以太坊 Solidity event 事件

1. event 的作用

event 事件 非常重要，可以让我们方便的访问EVM的日志，他是合约与外部沟通的桥梁。

例如，我们发送交易来调用某个合约，交易的处理是需要时间的，我们无法立即获得返回值，交易打包后，事件才真正发生，我们的前端就可以通过监听事件进行响应处理。

2. 示例

（1）创建项目

mkdir event_test
cd evnet_test

# 初始化
truffle init

# 安装 web3
npm install --save [email protected]

启动 Ganache，修改配置文件 truffle.js：

module.exports = {
  networks: {
    development: {
      host: "127.0.0.1",
      port: 7545,
      network_id: "*"
    }
  }
};

（2）合约代码

contracts/Coursetro.sol

pragma solidity ^0.4.22;

contract Coursetro {
    string fName;
    uint age;

    event Instructor(
        string name,        
        uint age);

    function setInstructor(string _fName, uint _age) public {
        fName = _fName;
        age = _age;
        emit Instructor(_fName, _age);  
    }
    
    function getInstructor() view public returns (string, uint) {
        return (fName, age);
    }
    
}

（3）编译发布

迁移脚本 migrations/2_deploy_Coursetro.js

var Coursetro = artifacts.require("Coursetro");

module.exports = function(deployer) {
  deployer.deploy(Coursetro);
};

truffle compile
truffle migrate # 输出信息中会有合约ID，需要记住

（4）测试代码

test/test.js

var Web3 = require("web3");

var web3 = new Web3();
web3.setProvider(new web3.providers.HttpProvider("http://localhost:7545"));

web3.eth.defaultAccount = web3.eth.accounts[0];

// 合约的ABI，在 build/Coursetro.json 中
var abi = [{"anonymous":false,"inputs":[{"indexed":false,"name":"name","type":"string"},{"indexed":false,"name":"age","type":"uint256"}],"name":"Instructor","type":"event"},{"constant":false,"inputs":[{"name":"_fName","type":"string"},{"name":"_age","type":"uint256"}],"name":"setInstructor","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"getInstructor","outputs":[{"name":"","type":"string"},{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"}]; 

var CoursetroContract = web3.eth.contract(abi);

// 合约ID，执行迁移命令的输出信息中有，Ganache 交易列表中也可以找到
var contractId = '0xDF32B5BAC77C2998587E81F787667B869fd06190';
var Coursetro = CoursetroContract.at(contractId);

var instructorEvent = Coursetro.Instructor();

instructorEvent.watch(function(error, result) {
    if (!error) {
        // 查看业务信息
        console.info(result.args.name + ' (' + result.args.age + ' years old)');
        // 查看全部信息
        console.info(result);
    } else {
        console.log(error);
    }
});

Coursetro.setInstructor("gates", "28");

（5）运行

$ node test/test.js
gates (28 years old)
{ logIndex: 0,
  transactionIndex: 0,
  transactionHash: '0x9867ac98df022d6a2c40eb101904a018b0069c1b06892144dd1cef98bb0d4721',
  blockHash: '0x6ca8abf99dc5295d5a742998c901372c3772499e77a371d7ebb2fc2b1fd41f14',
  blockNumber: 18,
  address: '0xa50246a6e751aeed6c0c101bb89e9890dd493908',
  type: 'mined',
  event: 'Instructor',
  args: { name: 'gates', age: BigNumber { s: 1, e: 1, c: [Array] } } }

Web3 Event 文档