在Part 1中我们创建了一个简单的voting dapp，并在本地部署了该合约。在Part 2我们使用了Truffle Framework创建，编译，重新部署了投票应用，且将合约部署到了公共的Ethereum测试网络。本章节，我将丰富一下合约内容，以便覆盖更多的内容：
在合约中使用
struct结构来组织数据，并将数据存储到区块链上了解tokens，及使用
学习使用Ether支付
我们知道，在真正的票选中，每个有选举权的公民都有一票，且只有一票。在类似董事会投票的场景里，拥有股份越多的人，可投票数越多。所以，要想获得更多的投票数，就需要购买或持有更多的股份。
我将把之前的voting dapp调整为董事会投票的类型，这需要新增一个方法，即可以通过购买票数获取更多的投票权。另外，再增加一个可以查看投票人信息的方法。在Ethereum blockchain里，这里的票数通常被叫做tokens。
第一步，需要声明一个可以存储所需数据的变量，请看下面的代码：
// struct 结构
struct voter {
address voterAddress; // voter的身份地址
uint tokensBought; // 拥有的token数量
uint[] tokensUsedPerCandidate; // 存储投票记录
}
mapping (bytes32 => uint) public votesReceived;
mapping (address => voter) public voterInfo;
bytes32[] public candidateList;
uint public totalTokens; // 合约发布时规定的总token数量
uint public balanceTokens; // 可购买的token数量
uint public tokenPrice; // token的价格
因为我增加了totalTokens，tokenPrice这些字段，所以构造函数需要更改一下：
function Voting(uint tokens, uint pricePerToken, bytes32[] candidateNames) public {
candidateList = candidateNames;
totalTokens = tokens;
balanceTokens = tokens;
tokenPrice = pricePerToken;
}
接着修改migrations路径下的1_deploy_contracts.js文件：
deployer.deploy(Voting, 1000, web3.toWei('0.1', 'ether'), ['Sam', 'Leslie', 'Jetty', 'Arkila', 'Piu']);
到此，已经完成了初始化tokens，并设置了token的价格，下面看一下别人如何通过支付ether来购买tokens:
/* 购买tokens的方法，注意关键字‘payable’。该方法添加到合约中后，你的合约就可以接受别人支付ether来购买token了，这就是购买ERC2.0代币的过程*/
function buy() payable public returns (uint) {
uint tokensToBuy = msg.value / tokenPrice;
if (tokensToBuy > balanceTokens) throw;
voterInfo[msg.sender].voterAddress = msg.sender;
voterInfo[msg.sender].tokensBought += tokensToBuy;
balanceTokens -= tokensToBuy;
return tokensToBuy;
}
让我们来看一下voter和contract之间的交互流程：
image.png
Voting.sol
pragma solidity ^0.4.18;
contract Voting {
struct voter {
address voterAddress;
uint tokensBought;
uint[] tokensUsedPerCandidate;
}
mapping (address => voter) public voterInfo;
mapping (bytes32 => uint) public votesReceived;
bytes32[] public candidateList;
uint public totalTokens;
uint public balanceTokens;
uint public tokenPrice;
function Voting(uint tokens, uint pricePerToken, bytes32[] candidateNames) public {
candidateList = candidateNames;
totalTokens = tokens;
balanceTokens = tokens;
tokenPrice = pricePerToken;
}
function totalVotesFor(bytes32 candidate) view public returns (uint) {
return votesReceived[candidate];
}
function voteForCandidate(bytes32 candidate, uint votesInTokens) public {
uint index = indexOfCandidate(candidate);
require(index != uint(-1));
if (voterInfo[msg.sender].tokensUsedPerCandidate.length == 0) {
for(uint i = 0; i < candidateList.length; i++) {
voterInfo[msg.sender].tokensUsedPerCandidate.push(0);
}
}
// Make sure this voter has enough tokens to cast the vote
uint availableTokens = voterInfo[msg.sender].tokensBought - totalTokensUsed(voterInfo[msg.sender].tokensUsedPerCandidate);
require(availableTokens >= votesInTokens);
votesReceived[candidate] += votesInTokens;
// Store how many tokens were used for this candidate
voterInfo[msg.sender].tokensUsedPerCandidate[index] += votesInTokens;
}
// Return the sum of all the tokens used by this voter.
function totalTokensUsed(uint[] _tokensUsedPerCandidate) private pure returns (uint) {
uint totalUsedTokens = 0;
for(uint i = 0; i < _tokensUsedPerCandidate.length; i++) {
totalUsedTokens += _tokensUsedPerCandidate[i];
}
return totalUsedTokens;
}
function indexOfCandidate(bytes32 candidate) view public returns (uint) {
for(uint i = 0; i < candidateList.length; i++) {
if (candidateList[i] == candidate) {
return i;
}
}
return uint(-1);
}
function buy() payable public returns (uint) {
uint tokensToBuy = msg.value / tokenPrice;
require(tokensToBuy <= balanceTokens);
voterInfo[msg.sender].voterAddress = msg.sender;
voterInfo[msg.sender].tokensBought += tokensToBuy;
balanceTokens -= tokensToBuy;
return tokensToBuy;
}
function tokensSold() view public returns (uint) {
return totalTokens - balanceTokens;
}
function voterDetails(address user) view public returns (uint, uint[]) {
return (voterInfo[user].tokensBought, voterInfo[user].tokensUsedPerCandidate);
}
function transferTo(address account) public {
account.transfer(this.balance);
}
function allCandidates() view public returns (bytes32[]) {
return candidateList;
}
}
同时要配合修改一下index.html里的内容，以支持填入要投票的token数量，和查看投票人信息的功能：
投票时，你需要指定给候选人投多少票(tokens)
新增一个可购买token的section
增加查看voter的功能
候选人信息不再写死，通过调用合约方法获得
index.html
Hello World DApp
A Simple Hello World Voting Application
app.js
// Import the page's CSS. Webpack will know what to do with it.
import "../stylesheets/app.css";
// Import libraries we need.
import { default as Web3} from 'web3';
import { default as contract } from 'truffle-contract'
import voting_artifacts from '../../build/contracts/Voting.json'
var Voting = contract(voting_artifacts);
let candidates = {}
let tokenPrice = null;
window.voteForCandidate = function(candidate) {
let candidateName = $("#candidate").val();
let voteTokens = $("#vote-tokens").val();
$("#msg").html("Vote has been submitted. The vote count will increment as soon as the vote is recorded on the blockchain. Please wait.")
$("#candidate").val("");
$("#vote-tokens").val("");
/* Voting.deployed() returns an instance of the contract. Every call
* in Truffle returns a promise which is why we have used then()
* everywhere we have a transaction call
*/
Voting.deployed().then(function(contractInstance) {
contractInstance.voteForCandidate(candidateName, voteTokens, {gas: 140000, from: web3.eth.accounts[0]}).then(function() {
let div_id = candidates[candidateName];
return contractInstance.totalVotesFor.call(candidateName).then(function(v) {
$("#" + div_id).html(v.toString());
$("#msg").html("");
});
});
});
}
/* The user enters the total no. of tokens to buy. We calculate the total cost and send it in
* the request. We have to send the value in Wei. So, we use the toWei helper method to convert
* from Ether to Wei.
*/
window.buyTokens = function() {
let tokensToBuy = $("#buy").val();
let price = tokensToBuy * tokenPrice;
$("#buy-msg").html("Purchase order has been submitted. Please wait.");
Voting.deployed().then(function(contractInstance) {
contractInstance.buy({value: web3.toWei(price, 'ether'), from: web3.eth.accounts[0]}).then(function(v) {
$("#buy-msg").html("");
web3.eth.getBalance(contractInstance.address, function(error, result) {
$("#contract-balance").html(web3.fromWei(result.toString()) + " Ether");
});
})
});
populateTokenData();
}
window.lookupVoterInfo = function() {
let address = $("#voter-info").val();
Voting.deployed().then(function(contractInstance) {
contractInstance.voterDetails.call(address).then(function(v) {
$("#tokens-bought").html("Total Tokens bought: " + v[0].toString());
let votesPerCandidate = v[1];
$("#votes-cast").empty();
$("#votes-cast").append("Votes cast per candidate:
");
let allCandidates = Object.keys(candidates);
for(let i=0; i < allCandidates.length; i++) {
$("#votes-cast").append(allCandidates[i] + ": " + votesPerCandidate[i] + "
");
}
});
});
}
/* Instead of hardcoding the candidates hash, we now fetch the candidate list from
* the blockchain and populate the array. Once we fetch the candidates, we setup the
* table in the UI with all the candidates and the votes they have received.
*/
function populateCandidates() {
Voting.deployed().then(function(contractInstance) {
contractInstance.allCandidates.call().then(function(candidateArray) {
for(let i=0; i < candidateArray.length; i++) {
/* We store the candidate names as bytes32 on the blockchain. We use the
* handy toUtf8 method to convert from bytes32 to string
*/
candidates[web3.toUtf8(candidateArray[i])] = "candidate-" + i;
}
setupCandidateRows();
populateCandidateVotes();
populateTokenData();
});
});
}
function populateCandidateVotes() {
let candidateNames = Object.keys(candidates);
for (var i = 0; i < candidateNames.length; i++) {
let name = candidateNames[i];
Voting.deployed().then(function(contractInstance) {
contractInstance.totalVotesFor.call(name).then(function(v) {
$("#" + candidates[name]).html(v.toString());
});
});
}
}
function setupCandidateRows() {
Object.keys(candidates).forEach(function (candidate) {
$("#candidate-rows").append("" + candidate + " 更新2_deploy_contracts.js代码，传入总token数量和token价格，并重新部署：
var Voting = artifacts.require("./Voting.sol");
module.exports = function(deployer) {
deployer.deploy(Voting, 1000， web3.toWei('0.1', 'ether'), ['Sam', 'Leslie', 'Jetty', 'Arkila', 'Piu']);
};
truffle migrate
部署成功后，启动本地server:
npm run dev
开始愉快的投票吧。。。
屏幕快照 2018-04-10 下午4.17.07.png