事件是合约外部通知,在logs中查看。一般用于dapp监听使用。
事件在合约中可被继承。当他们被调用时，会使参数被存储到交易的日志中 —— 一种区块链中的特殊数据结构。
成本较低

event Set(uint value);  
emit Set(x);  //触发事件

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract EventTest {
    event LogEvent(string indexed a, uint8 b);
    event TestEvent(string a);

    function eventFunction() public{
        emit LogEvent("hello", 101);
    }
    
    function test() public{
        emit TestEvent("world");
    }

    function getSig() public pure returns(bytes32, bytes32){
        bytes32 r1 = keccak256("TestEvent(string)");  //事件topic的值
        bytes32 r2 = keccak256("LogEvent(string,uint8)");
        return(r1, r2);
    }
}

其中 keccak256("TestEvent(string)")，即事件的签名，也是事件topic的值。
//logs-LogEvent
{
  "from": "0xa514f9ce4ceed99e4731b437123073f2d0c1745c",
  "topic": "0x449e54c0703954de7e4a92b7f921b71b2574e355474bdcfe8461f34d63e1e542",
  "event": "LogEvent",
  "args": {
    "0": {
      "hash": "0x1c8aff950685c2ed4bc3174f3472287b56d9517b9c948127319a09a7a36deac8",
      "type": "Indexed"
    },
    "1": 101,
    "a": {
      "hash": "0x1c8aff950685c2ed4bc3174f3472287b56d9517b9c948127319a09a7a36deac8",
      "type": "Indexed"
    },
    "b": 101,
    "length": 2
  }
  }

//logs-TestEvent
{
  "from": "0xa514f9ce4ceed99e4731b437123073f2d0c1745c",
  "topic": "0xe75028ff36bb6473da3731a30e1aeeae9988e2415dba2c4e91e0357955065fba",
  "event": "TestEvent",
  "args": {
    "0": "world",
    "a": "world",
    "length": 1
  }
  }