如何编写智能合约（Smart Contract）- 从零构建和部署去中心化投票App，decentralization Voting Dapp

孔壹学院：国内区块链职业教育领先品牌

作者：黎跃春，区块链、高可用架构工程师

微信：liyc1215 QQ群：348924182 博客：http://liyuechun.org

课程目标

了解区块链智能合约

学会搭建智能合约开发环境

学会如何编译智能合约

学会如何将智能合约部署到区块链

学会如何通过WebApp和智能合约尽心互动

掌握DApp（去中心化App）的整个开发部署流程

掌握去中心化在实战产品中应用的重大意义

项目效果图

编辑器选择

理论上讲任何编辑器都可以编写

Solidity

合约代码，比如：WebStorm，VSCode，Sublime，等等。我选择的是Atom，没有任何理由，因为Atom轻量并且界面漂亮。

移步https://atom.io/地址，下载安装Atom。

autocomplete-solidity

代码自动补齐

linter-solium

、

linter-solidity

代码错误检查

language-ethereum

支持

Solidity

代码高亮以及

Solidity

代码片段

安装所需工具

首先开发机上必须装好Node.js，再使用以下命令安装所需的工具：

$ npm install -g ethereumjs-testrpc truffle

liyuechun:~ yuechunli$ npm install -g ethereumjs-testrpc truffle
/usr/local/bin/testrpc -> /usr/local/lib/node_modules/ethereumjs-testrpc/build/cli.node.js
/usr/local/bin/truffle -> /usr/local/lib/node_modules/truffle/build/cli.bundled.js
+ truffle@3.4.9
+ ethereumjs-testrpc@4.1.3
added 1 package and updated 7 packages in 76.132s
liyuechun:~ yuechunli$

创建项目

/Users/liyuechun/Desktop/1012/Voting
liyuechun:Voting yuechunli$ ls
liyuechun:Voting yuechunli$ pwd
/Users/liyuechun/Desktop/1012/Voting
liyuechun:Voting yuechunli$ truffle unbox react-box
Downloading...
Unpacking...
Setting up...
Unbox successful. Sweet!

Commands:

Compile:              truffle compile
Migrate:              truffle migrate
Test contracts:       truffle test
Test dapp:            npm test
Run dev server:       npm run start
Build for production: npm run build
liyuechun:Voting yuechunli$

项目结构

contracts：

编写智能合约的文件夹，所有的智能合约文件都放置在这里

migrations：

部署合约配置的文件夹

src：

基于React的Web端源码

test：

智能合约测试用例文件夹

编写投票Dapp智能合约

在

contracts

文件夹下创建

Voting.sol

文件，将下面的代码拷贝到文件中。

pragma solidity ^0.4.4;

contract Voting {

// liyuechun -> 10
// xietingfeng -> 5
// liudehua -> 20
mapping (bytes32 => uint8) public votesReceived;

// 存储候选人名字的数组
bytes32[] public candidateList;

// 构造函数 初始化候选人名单
function Voting(bytes32[] candidateNames) {

candidateList = candidateNames;
}

// 查询某个候选人的总票数
function totalVotesFor(bytes32 candidate)  constant returns (uint8) {
require(validCandidate(candidate) == true);
// 或者
// assert(validCandidate(candidate) == true);
return votesReceived[candidate];
}

// 为某个候选人投票
function voteForCandidate(bytes32 candidate) {
assert(validCandidate(candidate) == true);
votesReceived[candidate] += 1;
}

// 检索投票的姓名是不是候选人的名字
function validCandidate(bytes32 candidate) constant returns (bool) {
for(uint i = 0; i < candidateList.length; i++) {
if (candidateList[i] == candidate) {
return true;
}
}
return false;
}
}

通过remix + metamask部署合约到Kovan Test Net

在Google浏览器里面安装

MetaMask

插件



打开https://remix.ethereum.org将合约代码拷贝到里面



确保

MetaMask

账号处于等于状态，并且有一定的以太币支付给矿工。

确保

Environment

是

Injected Web3

，如果切换不过来，关掉浏览器重新启动

在

create

函数中输入一个数组，数组里面的内容为候选人名单

点击

create

按钮，会弹出

MetaMask

界面让你确认，确认提交，过一会儿，合约就部署成功

可以测试给某个候选人投票，查询某个候选人的票数

拷贝合约地址

0xd3f33a2e553b363b432d7f81f721a2a6202ecc67

编译合约

liyuechun:Voting yuechunli$ truffle compile
Compiling ./contracts/Migrations.sol...
Compiling ./contracts/SimpleStorage.sol...
Compiling ./contracts/Voting.sol...
Writing artifacts to ./build/contracts

liyuechun:Voting yuechunli$

编译完合约以后在

build/contracts

文件夹下面会有一个

Voting.json

的

abi

文件。

查看Voting.json文件内容

{
"contract_name": "Voting",
"abi": [
{
"constant": true,
"inputs": [
{
"name": "candidate",
"type": "bytes32"
}
],
"name": "totalVotesFor",
"outputs": [
{
"name": "",
"type": "uint8"
}
],
"payable": false,
"type": "function"
},
{
"constant": true,
"inputs": [
{
"name": "candidate",
"type": "bytes32"
}
],
"name": "validCandidate",
"outputs": [
{
"name": "",
"type": "bool"
}
],
"payable": false,
"type": "function"
},
{
"constant": true,
"inputs": [
{
"name": "",
"type": "bytes32"
}
],
"name": "votesReceived",
"outputs": [
{
"name": "",
"type": "uint8"
}
],
"payable": false,
"type": "function"
},
{
"constant": true,
"inputs": [
{
"name": "",
"type": "uint256"
}
],
"name": "candidateList",
"outputs": [
{
"name": "",
"type": "bytes32"
}
],
"payable": false,
"type": "function"
},
{
"constant": false,
"inputs": [
{
"name": "candidate",
"type": "bytes32"
}
],
"name": "voteForCandidate",
"outputs": [],
"payable": false,
"type": "function"
},
{
"inputs": [
{
"name": "candidateNames",
"type": "bytes32[]"
}
],
"payable": false,
"type": "constructor"
}
],
"unlinked_binary": "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",
"networks": {},
"schema_version": "0.0.5",
"updated_at": 1507806214330
}

这个文件是编译后的abi文件，待会儿需要将这个文件的json导入到App.json中。

查看src/utils/getWeb3.js文件内容

import Web3 from 'web3'

let getWeb3 = new Promise(function(resolve, reject) {
// Wait for loading completion to avoid race conditions with web3 injection timing.
window.addEventListener('load', function() {
var results
var web3 = window.web3

// Checking if Web3 has been injected by the browser (Mist/MetaMask)
if (typeof web3 !== 'undefined') {
// Use Mist/MetaMask's provider.
web3 = new Web3(web3.currentProvider)

results = {
web3: web3
}

console.log('Injected web3 detected.');

resolve(results)
} else {
// Fallback to localhost if no web3 injection.
var provider = new Web3.providers.HttpProvider('http://localhost:8545')

web3 = new Web3(provider)

results = {
web3: web3
}

console.log('No web3 instance injected, using Local web3.');

resolve(results)
}
})
})

export default getWeb3

这个文件主要是封装了一个

getWeb3

的

promiss

供我们直接使用，可以从

getWeb3

直接获取到

web3

对象供

App.js

文件中使用。

修改app.js前端代码和合约进行互动

import React, { Component } from 'react'
import VotingContract from '../build/contracts/Voting.json'
import getWeb3 from './utils/getWeb3'

import './css/oswald.css'
import './css/open-sans.css'
import './css/pure-min.css'
import './App.css'

const contractAddress = "0xd3f33a2e553b363b432d7f81f721a2a6202ecc67";
var votingContractIns
dede
tance;

var _modifyVotingCount = (candidates,i,votingCount) => {

console.log("---------");
console.log(candidates);
console.log(i);
console.log(votingCount);

let obj = candidates[i];
obj.votingCount = votingCount;
return candidates;
}

class App extends Component {
constructor(props) {
super(props)

this.state = {
candidates: [
{
"name": "Rama",
"id": 100,
"votingCount": 0
},
{
"name": "Nick",
"id": 101,
"votingCount": 0
},
{
"name": "Jose",
"id": 102,
"votingCount": 0
},
{
"name": "liyuechun",
"id": 103,
"votingCount": 0
}
],
candidatesVoteCount: ["0","0","0","0"],
web3: null
}
}

componentWillMount() {
// Get network provider and web3 instance.
// See utils/getWeb3 for more info.

getWeb3
.then(results => {
this.setState({
web3: results.web3
})

// Instantiate contract once web3 provided.
this.instantiateContract()
})
.catch(() => {
console.log('Error finding web3.')
})
}

instantiateContract() {
/*
* SMART CONTRACT EXAMPLE
*
* Normally these functions would be called in the context of a
* state management library, but for convenience I've placed them here.
*/

const contract = require('truffle-contract')
const votingContract = contract(VotingContract)
votingContract.setProvider(this.state.web3.currentProvider)

// Declaring this for later so we can chain functions on SimpleStorage.

// Get accounts.
this.state.web3.eth.getAccounts((error, accounts) => {
votingContract.at(contractAddress).then((instance) => {

votingContractInstance = instance;
for (let i = 0; i < this.state.candidates.length; i++) {
let object = this.state.candidates[i];
console.log(accounts[0]);
console.log(votingContractInstance);
console.log(votingContractInstance.totalVotesFor(object.name));
votingContractInstance.totalVotesFor(object.name).then(result => {
console.log(i);
console.log(result.c[0]);
this.setState({
candidates: _modifyVotingCount(this.state.candidates,i,result.c[0])
});
});
}
})
})
}

render() {
return (
<div className="App">
<ul>
{
this.state.candidates.map((object) => {
console.log(object);
return (

<li key={object.id}>候选人：{object.name}          支持票数：{object.votingCount}</li>
)
})
}
</ul>

<input
style={{width: 200,height: 30,borderWidth: 2,marginLeft: 40}}
placeholder="请输入候选人姓名..."
ref="candidateInput"
/>

<button style={{height: 30,borderWidth: 2,marginLeft: 20}} onClick={() => {
console.log(this.refs.candidateInput);
console.log(this.refs.candidateInput.value);
let candidateName = this.refs.candidateInput.value;
console.log(this.state.web3.eth.accounts[0]);
votingContractInstance.voteForCandidate(candidateName).then((result => {
console.log(result);
console.log(candidateName);
let number = 0;
for(let i = 0; i < this.state.candidates.length; i++) {
let object = this.state.candidates[i];
if (object.name === candidateName) {
number = i;
break;
}
}
votingContractInstance.totalVotesFor(candidateName).then(result => {

this.setState({
candidates: _modifyVotingCount(this.state.candidates,number,result.c[0])
});
});

}));
}}>Voting</button>

</div>
);
}
}

export default App

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