Dapp简单的投票小例子

准备工作

Solidity合约

合约内容

构造函数，用来初始化一些候选者
用来投票的方法（对于票数加 1）
返回候选者获得的总的票数的方法

合约代码

pragma solidity ^0.4.22;contract Voting{mapping (bytes32 => uint8)public votesReceived;bytes32[] public candidateList;constructor(bytes32[] candidateNames)public{candidateList = candidateNames;}function totalVotesFor(bytes32 candidate) view  public returns(uint8){require(validCandidate(candidate));return votesReceived[candidate];}function voteForCandidate(bytes32 candidate) public {require(validCandidate(candidate));votesReceived[candidate] += 1;}function validCandidate(bytes32 candidate)view public returns(bool){for(uint i = 0;i <candidateList.length;i++){if(candidateList[i] == candidate){return true;}return false;}}
}

合约编译

编译前测试

查看安装的版本 npm list web3；npm list solc
node 进入命令行
var Web3 = require('web3')
var web3 = new Web3(new Web3.providers.HttpProvider('Http://localhost:8545'))//这个需要和ganache-cli设置的一致，才可以连接上。
使用web3.isConnected()//查看是否连接上，此时需要将ganache-cli启动
使用命令web3.eth.accounts,获取目前的账户，需要注意每次启动ganache-cli都会产生全新的账号和秘钥

正式开始编译

var solc = require('solc')
输入solc. 查看可以使用的命令
var sourceCode = fs.readFileSync('Voting.sol').toString() //需要注意，必须给与文件读取权限
输入sourceCode 可以查看编码文件
var compiledCode = solc.compile(sourceCode) //这个就是正式的编译过程
输入compiledCode可以查看字节码文件,编译结果如下图所示

这是一个js对象，最关键的是bytecode（字节码）和interface接口（ABI），这两个文件是部署合约最关键的文件
部署合约 var abi = JSON.parse(compiledCode.contracts[':Voting'].interface); 输入abi进行验证，是否取到正确的值，JSON.parse是为了Json格式化代码
var byteCode = compiledCode.contracts[':Voting'].bytecode 取到字节码其中:Voting是合约名称
创建合约对象 var VotingContract = web3.eth.contract(abi)
var deployTxObj = {data:byteCode,from:web3.eth.accounts[0],gas:30000000} 定义一笔交易
定义合约实例 var contractInstance = VotingContract.new(["Alice","Bob","Cary"],deployTxObj);
contractInstance = VotingContract.new(["Alice","Bob","Cary"],{data:byteCode,from:web3.eth.accounts[0],gas:5000000});
上面这一条命令是定义合约实例和定义一笔交易的结合体，如果分开，有可能会报错
取到合约实例的地址 contractInstance.address 这就将合约部署在区块链上，这个区块链是本地起的ganache-cli模拟区块链
contractInstance.vote("Alice",{from:web3.eth.accounts[0]}) 账号0给Alice投出一票
contractINstance.totalVotesFor("Alice").toString() 查看Alice的总共的票数
contractINstance.totalVotesFor.call("Alice").toString() 查看
contractInstance.vote.sendTransaction("Alice",{from:web3.eth.accounts[0]})

页面交互

页面 index.html

<!DOCTYPE html>
<html><head><title>Voting DApp</title><link href='https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css' rel='stylesheet' type='text/css'></head><body class="container"><h1>A Simple Voting Application</h1><div class="table-responsive"><table class="table table-bordered"><thead><tr><th>Candidate</th><th>Votes</th></tr></thead><tbody><tr><td>Alice</td><td id="candidate-1"></td></tr><tr><td>Bob</td><td id="candidate-2"></td></tr><tr><td>Cary</td><td id="candidate-3"></td></tr></tbody></table></div><input type="text" id="candidate" /><a href="#" onclick="voteForCandidate()" class="btn btn-primary">Vote</a></body><script src="https://cdn.jsdelivr.net/gh/ethereum/web3.js/dist/web3.min.js"></script><script src="https://code.jquery.com/jquery-3.1.1.slim.min.js"></script><script src="./index.js"></script></html>

注意

不再需要引入web3对象，因为页面里面就包含了web3对象，https://cdn.jsdelivr.net/gh/ethereum/web3.js/dist/web3.min.js，打开这个链接之后，在页面的最下面会显示，

index.js

var web3 = new Web3(new Web3.providers.HttpProvider("Http://localhost:8545"));
var abi = JSON.parse('[{"constant":true,"inputs":[{"name":"candidate","type":"bytes32"}],"name":"totalVotesFor","outputs":[{"name":"","type":"uint8"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"candidate","type":"bytes32"}],"name":"validCandidate","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"","type":"bytes32"}],"name":"votesReceived","outputs":[{"name":"","type":"uint8"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"","type":"uint256"}],"name":"candidateList","outputs":[{"name":"","type":"bytes32"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"candidate","type":"bytes32"}],"name":"voteForCandidate","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"inputs":[{"name":"candidateNames","type":"bytes32[]"}],"payable":false,"stateMutability":"nonpayable","type":"constructor"}]');
// var contractAddr = "0x3577f4d4902f4766753056468bd08ef68df6c623";
var VotingContract = new web3.eth.Contract(abi); 
// var contractInstance =new web3.eth.Contract(abi,contractAddr);
// contractInstance = VotingContract.at(contractAddr);
var contractInstance = VotingContract.at('0x3577f4d4902f4766753056468bd08ef68df6c623');
var candidates = {"Alice":"candidate-1","Bob":"candidate-2","Cary":"candidate-3"
};
function voteForCandidate(){let candidateName = $("#candidate").val();try{contractInstance.voteForCandidate(candidateName),{from:web3.eth.accounts[0]},(err,res)=>{if(err)console.log("Error:",err);else{let div_id = candidates[candidateName];let count = contractInstance.totalVotesFor(candidateName).toString();$("#" + id).html(count);}}}catch(err){}
};$(document).ready(function(){candidateNames = Object.keys(candidates);for(let i=0;i<candidateNames.length;i++){let name = candidateNames[i];let val = contractInstance.totalVotesFor.call(name).toString();$("#" + candidates[name]).html(val);}
});

报错

无解

at无法使用
如果使用 var contractInstance =new web3.eth.Contract(abi,contractAddr);避免了at，但是无法使用call回调函数，依然报错

部署服务

server.js

异步的方式启动，同步的会报错

var http = require('http');
var fs = require('fs');
var url = require('url');http.createServer(function(req,res){var pathName = url.parse(req.url).pathname;console.log("Request for:"+pathName + "received.");fs.readFile(pathName.substr(1),function(err,data){if(err){console.log(err);res.writeHead(400,{'Content-Type':"text/html"});}else{res.writeHead(200,{'Content-Type':"text/html"});res.write(data.toString());}res.end();});
}).listen(8888);