Dapp: Lottery Contract

Posted on | Post modified | In | Visitors

Overview

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Lottery Contract

# Variables
Name            Purpose
manager:    Address of person who created the contract
players:    Array of addresses of people who entered

# Function
Name            Purpose
enter:      Enters a player into lottery
pickWinner: Randomly picks a winner and sends them the prize pool

Solidity Variable

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Name                Notes                                    Examples
string          Sequence of characthers                     "hi there"
bool            Boolen value                                true, false
int             Integer, positive or negative.              0, -300000
uint            unsigned integer                            0, 3000000
fixed/ufixed    'Fixes' point number                        20.0001, -43.0002
address         Has methods tied to it for sending money    0x01C65bfDeD8c69ef3C28d4EF58F1dA

Reference type

  • fixed array: Array the contains a single type of element. length is fixed. int[3] –> [1,2,3]
  • dynamic array: Array the contains a single type of element. length can change over time. int[] –>[1,2,3]
  • mapping: Collection of key value pairs. mapping(string => int)
  • struct: Collection of key value pairs that can have different types

msg global variable

  • msg.data: ‘Data’ field from the call or transaction that invoked the current function
  • msg.gas : Amount of gas the current function invocation has available
  • msg.sender: Address of account that started the current function invocation
  • msg.value: Amount of ether (in wei) that was sent along with the function invocation

Lottery logic

logic

logic2

:contracts/Lottery.sol
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pragma solidity ^0.4.17;

contract Lottery {
    address public manager;
    address[] public players;
    
    constructor () public {
        manager = msg.sender;
    }
    
    function enter()  public payable{
        require(msg.value >= .01 ether, "send at least 0.01 ether");
        players.push(msg.sender);
    }

    function random() public view returns (uint) {
        return uint(sha3(block.difficulty, now, players));
    }
    
    // function modifier, reduce code we need to write
    modifier onlyManagerCanCall() {
        // only manage can pick winner
        require(msg.sender == manager, "you don't have authority");
        _; // all code in your function will replace the "_"
    }
    
    function pickWinner() public onlyManagerCanCall{
        uint index = random() % players.length;
        // this.balance has all the ether in current smart contract
        players[index].transfer(address(this).balance); 
        // inital dynamic array whose length is 0;
        players = new address[](0);
    }
    
    function getPlayers() public view returns (address[]) {
        return players;
    }
}

You can test this on remix.

Create a test case

:test/Lottery.test.js
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const assert = require('assert');
const ganache = require('ganache-cli');
const Web3 = require('web3')
const web3 = new Web3(ganache.provider());
const {interface, bytecode} = require('../compile')

let lottery;
let accounts;

beforeEach(async () => {
    // Get a list of all accounts
    accounts = await web3.eth.getAccounts();
    
    lottery = await new web3.eth.Contract(JSON.parse(interface))
    .deploy({data: bytecode})
    .send({from: accounts[0], gas: '1000000'})
});

describe('Lottery Contract', () => {
    it('deploys a contract', () => {
        assert.ok(lottery.options.address);
      });

    it('allow one account to enter', async() => {
        await lottery.methods.enter().send({from: accounts[0], value: web3.utils.toWei('0.02', 'ether')});
        
        const players = await lottery.methods.getPlayers().call({
            from: accounts[0]
        });

        assert.equal(accounts[0], players[0]);
        assert.equal(1, players.length);
    });

    it('allow multi account to enter', async() => {
        await lottery.methods.enter().send({from: accounts[0], value: web3.utils.toWei('0.02', 'ether')});
        await lottery.methods.enter().send({from: accounts[1], value: web3.utils.toWei('0.02', 'ether')});
        await lottery.methods.enter().send({from: accounts[2], value: web3.utils.toWei('0.02', 'ether')});
        const players = await lottery.methods.getPlayers().call({
            from: accounts[0]
        });

        assert.equal(accounts[0], players[0]);
        assert.equal(accounts[1], players[1]);
        assert.equal(accounts[2], players[2]);
        assert.equal(3, players.length);
    });

    it('requires a minimum amount of ether to enter', async() => {
        try {
            await lottery.methods.enter().send({
                from: accounts[0],
                value: 0
            });
            assert(false);
        } catch (err) {
            assert(err);
        }
    });

    it('only manage can call pickWinner', async() => {
        try {
            await lottery.methods.pickWinner().send({
                from: accounts[1],
            });
            assert(false);
        } catch (err) {
            assert(err);
        }
    });

    it('sends money to the winner and resets the players', async() => {
        await lottery.methods.enter().send({
            from: accounts[0],
            value: web3.utils.toWei('2', 'ether')
        });

        const initialBalance = await web3.eth.getBalance(accounts[0]);
        await lottery.methods.pickWinner().send({from: accounts[0]});
        const finalBalance = await web3.eth.getBalance(accounts[0]);
        const difference = finalBalance - initialBalance;
        // console.log(difference);
        assert(difference > web3.utils.toWei('1.8', 'ether'));
    });
});
:./compile.js
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const path = require('path');
const fs = require('fs');
const solc = require('solc');

const Path = path.resolve(__dirname, 'contracts', 'Lottery.sol');
const source = fs.readFileSync(Path, 'utf8');

module.exports = solc.compile(source, 1).contracts[':Lottery'];
// console.log(solc.compile(source, 1))

then rum npm run test

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mac@HansonMac  ~/Code/blockchain  npm run test
> inbox@ test /Users/mac/Code/blockchain
> mocha
Lottery Contract
(node:32122) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 data listeners added. Use emitter.setMaxListeners() to increase limit
    ✓ deploys a contract
    ✓ allow one account to enter (56ms)
    ✓ allow multi account to enter (103ms)
    ✓ requires a minimum amount of ether to enter
    ✓ only manage can call pickWinner
    ✓ sends money to the winner and resets the players (74ms)

Web of ethereum architecture

Metamask running in Chrome will inject web3 v2.0 automatically.

Our app will use web3 v1.0, and we want to hijack our provider into Metamask one.

install react, create new project and install web3

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mac@HansonMac  ~/Code: sudo npm install -g create-react-app
mac@HansonMac  ~/Code: create-react-app lottery-react

Creating a new React app in /Users/mac/Code/lottery-react.

Installing packages. This might take a couple of minutes.
Installing react, react-dom, and react-scripts...

yarn add v1.10.1
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
[3/4] 🔗  Linking dependencies...
...

mac@HansonMac  ~/Code: yarn add web3@1.0.0-beta.26
mac@HansonMac  ~/Code: npm run start

Hijack Metamask web3 to our version

:src/web3.js
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import Web3 from 'web3';

// inject our web3 v1.0
const web3 = new Web3(window.web3.currentProvider);

export default web3;
:src/App.js
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// add this to App.js
import web3 from './web3'

Deploy Lottery contract to Rinkeby

:./deploy.js
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const HDWalletProvider = require('truffle-hdwallet-provider');
const Web3 = require('web3');
const {interface, bytecode} = require('./compile');

const provider = new HDWalletProvider (
    'dinosaur erupt zoo ...',
    'https://rinkeby.infura.io/v3/451daf892abb4101b6845******'
);

const web3 = new Web3(provider);

const deploy = async () => {
    const accounts = await web3.eth.getAccounts();

    console.log('Attempting to deploy from account', accounts[0]);

    const result = await new web3.eth.Contract(JSON.parse(interface))
    .deploy({data: bytecode})
    .send({gas: '5000000', from: accounts[0]});

    console.log(interface);
    console.log('Contract deployed to', result.options.address);
};

deploy();

after run node deploy.js

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Attempting to deploy from account 0x01C65bfDeD8c69ef3C28d4EF58F1dA46DeAF13Cd
[{"constant":true,"inputs":[],"name":"manager","outputs":[{"name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[],"name":"pickWinner","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"random","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"getPlayers","outputs":[{"name":"","type":"address[]"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[],"name":"enter","outputs":[],"payable":true,"stateMutability":"payable","type":"function"},{"constant":true,"inputs":[{"name":"","type":"uint256"}],"name":"players","outputs":[{"name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"inputs":[],"payable":false,"stateMutability":"nonpayable","type":"constructor"}]
Contract deployed to 0x2a8683527b110f5f37502CD5c7C62f574A75499A

Import ABI

create a new file lottery.js in src folder. And copy all the ABI and deployed address.

:src/lottery.js
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import web3 from `./web3`;

const address = '0x2a8683527b110f5f37502CD5c7C62f574A75499A';
const abi = [{
    "constant": true,
    "inputs": [],
    "name": "manager",
    "outputs": [{
        "name": "",
        "type": "address"
    }],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
}, {
    "constant": false,
    "inputs": [],
    "name": "pickWinner",
    "outputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "function"
}, {
    "constant": true,
    "inputs": [],
    "name": "random",
    "outputs": [{
        "name": "",
        "type": "uint256"
    }],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
}, {
    "constant": true,
    "inputs": [],
    "name": "getPlayers",
    "outputs": [{
        "name": "",
        "type": "address[]"
    }],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
}, {
    "constant": false,
    "inputs": [],
    "name": "enter",
    "outputs": [],
    "payable": true,
    "stateMutability": "payable",
    "type": "function"
}, {
    "constant": true,
    "inputs": [{
        "name": "",
        "type": "uint256"
    }],
    "name": "players",
    "outputs": [{
        "name": "",
        "type": "address"
    }],
    "payable": false,
    "stateMutability": "view",
    "type": "function"
}, {
    "inputs": [],
    "payable": false,
    "stateMutability": "nonpayable",
    "type": "constructor"
}];

export default new web3.eth.Contract(abi, address);

abi

Rendering Contract Data

Modify src/App.js code. See source code here:
https://github.com/hansonzhao007/lottery/blob/master/src/App.js

Deploying React app to GitHub Pages

create a new repository, and connect your project.

install gh-pages package

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npm install gh-pages --save-dev

Add some properties to the app’s package.json file

:./package.json
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"homepage": "http://gitname.github.io/react-gh-pages"

"scripts": {
  //...
  "predeploy": "npm run build",
  "deploy": "gh-pages -d build"
}

Generate a production build of your app, and deploy it to GitHub Pages

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npm run deploy

You can see the example on http://xszhao.science/lottery

Reference

Deploying a React App* to GitHub Pages

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