now clearly storing your medical records on the theorem blockchain might not be the best idea ever okay so keep that in mind everybody has a copy of all the data it's not like it's I mean everybody can see it right away they still need to decode it and so on but I mean if the information is precious enough it will happen and as we said before it's not good at free usage so every single transaction that modifies the state of the blockchain that changes something in the database has to pay a price and this price in the case of a theorem is called gasps okay it's not directly paid in ether but basically it's it's a unit of complexity the more your code is complex the more gas units it will require to run and then each gas unit has a price in ether okay so when you send a transaction to the network especially when you call a function a an ether transversal just sending in crypto currency across to another account it's about 21,000 gasps but when you want to run more complex code then it's that it has a price and of course the heavier the the operation you're trying to run the more it costs and you can find plenty of information about that on this website and it's not very good as at fast protocol upgrades you've certainly heard of Forks soft Forks hard Forks whatever those are the ways that we change the way the consensus works and the way the network works when you did when you deploy a centralized application on the server somewhere you just change the application you upload a new version and nobody saw it with anything on this kind of networks where the software runs literally on every single node then you have to upgrade that software on the whole network and that takes time and sometimes it doesn't happen very very well sometimes some of the nodes decide to upgrade and others say no I'm staying with the old version and then problems start to appear so if you want to understand more about what a fork is there was one on a theme - yeah - three weeks ago and it went well everybody agreed with it so everything went smoothly but we explained everything in this video and the next thing that block chains are not very good at is pleasing centralized institutions and regulators so you might have seen this news of this guy at JPMorgan that says yeah but the Bitcoin is a scam and it's gonna disappear yeah it's a scam for you because it doesn't fit into your world but we're in a different world now you don't know it yet but it's different so that's something also to keep in mind you will have some resistance from either big companies even nation-states governments and so on because this is a completely new paradigm for them the last thing I want to insist on before we dive into some code is what you should not call a blockchain and again if you've seen my talk last year our talk last year so that I'm kind of obsessive with that but I think it's very important to balance the message and inform everyone on what a blockchain really is because you might sometimes hear of this thing called permission blockchains and notice the number of quotes around blockchains for us and for a lot of people actually in the blockchain community permission block chains are like planes on highways imagines if the Wright brothers came with the airplane and this yeah it's it flies and defeats flies possibly flies and then people said hey this is cool at being fast let's drive it on the highway no no it was designed to fly don't care don't need to fly invested in roads come on let's okay and that's what most companies are doing when they're using this permission block chains they're taking an amazing technology that was designed for a very complex and yeah very new landscape basically and they remove the part that don't agree with and they try to do something with it but then what the end up with is actually over overly complex solutions to existing problems yes banks have a problem to be efficient and to make our wire transfers more fast for example yeah sure but is the blockchain really the best solution if you start with the same assumptions and so on don't you find an easier solution then this okay there's no buzzword and so on and your share doesn't go through the roof just because you added blockchain to your name but still from an architectural standpoint doesn't make a sense so for us those permission block scenes are a huge diversion of money and energy when we would need definitely to have more brains and more people thinking about open block chains and how to solve the last problems that we have so yeah that's the problem and don't forget there's a side effect to that it's not like okay let them call it blockchain and they'll come back afterwards there's a risk for open block chains as well no this Gartner innovation cycle there's a true of disillusionment at some point well that's when those big companies will say yeah it doesn't work that's because of the blockchain remember they did exactly the same for agile they did exactly the same thing for the cloud okay when it doesn't work it's because of technology it's not because of their decisions so mark my words so as you guess today we'll be talking about open block chains and more specifically about a theme and we're going to show you what is adapt so adapt is a decentralized application and very basically it's a set of smart contracts and again we'll show you what it is with a user interface so you have smart contracts that are deployed to the blockchain and when they are deployed to the blockchain to get an address okay that's a fixed address that you can use to to access them those contracts can collaborate together okay and they all expose some API that you can use to call them and usually you call them from a UI because yeah calling smart contracts directly is like calling a rest a REST API directly it's not the most user friendly way I mean for normal people I guess you all use pull all day and so maybe that's not for real people anyway and you do all that use all that in a special kind of browser why is it special because it needs some keys to work it needs to send to sign some transactions for the network and it does that with a bunch of key pairs are stored in the browser and of course this browser needs to be connected somehow to the network on earth in order to spread new transactions to get new blocks and so on ok so that's what we're going to show you now let's get our hands dirty let's say we want to organize a new conference ok and it will be a conference where attendees can register using cryptocurrencies instead of paying with euros and dollars and stuff that's so boring and they can also vote for talks that they attend nothing fancy here but the difference is that in this conference speakers will be paid according to their reviews according to the good ratings that they get from attendees for portion aliy if you want now let's call it let's say decentralized Vogler let's call it D Vox do you think we'll have a problem anyway so that's going to be the name of the app we're going to show you today a few disclaimers this is completely original code so we of course we demonstrate another application in our online course but this is a completely new one and it's the first time we show it so there might be some quirks here and there but bear with us usual warning this is by no means usable introduction so if you want to organize your own conference you might have to work a little bit more than what we did this is really to show you how it works generally we are going to live code some of it review most of it the goal is not to for you to watch us to look at us type for three hours but still we're gonna show you some real code and show you how it integrates in the application progressively and of course we'll everything is already on a github repo that will give you the address at the end so you can check out the the entire project afterwards to see how it works and we'll focus on small contracts first and then at the end we'll show you a bit of web user interface to show you how to finish I would say the architecture of this step so well we'll the development environment look like first of all we're going to use test RPC it's a common line tool that is actually an idiom node emulator okay so when you're developing you don't want to develop with the real network where you have to pay real money to run your contracts and so on and also you don't have to - you don't want to wait real time for it to happen so on test RPC everything is instantaneous it's completely fake everything happens in memory mining is instantaneous and transactions are basically free so that's very good for development just a world of running this tool test RPC has been used for I think a year and a half and so on by a lot of people we are going to show you the truffle build a framework also and in the new version that was just released last week test RPC is actually integrated into truffle so you don't need to start it on the site anymore but okay we adapted our content and we wait for the documentation of truffle to be complete with truffle we'll use it to create the project to compile contracts to deploy contracts to the network and to test also our contracts and we will use I mean we are using a IntelliJ because we are all time Java developers and we love it but you can use any text editor with some sort of a solidity plugin for syntax colouring and stuff it works really most of those tools I mean work with a simple text editor another terminal anyway and I think it's coding time and this yeah it's coding time so I'm gonna hand over to Sayid it's going to show you how to create the project and get started okay let's go let's - to the to the code enough with slides so as we saw we need to set up an environment before creating our our tap our doesn't allows application and the first thing that we have to to install is a serial node so as Sebastian told you we'll use test RPC because it's fast and you know when we develop an application we hate to wait so to install test RPC it's quite easy it's the start be seen that it's no GS implementation and you can find it on our github this address github calm guitar MGS just RPC the installation is quite easy NPM node package manager install my energy to install it to globally ETA mgs - test RPC well we are not going to show you how to install no GS I think that guy that you are developers and I'm sure that you are able to to do it on your own so to install test RPC and PM not in uppercase air and p.m. install managing aetherium yes - this RPC hello so I will install it and the installation is quite fast it has been optimized since a few months now you know it uses only JavaScript libraries and dependencies so that's pretty fast okay sorry the installed if I want to start it test RPC and here we a lot of things first the version of the Star PC version 6.0.1 it's really important with the etherium or with any earlier technology to check the version because sometimes you have some you know some upgrades some minor fixes or some margin fixes and when you are working with the the ethereum blockchain some major changes can can break some some things in your just for comfort so you have to be really careful and to to be sure that the new version will not break anything on your on your code something in nothing new first for people working in a JavaScript environment anyway breaking changes in minor versions duh so test RPC is powered by a library called ganache core so ganache core now is bundled with within truffle dot 4.0 so with truffled 4.0 that that has been released the last week you will not have to start to start PC I try now well just just just just use this version just for PC so it's a test aetherium note it's creating for you 10 accounts and test accounts you can see here their public address and their private keys the private keys will be really useful if you have to import the account in a third party application for example metal mask that will be illustrated at the end of this of this session and test RPC is also creating 100f error for each of these accounts but don't try to sell these accounts on the main Network it will not work if anyone is monkey money exactly so test RPC check now we have to we need to have a framework to create to compile our smart comforts but also to deploy to this to this this in theory I'm not so we see that we will use a truffle as a development framework as you can imagine there are many frameworks but we think that the truffle is really a great solution this solution has been created by consensus the company beyond truffle and beyond a lot of tools that are used in in the interim ecosystem and we think that truffle is really great and if it's a solution that is I will say improving very fast very very fast the version truffle 4.0 is really promising but here from now we will use the previous version because we know that this version is really stable the documentation is up-to-date and well it's a demo we don't want to take risks yeah yeah offer for is has been released with documentation not yet good practice right anyway so we'll have to wait a few more weeks together up-to-date documentation on that side and it's really important because they added quite a few things so yeah so to install truffle same thing you have page here describing the framework as you can see you can you can compare smart contract deploy deploy it's basically the Gradle or maven before adapts install it npm not packet manager install you install it globally truffle but here then use this command because it will you it will install for you the last version so we will use will install the previous version the command is npm install - j trust and we'll use the version 3.4 dot 11 don't do it and here also the installations fast okay so truffle is installed if you want to check what you are using here truffle version and here we have two information that is really important first the version of the framework but also here in the version solidity version all 415 and solidity is the programming language that we will use to create the the smart contract so we have several programming languages I think that is Viper well there was there were already a couple of programming languages at the beginning that completely were thrown away like serpent if you might have seen something like that now the main languages being used is solidity the new one is called Viper it's not really ready for primetime yet but it's going to be soon and there's another one called bamboo so basically you're in known territory here because it's it's the same logic as with the JVM the the if your network work works with the EVM the etherium virtual machine so it's just by code and then you can use any kind of programming language that compiles down into this bytecode to work so solidity solidity is probably the most popular one with it exactly and you can find the documentation of solidity in this URL solidity dot read the docs at i/o and develop but again here the version of solidity that is provided in this page is the last version you can read the documentation that is related to your environment by you know switching here the version 0.4 dot 15 and you will have all the documentation related to your version and unlike some of an environment you might know new versions of the language don't have don't happen only every three years or four years it's more like every two months so yeah you have to keep track of that as well okay so now we have the framework we can create compile and deploy the smart contract we can also test it we have a a theory of node implementation with test RPC that will create a blockchain in in-memory watching this is why the star PC is quite fast but we have to edit the code of course so you can use the eye editors that you love for example not but or fin or whatever but effectiveness also has an interesting year yeah we can see in the ecosystem a lot of people using a toe as as an IDE text editor from github and if you want to to beautify your code your solidity code you will have to install a package in atom and this package is called I don't remember the name yeah it's a language - Assyria you can install for you a plug-in that produce if I your solidity code and if you use IntelliJ as as an IDE as your IDE you can install a plug-in that is maintained by the community that is cold including plug-in here solidity it's called individual region IntelliJ solidity this plug-in is quite interesting because of course you have all the other things beautify you can also you have the the auto completion that is quite interesting but of course you can use the power of IntelliJ to navigate between your spoiler alert or showing the full contract sorry spoiler alert you try any full contract okay anything okay so now we have the environment you are ready to create our project our effects project I don't know if you we are we allowed to use this name so ask for forgiveness alligator permission so first I have to create a folder of course call it a box and then I have to create a structural form for my contract hopefully truffle is providing a kind of boilerplate for you kind of start a project that that you are that you can use to create your DEP if you create if you want if you just want to create a smart contract work without any kind of front-end application you can use the command truffle in it shuffling it will create a project structure but just to just for your smart contract but here of course the plan is to create a real death with the backend part and a front-end part the backend part we will be your smart contract the frontal part will be your the the web application that will be used by your your users and there you can use any I mean any no you can use some of the front-end frameworks you're used to it react or angular or view Jas or whatever well some of them are clearly more used than others but yeah there are boxes because that's the feature that's going to be used for all those different frameworks so here I'm going to use a simple box and this box is called pet shop so it's uh I'm sure you've read that or use that somewhere the pet shop application ketchup application it's a it's a jQuery based application so to create the starter program I type travel unbox pet shop so here is creating the structure it's also setting up the NPM environment so it's downloading all the dependencies and you know how empty it gets downloads the internet so yeah it's just to show you the process but so I'm going to truffle framework boxes and as we see there are plenty of boxes available for you about for react a box integrating of the authentication mechanism but a box with the reactant new port such as it's kind of an SSO for the blockchain if you want yeah the patch of the ones that you are going to use a bot that will allow you to create a RC 20 token in order if you want to launch an ICO so this is the thing that you can use the web pack and you have some contribution here for status for angular 4 and for view for who I don't know what was a lot a lot of stuff but if you are brave you can create your own box by using this blueprint and to meet you - truffle ok so here the project is created ok we can open it and that's why we like also to work with IntelliJ because the support for those front-end frameworks is really good so you can work with your smart contracts and your front end at the same environment of course you can do that also with a Dom and so on but when you are used to to IntelliJ well so it's better so before starting coding we have to analyze the folders that have been creating for for us so far as creating these folders the contract folders migration node modules SRC tests and some some files so contracts will be the folder that will hold your smart contracts and you can see here that truffle has created the first contract called migrations so migration will be used part by the deployment mechanism of truffle to know exactly which contract still needs to be Royden watching so it's something that is really related to the truffle and migration will hold the configuration file that will be used to deploy your smart contract when when this my contract will be compiled this file will be used to deploy the the contract the bytecode to to the to the SE ryeong load deployment scripts basically so that's where you specify which contracts should be deployed in what order and some you can configure how you deploy them and so on non module so it's a no GS Thames starts all the dependencies that are required by your enrollment SRC will contain all the files that that will be required by your front-end application so the CSS the font images JavaScript this is where you will you will store your business logic the html5 saw the pet star J's and it's something that was provided with the pet shops and simple application and here enters folder test this is where you will create installed your test suite to to test your application it will be illustrated by Sebastian in the few nets and we have here a file called vs configured our JSON this is because a pet shop is using a light server and light server comes also with the processing so it's a library that will optimize the realm of your web application we have the package that JSON where you will put your dependencies and also to have some comments for example and p.m. run dev will start here the light server and and this file is also interesting it's really important for travel it's the truffle de geus so truffle DOJ's will be used for travel to to locate the target our environment where to deploy your smart contract so here the Folies empty but we will have to to fill it in a few moments so that's it for the explanation of these folders but now we have to create the smart contract and we'll call it conference so we create a new file on the contract folders call it conference the sol sol is the file extension for solidity and then that's it let's start the writing we have here to define a keyword pragma so DDT we have here a version 0.4 dot 15 ok and instead of typing here class will type contract and the name of the contract the conference so here the the pragma directive it just used to ensure that the smart contract will be compiled with a computer we will will with them to not break any rules on on your contract for example here the compiler that is below the version of 0.4 dot 15 of the solidity will not be accepted and all version of the solidity compiler starting from the version ordered 5 will not be accepted to this this is done to ensure that yozma contract well will still work and we still be able to be compiled so here I have a contract called conference now I have to create the state variables that will hold my my data so the state variable will be the contract state and here I have yeah this one so the conference will have to store some information to identify at all and this is what we are going to do here so we will create that there what's up in an interactive approach so the first step will have to store at all and to read the details of the talk the talk will be identified by a name a title sorry a location a title for example just talk one a location room for a start time and an end time that will be a coded in a unit size it will be compatible with UNIX timestamps the number of seconds since January 1st 1970 and we will have one speaker per told and the speaker will be identified by its address and a name a full name here you can see that in solidity we have the the data table address that that is a pretty wide value 20 mile value used to identify the the public address of of the accounts and here you can see that it's quite similar to any any other programming language just precision you might hear us talk about addresses and public keys interchangeably that's because they are basically based on the same information the address is a hash of the public key so it's a shorter version I would say of the public key so we have strengths you you int and signed int and of course solid it is providing other kind of data types so we need here to add at all and for that we have to do to create a function that will be called I'm told but I think that I already know you have to that's the constructor yeah start with this so the app dog we're going to analyze this function so as we can see it's a function and it's public that means that this function can be can be a cold mouth of the contract exactly outside of the contract it will receive a set of parameters a title a location start time and time speaker address and speaker name forgot this for for now and here we are just assigning these input parameters to my state variables this means that this function it is changing the state of my contract and it's really important because you will see that if you call this function you will have to pay some gas Agito and storing data in the state of the contract is one of the most expensive operations so that's also something to keep in mind and of course you don't want that anyone will be about to add at all so this function will be allowed only for the contract owner and the contract owner will be identified by this state variable variable address owner so how to retrieve this contract owner we will reach with this information from the the constructor so when you create a contract when you deploy it to the shin the constructor will will be called the constructor is an optional function you can declare it or not but here we are going to use it in order to to keep to retrieve the address of the contract owner and this address is provided by this global properties global variable school called MSG dot sender you have a set of variables but this one is really useful because you will have the address of the contract order so and this can be used in any function that is non constants will see the difference afterwards but that's a from within any transaction you can know which account triggered that transaction and this is in the MSD descender so here we know who has deployed the contract who is a contract owner and each time someone is calling the adult we will retrieve the address of the function color to see hey guys are you the contract owner yes or not if not so you are not allowed to to alter the counter the state of the contract if yes we will get all the parameters and we'll change the contract it's instant state okay so I told it's done so J of course you as you see here we have only one talk and one speaker so it's basically meet up we'll add more talks and speakers afterwards a new lock also to retrieve the details of the talk so here we have again a function it's called yet talk it's public and we have something new here constant constant means that this function is not allowed to change the state of the contract and it's also mean that this function is free you can use it without paying any any gas and this function will return a set of properties the title the location start time and time speech address speaker name the solidity supports multiple returns yeah so that's it we have a smart contract called conference that has function that will after the contract state a getter that we return the detail of the contract and when we deploy the contractual web we will be able to identify the contractor owner so I will say that from now that's okay so we are ready to deploy it but to deploy it we need here to create a deployment file to inform truffle that this file confirms that Seoul will have to be deployed to to the to the to the blockchain so here we have to create a new file under migrations and this file will be called to the naming is important especially the prefix because the way that truffle works is kind of unusual for us Java guys but the migration scripts are ordered so they are run in sequence based on this prefix and and then that's what is used in the migrations contract to keep track of where the last script was run so here I have created another my Grosh falter a file called to underscore g pro contract that contract so again the naming is important especially the prefix part so yeah done and here we will inform truffle that the smart controls that is located in this file conference that's all will have to be deployed that's it and everything will be managed by truffle so how so it's time to to deploy so it will it will compile so dtc file into a binary file create the transaction with the network sign it send it to the network everything is handled by by truffle for us well well but before deploying the smart contract we have to inform the target environment truffle here is providing a file called truffle de Geus this is where you are going to to store all the the target enrollment for example here we have a project that was able to deploy contract in different environment for example a development environment located in localhost for 8545 or to the ring be test public test network or also to the live network to the main network an interesting thing to note here is that you can see that the the host doesn't change it's always localhost and that's a key difference with the the environments you might be used to because actually even if you're deploying to the main chain so to the production network if you want the host you're deploying to is basically the local node you're running on your own machine and then it spreads across the network automatically so our here I will inform travel that the contract that has been compiled needs to be deployed on localhost for 8545 and it's a different development platform so before to do that I need to start test RPC a lot of it's already started maybe somewhere so just RPC if it's already started will complain anyway so tests are PCs running I go to here until eg and I will types up some comment here truffle migrate and we can see that truffle is compiling the contract located under the contracts folders and here it has deployed all the other contracts the migration that is used by truffle to better mind if another contract needs to be deployed but what is really important for us it's this information here deep loyal conference blah blah blah with here and address here it's the edge code of the transaction because we have sent a transaction to the to the to the to the ethereum notes saying hey guys I want to deploy the contract and I will pass as value the bytecode of of the of the smart contract and this is something that we have also to mention when we build a smart contract well we'll we'll create a byte code that will be executed by the Italian virtual machine and this bytecode is sent within the transaction can you show the console for test RPC yeah if you want to adjust to the test RPC terminal we're not sorry just switch yeah so here you can see all the transactions has been been created and how much gas they cost you know gas usage and so on so that's also some information you get from test agassi and he treated for transactions one to deploy the migrations contract when to update the migrations contract saying the migrations contract was deployed another one to deploy the conference contract and the last one is to inform migrations that the conference contract is deployed so this is the migrations is like the the deployment state if you want so now if we want to interact with our smart contract we have to use a console so we'll use the truffle console here okay maybe you can reduce show move up the console no the I mean just reduce the window or something yeah before interacting with the contract we need to getting instance of this contract and here to get instance I have to name my contract conference deployed so just well is typing the the truffle provide in addition to all those build and test tools provides us with an abstraction on top of the notion of contract called truffle contract and this is this abstraction that side is using now to the deployed function and all that is provided by the truffle contract dependency there on hit ensure I have a lob Jack that I can use to introduce s-type just like that to show that it's just a JSON object containing the structure of the of the contract and all the functions that you can call on it and I think your screen is to begin II to reduce okay yeah so let's start by reading our instance of the contract by calling the get dog if I type up get dog we can see here that we receive some information but all the information are empty because by default the the instance of the contract are set to empty values for example here it's normally it's the total title description the start time the end time the speaker address and speaker name so this value are are empty its default initialized if I want to see the address of my contract I can type conference that address and have the contract of the contract this is the address where the contract can be found inside the blockchain and if I want to add at all so I just have to type the adult function so the first permanent will be the tour title bulk one for example before that I have to create the start time and end time yeah so L is doing that just to explain the difference before you called yet talk and as we defined it yet talk is a constant function so it is just reading States from the blockchain and it doesn't need to send a transaction for that it has a full copy of the of the ledger locally so you can just see what the data is in in the local instance whereas here what we are going to be doing is calling the add talk function and the add talk function actually modifies the state so we need to put in a transaction to spread it across the network needs to be mined and so on and so forth so that's there's a big difference it's kind of CQRS styles of things there is a big difference between reading data and modifying it oh my god it's better time so here I'm creating my start time just get here number of seconds so I have here two variable start time and time and just another word of running also as you can see here those two values string stuff it's the start time and end time but an unsigned integer in the context of the the virtual blockchain sorry if your virtual machine is actually a 256 bit integer that's quite a huge and clearly JavaScript doesn't have that kind of data type so it's using a an alternate data type called a big number and that's his way of representing it's that's basically like your your big decimal and in Java or something so I'm ready to call the at all so I need a tour title location start time and time a speaker address so I will use one that has been generated by a desktop PC for example this one okay and a name John do I know that everyone is knowing this guy so I'm ready to a call no you're not ready to go no I'm not ready it's called because if I do that the problem is that we don't know who has to pay for that you know the adult function is a function that will alter the state of the contract so someone has to pay for that so we need to identify who is calling this function and for that I will add an additional parameter or call from and I will say you remember that the adult has is only allowed by the contract owner right now I can tell you that the contract owner is the account zero so when you run truffle migrate it will actually automatically deploy it if you don't specify you can specify the sender address in the truffle dot JS file if you don't specify it it will take the first account of the note it's connecting to so here and test RPC generated the ten accounts and we can access them with web 3.8 eh that accounts and we take the first one because we know that's the one it was that was used to deploy the contract and we add that with kind of a JSON object that is the last parameter so there are many different metadata elements that you can put in there here we just need the from yeah so suspense I type ooh I have a transaction so that means that we have called the I told function and a transaction has been created this translate transaction needs to be mined by the node and we have here the ash of the transaction and we know that the transaction has been mined it's located in the block number five in the chain data it caused this case this number of gasps sorry and that's it right now and for that issue again you can specify that but here it's using the default gas price which is a 20 or 40 gigabytes I don't remember anyway so the theorem one ether is divided into 10 to the power of 18 ways that's the minimum unit of value and there is a price in way for each gas unit so that's how you determine the the actual cost in ether sorry I'm going to see if my contract state has been changed or not I call up kettle and voila so here inside the smart contract somewhere in the blockchain the State has been changed I have bulk won the room fourth so the location has been changed I have my start time end time I have the address of the speaker and the name of the speaker and remember this this address you know wet 3et Ashe account zero as you can see here test RPC is creating for us 10/10 public key and there are available here on the console by typing web 380 H that counts we have an array of this accounts you can see that they are the same okay hopefully and to identify the owner of the contract you can type web 380 H dot conveys and conveys is the default account that owns the contract and here the conveys or ether base in ethereum it's their dress or 8 or 17 blah blah blah and by default we always use the first cancel unless you override it intro for the GS turns ok so and of course here you can retrieve information from the get old can hear you received here an array of information and I think that can you can type get talk the function beta so the truffle contract obstruction uses promises all over the place yeah but when you return multiple values from from a function then you can access them in an array for example here I have achieved the the position 2 of my array and it's the start time okay so in summary we have what we have done here first we have created a contract called conference we said hey guys the recommendation is to use a compiler that is compliant with the version of my smart contract we have created a set of variables this is this is the state of my contract the first iteration of this dub is to say we are just creating one tool and to get the detail of this talk so we have here a function called adult that is public and this function is altering the state of the contract so we'll have to pay some gas for that but this function is only a load for the contract owner as the contract owner has been stored when the contract has been deployed we have retrieved the the address of the contract owner and store it into the owner address variable and then we are able to retrieve information from the smart contract thanks to the function get dog if the function is public and it's a constant that means that I can call this function 1,000 or 1 million times I will not pay for that it's free because it's a read-only function since this function is not too low to alter the the contract and we have of course here created a file to see when we will deploy the contract I need to retrieve the contract located in this file and then by default when I typed truffle migrate please deploy the contract that I have just built into the ethereum node located under local host port 80 45 and the pool this this node is the test RPC that we have started doing before that's so now we wrote our first smart contract we deployed it everything but it's very important to remember that block chains are immutable so when we when you deploy something when you deploy a contract which is as I said before like a web service of sorts to the blockchain it's immutable so it cannot be modified after it's been deployed sorry I forgot to mirror on my screen so it cannot be modified after it's been deployed so it's very it's even more critical in a blockchain environment to actually test everything that you do before you go live and that's what we're going to see now how do you unit test such a contract so that's exactly the contract that say it showed you and I'm gonna show you how to test it so there are mostly three strategies for testing small contracts of course you can run unit tests and usually you do that on test RPC you don't want to be running unit tests in a main chain because then you will pay for running your tests and then you will not do it which is bad so let's do that on test RPC once you have unit tested your contracts you can also integration test them and you can do that either on your own private blockchain so you can create kind of the your own development Network if you want or you can do it on one of the public test chains so the theorem in addition to the main chain there are a certain number of test nets that you can use that our public that you can share with everybody so when you deploy something in there the ether is not real again it's fake money but but still you can deploy your contracts there and share them with your friends for for user testing for example and now what we're going to show you is how to do it on test on PC again when you do it in test RPC there are when you unit test your contract you can do it in two ways either using Java scripts and that's what we're going to do you can also use solidity so you can have a contract that's used as a test that calls your contract but that technique is kind of new and not very well documented so far so I'm gonna focus on on the JavaScript part what else would I want to say yeah so the first thing we're gonna do is just start to star PC again so I'm going to front machine so I'm doing it directly in there okay again 10 new accounts the the accounts are random by the way you can start the stuff we see with some common line arguments so that it always creates the same accounts which can be practical in some in some ways but yeah so test obviously is running in the background we don't need to care about it for now now I'm going to create a new test in that test folder and that's gonna be a JavaScript file so just call it conference conference test dot J s okay and there we are going to copy some code so show you what it does same thing as in the deployment script we need to import the conference smart contracts the source just to let know to let refer know that it needs to compile that and it needs to deploy that before we can run the test and then truffle integrates with mocha and chai so I think it's mocha for the test framework and chai for the assertions or something like that yeah and and you can use just that for testing your small contracts so instead of having a test here we have a contract directive that takes the just the name of the contract that we want to run this can be anything but the name of the contract and the function that will be used to test takes a list of accounts and parameter and that's the accounts that are available on the node that we are using and this is very useful because when we will test some actual calls with transactions we'll need those accounts to send I mean to pay for for the execution of those functions then we use actually it's for Casey here conference that deployed same thing as we did in the traffic console it's really the same instruction then function we get an instance and then once we have an instance of a deployed contract we can call the get talk function on it okay and then in the return and by the way this when you when you call a function that triggers the transaction this then is only executed after the block has been mined here there is no block that needs to be mined because it's a it's a constant function and then we can use asserts just to check that theoretical values match the real values that we get okay and that's really all there is to it now that we have that we can go back to the terminal open a new window and here use truffle test and what truffle test will do is compile the contracts that have not been compiled in a while deploy them to the to the I mean to test our PC in our case and and run the tests and it will redeploy the contract for every test suite and it's a suite is just one of the JavaScript files so you can have several test cases in stage suite and each test suite gets a a blank slate in terms of state so here I'm running truffle tests it compiles the contracts and deploys them and here we have conference is not defined yet because I was actually using the wrong case in both so now I can run the tests again redeploy the contracts and we could we have a passing test yay first one so and here of course if I check for example for a non-existing value and I run the test it will fail and sometimes the error messages are very useful sometimes they're a little less than that but still so the test passes now I want to show you how to test the add a talk so for that we'll add a couple of variables to our tests so inside of the contract structure I will add a couple of variables okay so we'll use a variable to store the contract instance want to store the owner of the contract just all over the the fields that will pass to the get to the at talk function and that's basically it okay so now we can add a new test case to the test suite right after here okay so let's go over it this one will test what happens when we add a talk so again we'll need to call a conference that deployed okay and then we get an instance that's always the same mechanism this time we're saving it and then on it we're calling contract instance that at talk okay with all the values that we've defined above and the special parameter at the end that we use to specify who we want to deploy the contract and in that case that's the owner which corresponds to the first address of the first account okay when we call the add talk function we wait for the block to be mine so again this abstraction make sure that we only get notified when important information is available so once the this transaction has been triggered and mind then we can query the state of the contract by using contract incidents that get talked and then we should have all the same you as we had stored okay so pretty straightforward by the way you can see that I can use the two number function on the big number to transform it into a JavaScript number so that I can test it of course I take the risk that if my number is really big then I'm going to lose some information but here is just the start time and end time should be good okay I'm run truffle tests again and here we have two passing tests my god I'm on a roll this was not prepared at all of course and that's it that's how you unit test your code so the end smart contracts is really important to test them once you release them into the wild then your if something happens you're screwed by the way something like that happened in June 2016 some contracts were poorly tested and they were released in production and it resulted in fifty million dollars being lost or something so yeah unit tests are your friends now the other thing I want to show you in addition to test is how to get notified when something happens in your contract so with events your contracts can communicate with your application with your especially with your front-end so let's say for example I'm running the front-end I'm going to show you later and somebody adds a talk I would like to be notified so that I can reload the screen and show the ad talk there is a way to do that in solidity it's called events and what's important to understand that events are actually like logs so they are added to the blockchain but not they are not part of the blockchain state they are in a different data structure in the blockchain that you can watch for events and interact with but only external account sorry only external applications can view those events contracts themselves have no access to them they can send events but then they don't know who is pressing is processing them and so on and more importantly they cannot watch events okay so here we are going back to our conference contracts and we are going to define an event that will be used to notify Watchers when a new talk is added so to do that I'm just going to go right here and add this event so that's the way you declare an event with the event keyword that's the name of the events and then I can associate a couple of fields to my events and that will be useful for the Watchers to know what what happened exactly and that's it for the declaration now of course I need to use that event and I will do that directly in the ad talk function so once the ad talk function is successfully saved everything I can go there and call the ad talk event and pass it the title the start time and the end time and that's it when I do that there later I can watch this event and and react on it okay I will save this and I will redeploy this contract now they start fishing still running in the backend it's in memory but it's still running in the backend so it still has the old versions of the contracts deployed and we actually already run both migration scripts one and two okay so if I run just truffle migrate now which I will do okay if I just want truffle my great actually sweet do we need way because I changed the contract and it detected yeah okay so if I just run after the contracts have already been deployed it will say okay network is up-to-date I already run both migrant migration scripts as confirmed by the migration state so it should be good if you want to force the latest version to be deployed you have to use - - - reset and then will force everything to be deployed again okay now contracts are deployed and everything so I can run traffic console again okay just push that up a bit and with that I will be able to interact with my contract again so the same thing as before we started a new console here so we need to get a new instance of our contract conference deployed then function instance of equals instance oops yeah I made a steak I thought there was one too much disease yeah that's it okay nope that's better okay so now we have app that contains the interface to our contract and we can actually watch our events so for example I will add app Talk event okay so I want to watch this event and then so this is the way I get the access to it event and then I call the watch function on it and this will have unfortunately that's not very consistent here it's a good old error data callback it's not a promise and here I will just console.log it that's one of the main difficulties when you're working with this set of tools right now it's still very inconsistent sometimes you have promises sometimes you have callbacks sometimes you have nothing and yeah it's kind of frustrating but you get used to it and the things that we can add to the document is a way to filter which events we want to watch so you can use a first object that can be used to filter on the values of the field that we have added to the events here we'll just take all events and you can also say the which range of block you want to of luxury that you want to observe so here we want to observe only the latest block and not the whole chain okay I don't think this is right is it yeah should be okay we'll see so now that we have that let's recreate our start time equals new dates okay stay here whatever just keep it that way yeah that's a start time and we'll do the same for end time let's say we have a talk that lasts a full day come on and then we can call the at talk again okay so let's call it talk one it will be in room one start at start time and at any time I can use one of the accounts you can change your own terminal just or just by using what three th accounts oh yeah all right Group three third ETH accounts six for example over here so that's my speaker address and then my speaker name John Doe again of course I need to specify from which account I want to send that and that would be web 3.8 eh dot conveys accounts one of course I can do that right wink wink so it sent a transaction but we see that the logs are empty so no event was triggered and we didn't get the call back either okay so that's the kind of thing that can happen you have something that gets an error but you don't get any obvious sign that something wrong happened but here something wrong obviously happened because we didn't use the right account to send a transaction from and when you do that oh we get a transaction and an event so this time I'm just going to show you again the okay so here what you can see is that after we called the function here okay so this one will send with the account 0 we get the transaction receipt with the transaction hash all the data we have some logs and logs contain actually the events that were triggered inside this transaction and here we have the ad talk event type and since we registered a watcher on the on that event we also have a console log afterwards that contains exactly the same thing as the logs here and gives us information about a talk event and we have the arcs that contain the values that were sense with the events okay so that's how you can watch an event and later on you can do that also in JavaScript in your front-end to be notified when something happens by the way this mechanism of events is very important because you've seen that we used returns for constant functions but there is no way to return anything from a non-constant function so when your function is actually modifying the state of the of the blockchain it does it in an asynchronous way so you need to send a transaction transaction needs to be mined and so on so you don't have direct access to the return values yet they are incremental e adding this kind of feature to the to the language and to the EVM but for now there is no way so events are the only way by which you can make sure that you get some some real value okay now that we have added this event as I said before we need to test everything so let's go back to our conference test and we'll add a new test case that will test that an event is currently trigger when we call our talk okay so this will be okay so same thing as before here we'll just we just call in conference that deployed definitely need to change that later on we just need to call conference that deploy to get a deployed instance of our contract we call the ad hoc function again from the owner same as before but this time we get a function receipt so earlier we didn't we didn't use that yeah here we didn't use it okay but now we can access the transaction receipt and this transaction receipt among other things contains a logs field so we can check that we indeed received just one event and we can check the data will the name of the events and the arcs of the seventh okay when I do that go back here actually we'll open a new terminal and run truffle test and we have three passing tests so that's it we show you how to unit test your smart contracts and how to use events to get notified this will be very important in the front-end later on and I think the next step will be to show you how to register to register an attendee but would probably do that after the break right now 15 minutes break so 15 minutes break let's come back at 11:25 or something and and then we'll show you the rest before we start again I just wanted to answer for everybody one of the questions that we had at the break because I realize now that can be disturbing when we when we run a transaction when we call the add Talk function we specify the account we send the transactions from okay and we just indicate the public key the address of that account you might think okay but then that means that anybody can call this function without any other form of it but I'm taking the authentication well actually not what happens behind the scenes that we are using test our PC and test our pcs development environment it's an emulator so by default all the accounts that are created for us the 10 accounts are created for us at the beginning are unlocked all the time ok so you don't need to specify any sort of authentication it can use the accounts directly on a real chain all these accounts would be locked with a private key so you need to have access to the private key in order to be able to assign the transaction and then to call the function and of course we you cannot pass the private key directly as a parameter that would be a security issue a security risk but what you can do is unlock the account before sending the transaction and you can do that with some commands in a trifle console for example and and then after that you can call the function and it will use the the private key to sign the contract the local so that's also something I wanted to clarify so now we are going to show you how to register attendees for our talks which will be useful because if you remember people will be able to register using cryptocurrency ok so in this step we are bigger we're going to register an attendee why because here we are creating a voting system so you guys you will be able to vote for a for talk but the the requirement is that an attendee has to pay a fee to attend the cont turns so to be an attendee you have to pay fee for the conference this is what we are going to to implement soar here we will see an interesting notion called the value the value that will be sent with the transaction so here we'll start from this version of the project we have the death toll and I get all can the events and the testing that's great and now we are going to create a payable function called register will see what is a PayPal function so here I'm going to create some variables - for the first one will be used to hold the address of the attendee so your address your ethereum address the public address your name and here we have a constant variable that will hold the price and you can see here that the price is not expressed in in ether but in way so ether is is the the value that is used in ethereum but inside the VM we have a kind of metric system used to to codify these these values and the base unit in etherium is called way you can use some website to convert a an ether for example in way for example one ether is I would say one exponent 1810 1818 away so the registration fee will cost the check one point 1.8 ether and now we are going to create the register function so here it's just as a point of information as of right now a theorem is three hundred dollars when he forgives three hundred dollars so here the requirement is to say if you want to restart to a conference you will have to pay fee the registrants the registration price and the value that we'll send to the smart contract will be must be equal to this registration price and also you you you are not allowed to register twice so will check if you are not already register to the conference and then if everything is okay we'll be able to alter the state of the contract as you see okay you are allowed to register to the conference and here we have a new function called register and if we check this function it will receive only one parameter the full name and here we can see that it's a function called register with parameter the function is public and we have something new here payable payable means that this function is allowed to receive value is allowed to receive money from the function color for example at all here it's a function public function but it's not allowed to receive any value from from the function color and it's important to understand that this value that you can send to a function call is separate from the gas ok the gas is the operational cost of your call but here is really a payment that you can associate with your call so here we are using the equal function that will ensure that the value that you have sent is equal to the registration price that is the constant that we have defined on top of the code so here we have another global value that is provided in the in our code you remember the message that sender that contains the address of the function color and here message that value contained the value sent by the function color color to the to the to the so here if it's okay great we can we can render that the next line and here we will ensure that the message sender is not already registered so again this is a small conference one talk one speaker when you call attendee so it's really sad it's my star turtle is an MVP okay later on we'll add more registers and if the rules are okay so you can store the message sender to the attendee address the foo name in the attendee name and then - to trigger an event a register event so I have here to create this event okay so I have an event called a register event that will that will provide the address of the the attendee and his name okay that's it I'm just yeah one thing that he didn't show you is when you when you specify the arguments of an event you can mark them an indexed and when they are indexed it makes it faster and easier to filter events just by this value so for example if you want to filter events that are reduced where a specific attendee registered then you can use this indexed argument and here we are also to create an additional function constant function so a function that is free free of charge and this for chun is called is registered is register will be used to know if you are already registered to the to the conference or not and here we received an address and that is the the account your account for example and to check if this account is already registered or not for example this this this function can be used by the the organ organizer of the conference just to to check if you are if you are already registered or not and that's it if you are not registered it's return false otherwise it returns true when also another thing that is important you consider this require a call that's also something new it's basically like an assert it's provided by by solidity and what it means is that if the condition is not verified then it throws an exception and that will be somewhat better than what we had before when you remember in the ad talk if the owner of the contract was not the message sender then we simply returned and we didn't have any exception as you saw we just had a successful transaction with the exception the transaction will actually fail and we'll get an exception so this require thing is is important ok that's it we can build and deploy this version of our contract in this terminal I have start test RPC and here I'm going to migrate my contract migrate and check here okay the contract has been deployed and then I can open the console refer console I will get an instance of my contract okay and now I will I would like to listen for my my event just to be sure that the register has has been done properly so how will create register event variable I'd like to hear for my event this one so what is doing here by recording the reference that's registers return from the watch function it's just a way to keep a handle on the event so that you can unwatched later it's also a good practice especially when you're running this from the front end to not watch event all the times because otherwise you can you don't know what happens if you're not there to listen so yeah then later on we can use register event reference to and watch the event and here I will filter my event for the account 3 because it will be the account that I will use as as the the attendee of the conference watch and you don't you arrange sorry no range of blocks okay you are not obliged to specify a range by default it will be from letters to to latest so here is using the filter parameter that we left empty before now I will display my event here event okay first time so now I'm ready to register the account 3 but before doing that we'll check the balance of this of this account so I will call this function web 3 ETH get balance for the account with 3 that ETH that accounts 3 and we can see here that the account 3 has a lot of weight a lot of way that's the problem and so all the unit's manipulated by the the contracts are also always in a way and fortunately for us there are a few functions that are available on the web 3 library that allow us to convert way to ether and ether to weigh its own so I'll always remember that because sometimes you're you don't understand what I I send one ether to my contract and it's not working no because you send one way it's not enough and here I have used web 3 from way for the balance so I can see that here this function is converting a way to ether so I can see that the balance of the accounts 3 contains 100 ether so can you check also the the balance of the contract yeah so the that's also an important thing to understand is that every accounts that you create has a balance an ether but every contract also has a balance so you can actually send money to a contract and that's what we're going to do with the valuable function so here we can just call the get balance function on the contract address in the same way as we did it for for a real account I mean for an external account as it's called and right right now the contract has no ether in its balance now I'm going to register this guy so register I have to give a name so Rick the cart plan to attend the conference and the attendee will be easy again 3 yeah and here we have to pass another parameter called value this is the value that you are willing to pay that you are willing to send to the to the transaction and here remember the conference cost one put on eight ether so I will use here web three two way because I don't want to write this value in way I will type 1.8 and to see this value is given in ether yeah okay register read the card from a country I sent 1.8 ether from my wallet and I'm ready to go and you can see here we have received an event this event is called has been triggered by register event and the address of the attendee is oh it's 8 5 blah blah blah and the name is read the card if I check here my you can just call it back now you can just going back with the upper rows you want to show the balance yeah you can just of course so if I change the balance of the account 3 you can see here that we have paid something so 1.8 ether for the conference and some additional ethers for for the guests in order to to to validate the transaction and if I want to check the balance of the contract here we can see that the contract has received 1.8 ether ok so now if we want to be sure that everything is done properly we have to add some testing we have to prove the test in order to be sure that everything is okay so I will take this file confirm test and I will add some additional parameter for judging to make it bigger so here I will add some variable the registration price the attendee here I will use the account one while it does not matter you can put three if you want a full name and we will check the balance before and after the call and at the end I will add my new test case so here we will check if we can registered to a conference so we will get first we'll get here the balance before the call we'll get the balance for the attendee and for the conference and we'll keep it in these variables so balance certainly before can't write before and then we will try to register this this attendee this this account or providing the the full name and then here the second part is to is to first we call their register and we want to be sure that an event has been triggered by the function because as you can see here if the event is triggered that means that everything has has been executed properly because it's the last instruction of your function and if if a required condition is not met and it throws an exception and of course it interrupts the kitchen of the of the of the function and even better than that sits transactions are supposed to be atomic it reverts all the change all the changes that you already did to the contract State so it really goes back to how it was before the only price you have to pay if you want is the gas up to that point so if there is some gas consumes before you throw an exception then this gas is lost is it go it's lost it's not lost for everyone it's actually paid to the miner who mined the block containing that transaction but that's a price to pay for throwing exception if you want so here I have received my register event I checked the address of the attendee and the full name just to be sure that everything is has been done correctly and then I will call here I will check if the attendee is properly registered so I will call the constant function is registered and normally of course the value should be true and here we will check the balance after the call so we will retrieve the balance from the attendee from the conference and we will make a check just to be sure that the assertion is is correct and here you see that the test is using so for the contract balance it would be exactly 1.8 ether because that's what what was sent by the the attendee but the the balance of the attendee itself is less than the amount he paid I mean it's a hundred minus 1.8 to pay for the fee minus a small amount for the gas and this amount is kind of hard to predict because it depends on the complexity of the code that was effectively executed so this is kind of a loose test to make sure that it's yeah the comparison is okay so let's try let's test our project trifle test suspense great we are able to register an attendee cool magical and the the last step regarding the registration is that you Europe you can also test the exception for example here you can add another test suite called Conference exceptions that GS and this guy is this one okay what we're going to do here is to a simulate an error and here we're going to try to register an attendee but with the wrong price as you can see here we should receive an exception if we receive an exception and in our case because we are using require so if we receive a revert from the contract that's okay this means that the smart contract is processing the exception properly but if the function has managed the call I will say and it was about to register the attendee it means that will be in the then section and that's not okay so we have to to fail the test case by the way and something that might be disturbing also for you Java developers in the room when you throw an exception in Java you can associate a message with it not in solidity not yet not yet it's gonna come but it has a cost in terms of development and stuff so actually the last hard fork that happens two weeks ago is going to enable that but for now the only thing you know that something happened and then you have to figure it out by yourself by using console logs and stuff or by using some debugger that's also been released recently so as you can see your something that our myth missing but at least we can test that an exception occurred now I can run truffle tests a tougher test will run all the tests sheets that are available under the the test folders as you can see here of course we can add at all register an attendee and we can also where we have verified that if we try to resist an attendee with the wrong price the smart contract is working as expected and well you can if you want you can execute you can test a specific test suite for the truffle test from this folder Conference exceptions by providing the name as an input parameter it will run only a specific test read so that's it guys you are ready to attend the conference and to to vote we have a question here why don't we need to oh yeah good question well just to show you that you can yeah mostly you can define a gas or you can use the default value well what's important to understand here is when you specify the gas it's not the real gas it's the maximum gas you're ready to spend yeah so by default when you send when you call a contract like that truffle will insert a default gas value and again that's a max that's not the exact amount because the exact amount is unpredictable thanks to them some algorithmic T theorem I won't go into it's impossible to predict the complexity of some of running some code in the dance so here is just a way to say okay for this call I'm willing to spend use 500,000 gas units right now if we go over that if the execs equation of the code reaches that limit it's like it will throw an exception indeed so all the state will be reverted and if I added if I send some value along I will get it back but I will pay the gas anyway so that's a way again the the the gas mechanism is a way to avoid spy spamming the network if I didn't have to pay for exception throwing then I would just I could just send invalid transactions to the network all the time and I will I would spam the network with invalid transactions so it's a way to disincentivize wrong transactions it's really interesting question for example here I have simulated an error I've provided the gas that is really really low 1000 and when I try to run my might this case it tell me well something wrong happened but here I want to know exactly what happens so I have used console.log and to display the error and the error message is this view exams game gives me base fee it says yes limit and it can be it can be a way to identify how many gas gas will be required by by your real function for example okay so that's it so when you specify by the way again this is a maximum so if you specify 500 thousand and only a hundred thousand is used you get the 400 thousand back it's really just a way to to protect yourself against infinite loops and stuff like that okay so now we have one attendee one speaker one talk it's time to boost our contract and make it more useful by adding the possibility to create and retrieve several talks because usually in a conference you have several talks okay so first of all we'll add the possibility to keep track of several talks in the contract stay and we'll modify of course the our TOC function to update that list we'll retrieve the list of ID's for each stock and for that we'll demonstrate the use of structure types and mappings so structure types are a way for contracts to is a screen so structure types are a way to keep the fields that are related to a single entity together okay so as we said before a contract can be seen like a class but it's really not it's just the same level of abstraction I would say and structure types are just like structs in other languages it's just for storing data I mean data objects if you want and mappings are kind of like hash maps in Java so they're associative arrays where you can associate a key to a value but you will see that they have some specificities in solidity but first of all we will clean up the contract a little bit because there is kind of mixed responsibilities in there if you remember early on we used in the add talk function here we use this this test that checks if the message sender is the owner of the contract and if not returns what we should do here is actually require use the require function so that it throws an exception that's one thing and another thing is that this has nothing to do with the business with the business of our function it's mostly authorization code so we'll put that somewhere else where it's more adapted and where it can be reused and for that we'll use a new feature well it's not new in the sense that it's been there for a long time but you will show it to you it's called modifiers ok so here inside the the contract at the beginning I will add a new modifier that it's called only owner okay so modifiers and solidity are kind of like very very aspects okay there a way to surround your function code with some other code and do it in a reusable way so here the only owner will check that the message sender is the owner of the contract if not it will throw an exception if this passes then we use the underscore character here to represent the code of our function okay so I could also do something after so it's really like again a very simple aspect okay and this is a reusable authorization modifier that I can then apply to every function that needs it so for example in the ad Talk function we will remove this test now and here after the public will add the only owner modifier okay so that will apply our modifier to our function quite literally the that is that the the compiler will add the code of the modifier injected into the byte code of our function okay so it's very very a roll like that and thanks to that if we try to call add talk from another account than the contract owner then it will fail we'll get an exception okay now another thing that is important but I mean it's an occasion to show you is that this authorization pattern is actually quite common and yeah there are a lot of situations occasions where you want to check that the color of a function is really the one that deployed the contract so to do that to make it more reusable even we can use inheritance so yes solidity supports inheritance and it supports multiple inheritance so one contract can inherit from several other contracts here who will create a new contract called ownable no don't want it okay and I will actually replace the code so again same thing we specify the the pragma for the solidity compiler version and then we keep track of the owner address in here okay in the in what we will use soon as the support at the superclass we declare the on the owner modifier right here and we also declare the constructor of our ownable contract that will register I mean keep track of the owner of the contract okay and then inside conference we can modify the conference contract to inherit from ownable but of course here it doesn't know what ownable is because we need to import that contract so here at the beginning we can just add the import honorable that's all it's in the same directory so it's just a way to let him know and then we can say conference is honorable thanks to that we can remove this state variable we don't need to keep it track of it here and that's pretty much it I think yeah we can we don't have any constructor here but since it's it's the same as Java to some extent there is a default constructor that's going to be generated for us by the compiler and it will call the super construct a constructor automatically so it will keep track of this owner state variable okay and I think that's it so now we didn't change anything to our functionality so truffle test should work just the same and it does okay so would actually add a test case now that checks that an exception is thrown when you try to call the at talk function from the wrong account but we'll leave it to you as an exercise now that you know how to catch exception in you t-test and so on so now that we've done this little bit of cleanup let's actually make it possible to store several talks in our contract to do that we will define a new structure type so we'll do that at the beginning of the contract okay actually two different types one for talks and one for speakers okay so talks will contain the title location start time and tank I added this cancelled field that's going to be used later and a list of speakers an array of addresses okay and the ID is new it's just a way now that we will have several talks to uniquely identify each talk will have also a structure type for speakers with again the address of the account and the full name and this time the full name will store it as a bytes 32 yeah before that it was stored as a string and by 32 is a special datatype in in solidity that contains a fixed size array of characters okay so it's always going to be maximum 32 characters whereas a string can be a variable size array again one thing to understand is that we are running in an environment where every byte counts every data that you store in the contract States has a cost in terms of real money so it's very important to choose your data types very carefully and here we're using buy stores too because we know that the full name of a speaker is never going to be that long and also because there are some limitations in solidity that prevent us from doing some things with some data types and especially returning a viable array of viable size types is not allowed yet so that's going to be an issue for later okay so we have both our structured types and now we will be able to store a collection of talks we'll do it for four talks speakers it's the same logic afterwards so to do that we'll add so first of all we can get rid of those variables for speakers and talks will keep the attendee stuff for now okay and here we'll add some variables to keep track of things this was before so now we added three state variables okay talks speakers and total talks for that we're using the mapping type now as I said the mapping type is an associative array kind of like a hash map where you can associate a value to a key the value can be any data type including a structure type which is what we used here and the key can be anything that is unique of course and here we are using integer IDs for talks and we're using the address of the speaker four speakers one thing that is important to understand and again it's a little weird is that mappings are associative arrays but there is no way to check if a key exists if it's been assigned a value and there is no way to iterate over it as a consequence of that okay so the only way to know which talks exists for us will be to keep track of the number of talks and then we know that every ID from one to the number of talks has been assigned and the others when if we try to for example if there are two talks and we want the title of the third one we'll just get an empty value not know not an exception just an empty value okay so again that's a weird thing in in the in the in the e/m but that's something that's necessary because of how it works okay now that we've that we can modify the the ad talk to take that into account actually I will completely replace this function here some room for it so we have the ad Talk function it takes the same parameters as before at the beginning the title the location the start time in the end time but this time it's all can have several speakers unfortunately there's one thing that we cannot do with solidity yet is to send a parameter that is a list or mapping of structure types so here we can only send a list of primitive types and in that case that will be addresses and bytes 32 okay so we have to space it as different arrays and then we'll have to check that those arrays have the right size and so on it's again using the only owner modifier that we declared in the ownable superclass then we check the input values using require so we check that we actually have a title that the start time is over is above zero that the end time is after the start time and that we actually have some address with some addresses for the speakers again normally you would also have to check that the array of speakers dresses is the same size as the array of speaker names and so on and so forth but you get the gist then once all the input parameters are okay we first increment the total talks counter because we would use it to assign some real values to our mapping and again a mapping is just an associative array so we can just do this okay so talks I pass the key and I can assign each field of my structure type by the way structure types are syntactic sugar added by solidity but the EVM as no knowledge of it which is why there are some limitations with it and that's it and then we iterate over the speaker addresses array and for each one we make sure that there is an address and that it's not new and and then we again create a new speaker with this data and we add it to the talks okay so we establish this many-to-many relationship between talks and speakers and when everything works then we can trigger the add talk event but this time we pass another parameter total talks that will correspond to the ID of the new talk that was added so that means that we need to update the the event as well so if we go up a talk event here we will need to add you int indexed I ID okay and this way we can passed the ID and we can even again filter events on a specific value of ID okay last thing we want to do is to add actually a couple of more things is to add a function to return the number of talks okay so get number of talks is a public constant function that returns an unsigned integer simply returns the value of total talks by the way this could have been replaced by simply declaring the state variable itself here as public if you do that then the compiler will generate a getter for this viable just together no setter there is no way to modify a contract state variable in a generated function of any kind and only in your functions that you define yourself okay in this case I will just use my my function here because I want to change its name of the waste has the same name as the so it will be called total tox and that's it now of course I have to change the implementation of get top okay because it those variables that returns the don't exist anymore and here again we are limited by what we can do or not do with solidity we cannot return an array of structured types because again that's because the EVM doesn't have any knowledge of structure types this is something that will change soon normally in the next version of solidity there should be some some changes to that but yeah for now it's not possible and so yeah again this function just talked it just takes the talk ID as an input parameter and it returns all the values so we cannot return a structural type itself so we cannot return talk here we have to return the unpackaged version of it okay so we check that the stock ID corresponds to a talk we could have done the same with just checking that talk ID is less or equal than total talks for example we check that the talk has not been cancelled and then we can load this talk so within solidity we can use I mean inside a function we can use structure types however we want so we just load it and this memory thing is again very important so by default when you load something from the contract state you actually load a reference to the contract state ok and it's different than if you want to modify it in memory and here we just want to return it from memory okay we don't want to take any chance that will modify the state so we just load this viable from the contract state into memory and then all the subsequent operations on talk will be a little cheaper because we were manipulating memory instead of contracts instead of blockchain persistence if you want so here we're loading this in memory we're doing the same for the speaker's address and speaker's name and here we are actually assigning I mean creating new arrays with the right size and we are iterating over the top speakers and assigning them to copying them into our arrays that then we can return okay so we turn take took title location start time end time and those two arrays that we've just built okay so now that we have that again it's a constant function doesn't modify the state and we're good the last thing that we want to do I think yeah so here we have a function to return the details of a talk now we need also a function to return a list of talks actually what we will do again we cannot return I mean big array of anything of unknown size so to avoid that what we will do is return on our list of IDs and then the front end can iterate over those IDs and retrieve the details for each for each talk you might think okay that's clearly suboptimal we are dealing with an N plus one situation here and that's the kind of thing that you want to avoid on database and you would be right but here again this is a constant function it's just querying state directly from a local node okay so the cost is not that big and yeah and in that case we we don't have a choice we have to do that so let's add this gift aux function so here we added a little bit of funky stuff to it to filter if we only want Stephen the talks that have been cancelled or not so if you pass it false you will get all the talks that have not been canceled sorry and and it returns an array of anti need teachers that corresponds to talk IDs okay of course this function call will only be valid if we have some talks and then we'll build an array of all talk IDs that will be initialized as an array containing total talks elements unfortunately from within a so dtt function especially when you're dealing with an in-memory array you have to specify the size again kind of cumbersome at at the beginning but that's the thing if you have to do so that means that we will store some talk IDs because we have to filter them according to their concert state and if we have some canceled talks and we only wanted the non canceled talks then the resulting array will be smaller than the total number of of talks so we'll need to do a little bit of algorithmic magic to shrink the table after that so again we check the status of the talks depending on the value we get passed we add we copy the talk IDs in our array if number of talks is equal to total talks it means that no talks was cancelled and the array size is the same so we can just return it directly otherwise we create a new array a smaller one containing just a number of talks that have not been cancelled and then we copy over very low level stuff very weird stuff to do when you're used to more expressive languages but still okay and that's it so we could also add some functions to for example returns the the only the talks of a given speaker and so on but again that's basically the same logic over again so it doesn't bring anything new to the table now that we have that let's run for my great for him nothing has changed of course because the migration state said that we already run all the migration scripts so we have to run truffle my grades - - reset to force the latest version of our contracts to be deployed and it's working and now we can run truffle console actually you know what like that so that you can see there okay so here the same as before deployed we get an instance of our deployed contract and we save it in a local variable tick so app contains our contract and then we will add a new we'll watch the ad talk event to make sure that the the event is triggered correctly so far ad talk event equals F dot our talk event dot watch I feel that it now so here I'm not gonna filter on values and I'm just gonna go from block latest to block latest okay and then in watch I get a function and this function is an error and an event simply console.log the events unexpected string of course I made a mistake oh yeah to block to block I forgot the Cullen there we go okay now let's do a start time again you did get time oops okay and we'll do the same for end time I will just be easier to use later on okay and now we can add a top well actually before I add a talk that is just our speakers it talks oh yeah I get to define a parameter oh no that's not what I want you to do get get number of talks so if I call this constant function to get the total number of talks I get zero okay and now I can add a talk so I can call the add talk function just remember that I need to specify some metadata after it so this talk will be called talk 1 it will happen in room 4 start at start time and at any time and here we have to pass an array of speak your dresses and an array of speaker names into it directly here okay so for the speaker addresses let's say that it will be 3 dot e th dot accounts 1 and with three the eth that accounts to for example okay and the name will be no Sayid that's the last one for example okay and now in the metadata of course we need to specify who is adding the talk and here I will use web three that ET h dot accounts one and you human compilers will already know that there is a problem if I do that I get a VM exception and the VM exception in my case just returns invalid of code because I'm using an old version of it of test RPC it should would say revert trigger the whatever but just to show you that here we have an exception and that's it we don't know why we don't know no math no error message nor whatever just something wrong happened with your code and of course if I run now from account zero which is really the account the contract owner then I get a transaction receipt with some logs and the log the event is log to the console so I can see that I have ad talk event it has an ID a name and start time and end time okay so now if I run up yet number of talks again everything works and if you get talk one we give the details for top one and yeah I think we wanted to show you more like how to register I mean how to reward speakers and also the front end but we won't have time it's already ten minutes left so one thing that is important right now you saw that with the register function we added some con we added some value some ether to the contract balance and then of course the contract balance will be able to transfer that value to other accounts whether other contracts or other users and in our case it will be speakers so you can use that logic to actually pay the speakers with actual ether and that's where the power of smart contracts is they have they have they contain value they can send value they can receive that okay so that's something that you will be able to see in the github repository and also for the front-end we just wanted to show you it's very simple but you can develop any kind of JavaScript framework that connects using the same web three functional functions that we use as we are using in the truffle test so the logic remains the same you just have to take into account that a lot of the actions are asynchronous so never take something into account directly and you always have to yeah show the user that something is happening in the background I will just show that that transfer mode so here quickly if a speaker wants to be we were we roared it so you will call the withdrawal reward and here he will have some logics that will count your rewards and what is really important here is this line because this line will transfer a number of money of ethers to the speaker so here the logic is the speaker that is the message sender will receive the number of rewards regarding the the toll that that is given during the conference and if the transfer failed everything that has been changed here will be reverted so your contract instances will not be changed if the transfer filter and that we trigger an event with a reward event that's it okay so just that was just to show you that you can transfer value out of a contract and then just show you quickly three so that is the the code of the front end so again when you unbox the petshop example you get a full jQuery application at first and and you can customize it and here for example in Abdo GS which is the javascript of our application for example the function that we load the talks and displays them can access the contract in the same way as we did in traffic console and in the tests so using the deployed function we get an instance we call the get talks function with false saying we want only the talks that have not been cancelled we get a list of talk IDs and then we can simply well using jQuery we re reinitialize our state and we can iterate over those IDs and for each one called the get talk function again and in the return we call display talk which is in this case is simply a JavaScript function that will update individual elements of our list of talks okay all of that will run in a traditional browser provided that you use something called meta mask so meta mask is a Chrome and Firefox I think extension yeah the plug-in that you can install on your browser and that will actually act as a proxy to the real blockchain okay so it will hold all your accounts and actually I can show you here the terminal is here test RPC is here let me force an upgrade my great - - reset so I'm really playing my contracts and once it is done I can run the light serve HTTP server provided by the contract so I'm peein run dev and pay install oh okay so I have to install dependencies first of course and this is just to illustrate that you can store your HTML Javascript CSS files anywhere you want of course the easiest way it is to store them on a central server but then again centralization and stuff and unfortunately there is no easy option to do it in a decentralized way for now there is something called ipfs allows you to store your files in a decentralized way but yeah it's kind of cumbersome to use and soon there will be swarm that's an official project from a theorem that will make that very easy but it's not ready yet for now let's run npm run dev we serve our files from our local server and here you see it loads in the browser quite normally and I will use meta mask so this is the extension where I can connect to any number of network and there is a notion of a vault that is a safe where it keeps all my accounts and all my keys and here I can switch to my local network and I can well I can use this extension to view my accounts to send transactions and when I try to send a transaction from from a page meta mask will intercept that call and will say ok this account is locked are you sure you want to confirm the transaction and if so I will sign it if not we stopped it there ok so this is an extension that's very powerful there's also the mist browser where you can do the same sort of things but it's not very practical for development purposes so that's meta mask and the front-end again everything is available in the chain skills slash devoxx repository on github so you will be able to check that code run it yourself there's readme explaining everything just a few last things to conclude so overall what's important to remember is that none of that code that we showed you was production ready I said especially with all the jQuery stuff it's not really readable and maintainable in the end and again you can use reacts that's the most used option right now to do that sort of things angular as well to UJS as well so we created a project with a scratch from scratch with using truffle acuity the smart contracts with a few advanced data types like mappings and structure types our contract can receive and send cryptocurrency and it can also authorize access to certain methods depending on who is connected and we almost showed you a basic way to connect a front-end to our contract was almost a small very small report there was the DEFCON 3 developer conference just last week in Cancun actually I landed yesterday and we were there to kind of follow the last news so a few last news from the network the community is growing really fast I think this year was like five times last year in terms of number of attendees so it's growing really really fast Vitalik bittering is kind of the inventor of a theorem if you want is the creator of the initial project and it proposed a very interesting role map strategy that I really encourage you to to have a look at there was a very few interesting tools very yeah some some very interesting tools that we discovered and we're gonna cover them on the blog and the general impression from the whole community is that we are still playing with toys that are really early pre-production stuff so it's gonna come production ready soon but we are still dealing with some scalability in a and productivity issues another important message from the conference everybody all the developers from the core team said ok stop this ICO madness you're spending you're wasting way too much time on that we need more brain time on the tools and on the the infrastructure itself regarding the remark above and yeah Mexico so that's it the last thing I wanted to say again we have this online course I think I might have mentioned it a couple of times or hatred it's on udemy and if you used devoxx 2017 code you get the whole course for 15 euros and that's that it also for the people who are watching us on YouTube live right now and we demonstrate how to create a front-end application yeah there we have time we have 10 hours we have time thanks a lot thank you very much [Applause] [Music]

Channel: Devoxx

Views: 7,697

Keywords: DV17, Devoxx

Id: 7LCHlkdibWg

Length: 167min 25sec (10045 seconds)

Published: Wed Nov 08 2017