W3-2 合约事件
W3-2 合约事件
理解事件
作用
- 合约与外部世界的重要接⼝，通知外部世界链上状态的变化
- 事件有时也作为便宜的存储
编码特征
- 使⽤关键字 event 定义事件，事件不需要实现
- 使⽤关键字 emit 触发事件
- 事件中使⽤
indexed修饰，表示对这个字段建⽴索引，⽅便外部对该字段过滤查找
pragma solidity ^0.8.0;
contract testEvent {
constructor() {}
event Deposit(address indexed _from, uint _value);
function deposit(uint value) public {
emit Deposit(msg.sender, value);
}
}
保存事件/数据上链
数据编码过程
事件签名为第一个 Topic ，在黄皮书种称作 LOG0
索引事件进行 ABI 编码后用 Topic 部分 (topic[1,2,3..]) 记录下来，详细见👇代码
⾮索引事件进行 ABI 编码后作为 Data 部分保存下来
- 代码示例
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// 假定 events 事件格式 event Transfer (address indexed _from, address index _to, uint256 value); // 去除关键词 events，indexed 和 参数名称 _from, _to , value let transferTopic = ethers.utils.keccak256( ethers.utils.toUtf8Bytes("Transfer(address,address,uint256)")); // 等价于 ethers.utils.id("Transfer(address,address,uint256)")
代码示例
事件解码 decoded
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var ethers = require("ethers")
// 定义要解码的事件的ABI
const eventAbi = [
{
"anonymous": false,
"inputs": [
{
"indexed": true,
"name": "from",
"type": "address"
},
{
"indexed": true,
"name": "to",
"type": "address"
},
{
"indexed": false,
"name": "value",
"type": "uint256"
}
],
"name": "Transfer",
"type": "event"
}
]
// 创建一个新的ethers.ethers.utils.Interface对象
const contractInterface = new ethers.utils.Interface(eventAbi);
// 要解码的topics数组
const topics = [
"0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef",
"0x0000000000000000000000004f9fbb3f1e99b56e0fe2892e623ed36a76fc605d",
"0x000000000000000000000000b8a9fada75c6d891fb77a7988ff9bad9e485ca1c"
];
const data = '0x00000000000000000000000000000000000000000000087f552f6cf833c0c200'
const decodedLog = contractInterface.parseLog({data, topics});
console.log(decodedLog)
事件编码 encoded
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// 以事件中的 data 部分，类型为 unit256，数值为 40127806604948680000000 进行编码
var ethers = require("ethers")
const abiCoder = new ethers.utils.AbiCoder();
const encodedLog = abiCoder.encode(["uint256"], ["40127806604948680000000"]);
console.log(encodedLog)
算法
监听事件
- 监听（实时）事件（使⽤ webSocket 链接）
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let erc20 = new ethers.Contract(erc20地址, ABI, provider);
// indexed 的字段可以作为参数加⼊到过滤器中
let filter = erc20.filters.Transfer()
wssprovider.on(filter, (log) => {
console.log(log);
})
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event = {
blockNumber: 9,
blockHash: '0x5eaa8438d80d30e521b6c48db8c22d5b40255cd472d71cfd387c314486a7e679',
transactionIndex: 0,
removed: false,
address: '0x5FbDB2315678afecb367f032d93F642f64180aa3',
data: '0x00000000000000000000000000000000000000000000000000000000000003e8',
topics: [
'0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef',
'0x000000000000000000000000f39fd6e51aad88f6f4ce6ab8827279cfffb92266',
'0x00000000000000000000000070997970c51812dc3a010c7d01b50e0d17dc79c8'
],
transactionHash: '0x1eda2652cde3d9b3d8dbb1e0441a2139ebd25ba25a97fab3158406a695e33ba9',
logIndex: 0
}
获取事件
获取历史事件
调用方法
- https://docs.ethers.io/v5/api/providers/provider/#Provider-getLogs
- https://docs.ethers.io/v5/concepts/events/#events–filters
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let erc20 = new ethers.Contract(erc20地址, ABI, provider);
let filter = erc20.filters.Transfer()
// 选择区块区间
filter.fromBlock = 1000;
filter.toBlock = 2000; // "latest";
// 获取⽇志
let logs = await provider.getLogs(filter);
let logs = await erc20.queryFilter(filterTo, -10, "latest");
处理事件
解析事件
- 使⽤ ethers.utils.Interface 进⾏编解码
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const TransferEvent = new ethers.utils.Interface(["event Transfer(address indexed from,address indexed to,uint256 value)"]); const data = TransferEvent.parseLog(logs[i]) console.log("from:" + data.args.from) console.log("from:" + data.args.to)
- Interface (ethers.io)
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