Blockchain Coding Dojo - BlockchainHub Graz

Blockchain Coding Dojo Ethereum Smart Contract with Solidity & web3.JS

First Up: ● The Who / The Why / The How / The What ● Short Presentation ○ Example we’re going to build ○ Setup & Tools ○ Solidity crash course ○ Web3.js crash course ● Q & A Welcome To The Dojo!

● Get Programmers more involved in Blockchain technology ○ Get a feeling for what creating DApps is all about ○ Get to know the tools & ecosystem ○ Share experiences & learn The Why

● Coding Dojo! ○ Split up in N % (3|4) groups ■ (groups of 3 or 4) ○ Driver/Navigator Pair Programming (switch every ~8 Minutes - don’t hug the keyboard!) ■ Rest of the group contributes via discussion ○ Didi & Mario are available for help (as far as we can (not far! :D)) ○ Start Coding! ■ An example including a smart contract and a UI for using it ○ Victory! ■ Deploy the contract to the testchain ■ (If you’re done and still motivated, feel free to experiment with / expand the example!) The How

The What Winner-Takes-All Crowdfunding on the Ethereum Blockchain!

Example: Winner-Takes-All Crowdfunding Basically: A Crowdfunding System, where people can submit their Projects until a Deadline, with an entry fee to avoid spam. After this Deadline, people can “vote” for projects using Ether until a second Deadline. After the second Deadline, the Contract can be finished and the project with the most “votes” (i.e. Ether) wins the WHOLE pot (all entry fees + all pledged Ether).

Winner-Takes-All Crowdfunding - Tasks ● Create contract with proposal date, campaign date in the future and minimum entry fee ● People can enter projects (with an initial money transfer of X ether, so there's no spam) ○ until proposal date ● Now, after proposal date, other people can "vote" with ether for the projects ● When the target date is here and the contract gets finished, ALL the money goes to the project with the highest votes (most money contributed) ○ and contract is closed thereafter

Winner-Takes-All Crowdfunding - “Layout”

Setup & Tools ● Repository: ○ https://github.com/zupzup/blockchainhub-solidity-dojo ● Includes: ○ app.js - the UI JavaScript source ○ contract.sol - the smart contract source ○ contract.js - contract in one-line-form for web3.js ○ index.html - the HTML for the UI ○ js/ - static js ○ css/ - static css ○ LICENSE - the license (duh!) ○ README.md - setup & task list

Setup & Tools ● Docker ○ https://www.docker.com/ ● Solidity (Solc) ○ https://solidity.readthedocs.io/en/develop/# ○ https://ethereum.github.io/browser-solidity ● testRPC ○ https://github.com/ethereumjs/testrpc ● Web3.JS ○ https://github.com/ethereum/web3.js ○ There are some high-level frameworks as well ■ https://truffle.readthedocs.io/en/latest/ ■ https://github.com/iurimatias/embark-framework ■ http://populus.readthedocs.io/en/latest/ ■ ...

Setup & Tools ● Bothersome to set up locally, so we’ll use Docker ● TestRPC (our local blockchain, which auto-mines) ○ docker pull harshjv/testrpc ○ docker run -d -p 8545:8545 harshjv/testrpc ● Building (compile and make ready to use with web3.js) ○ docker pull mzupzup/soliditybuilder ○ docker run -v /path/to/this/folder:/sol mzupzup/soliditybuilder ○ OR (on Linux + macOS) with file-watcher ■ docker pull mzupzup/soliditywatcher ■ docker run -v /path/to/this/folder:/sol mzupzup/soliditywatcher

Setup & Tools ● Once testRPC and the build-container are running ○ Open index.html and see what’s what ○ Explore contract.sol and app.js ○ Get to work! :) ● Documentation: ○ Solidity ■ http://solidity.readthedocs.io/en/develop/ ○ Web3.JS ■ https://github.com/ethereum/wiki/wiki/JavaScript-API ○ Async.js ■ http://caolan.github.io/async/docs.html ○ jQuery / HTML / CSS / JavaScript ■ https://duckduckgo.com/?q=jquery ;)

Solidity ● High-level language for smart contracts ○ Syntax similar to JavaScript ○ Statically typed, Inheritance, Libraries can be imported ● https://ethereum.github.io/browser-solidity ● Docs:http://solidity.readthedocs.io/en/develop/index.html ● We will use 0.4.6 (0.4.10 now, moves VERY fast) ● Still very much in development, so some random things just don’t work (yet) ○ http://solidity.readthedocs.io/en/develop/frequently-asked-questions. html

Solidity - Smart Contracts Basics ● Contract is basically your “backend application” ○ Running on the Blockchain ● Smart Contracts need Gas (i.e.: Money) to run ○ No real money on our testRPC setup of course ○ You can estimate the gas cost via web3 / browser solidity ○ Only state-changing operations consume Gas ■ Just asking for a variable doesn’t ○ Unused Gas is refunded ■ E.g.: you send a transaction and add 10000 Gas. ● The transaction only needs 5000, so you get 5000 back immediately. ○ Incentive to leave a minimal footprint

Solidity - Types ● Integers ○ uint8 - uint256 ○ int8 - int256 ● Strings ● Address ○ .balance ○ .send() ● Booleans ● ...

Solidity - Data Structures ● Array (fixed & dynamic) ○ Mostly as one would expect (.length, array[0...N]) ● Mapping (similar to hashmap) ○ Can’t iterate over! ● Struct

Solidity - Events Used for debugging only (logs to the blockchain)

Solidity - Special Variables ● Msg.value - value of the transaction ● Msg.sender - address of the transaction’s sender ● http://solidity.readthedocs.io/en/develop/units-and-globa l-variables.html#block-and-transaction-properties

Solidity - Throw Reverts transfer, returns “error”

Solidity - Example Contracts http://solidity.readthedocs.io/en/develop/solidity-by-exampl e.html Remember, maybe we can’t use ALL new features of 0.4.10, because we use the 0.4.6 compiler.

Web3 - the Ethereum JavaScript API ● https://github.com/ethereum/web3.js ● Used for: ○ Connecting to the Blockchain ○ Compiling contracts ○ Instantiating Contracts ○ Calling methods on contracts ○ Retrieve public values from contracts ○ Send Transactions ○ Convert to/from Wei ● Basically a full interface for the Ethereum Blockchain ○ Docs: https://github.com/ethereum/wiki/wiki/JavaScript-API

● Addresses of available accounts: ○ web3.eth.accounts ● Get Balance for an account in Ether: ○ web3.fromWei(web3.toDecimal(web3.eth.getBalance(web3.eth.accounts[0]) ), ‘ether’) ● Get a “public” attribute of the contract: ○ contractInstance.{publicAttribute}(function(err, data) { ..callback function.. }) ● Sending a simple transaction: ○ contractInstance.{payableFunction}(function params.., { from: fromAddress, value: valueToSendInWei, gas: gasCostOfTransaction }, function(err, data) { ..callback function.. }) Web3 - Examples (just to get a feeling for the API)

● Debugging (very important!) ○ Browser Dev Tools ○ Solidity Events ■ Can listen to them in web3.js ■ https://github.com/ethereum/wiki/wiki/JavaScript-API#contract-all events ■ Logs all events within the contract Web3

These Slides: https://goo.gl/Msofql https://github.com/zupzup/blockchainhub-solidit y-dojo Let’s Roll!

Blockchain Coding Dojo - BlockchainHub Graz

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