《纸上谈兵·solidity》第 3 课：事件（Event）机制与链上日志——不是 print，是广播！
本课目标
- 理解事件（
event）的作用 - 掌握事件的声明、触发和监听方法
一、事件是什么？
在 Solidity 中，事件（event）不是控制台输出，也不是日志记录函数，而是链上广播机制。
event是以太坊虚拟机（EVM）内置的 日志系统（logs）- 事件可以在合约执行过程中发出并被监听
- 它们存在于 交易收据（transaction receipt） 中，而不是链上状态
event Transfer(address indexed from, address indexed to, uint256 amount);二、事件的声明与触发
示例合约：EventDemo.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
contract EventDemo {
event Greeted(address indexed sender, string message);
function greet(string memory message) public {
emit Greeted(msg.sender, message);
}
}三、事件与 console.log() 有何不同？
比较项 |
|
|
|---|---|---|
是否上链 | ❌ 否，仅本地运行 | ✅ 是，写入交易日志 |
是否可被监听 | ❌ 否 | ✅ 是，可被前端/Web3 监听 |
运行环境 | 本地、测试时可用 | 任意 EVM 环境均可使用 |
实用场景 | 调试临时输出 | 广播链上动作供外部使用 |
四、indexed 的作用：支持按条件过滤
你可以为事件中的参数加上 indexed，最多三个，这样它们就会变成 事件主题（topics），可在链上快速筛选。
event Transfer(address indexed from, address indexed to, uint256 amount);这使得你可以仅查询某地址的所有 Transfer 事件，而不需要扫描全链日志。
五、测试与监听事件
1. Foundry 中测试事件
下面以经典的 Counter 合约为例，新增一个事件用于测试：
合约：Counter.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
contract Counter {
uint256 public count;
// 添加事件声明
event Incremented(uint256 newCount);
function increment() public {
count += 1;
// 触发事件
emit Incremented(count);
}
function getCount() public view returns (uint256) {
return count;
}
}测试文件：test/Counter.t.sol
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "forge-std/Test.sol";
import "../src/Counter.sol";
contract CounterTest is Test {
Counter public counter;
function setUp() public {
counter = new Counter();
}
function testInitialCountIsZero() public {
assertEq(counter.getCount(), 0);
}
function testIncrementIncreasesCount() public {
counter.increment();
assertEq(counter.getCount(), 1);
}
// 事件触发测试
function testIncrementEmitsEvent() public {
vm.expectEmit(false, false, false, true); // 只验证数据部分（不是 indexed 的 topic）
emit Counter.Incremented(1);
counter.increment();
}
}2. 脚本中打印日志
测试文件：scripts/Counter.s.sol：
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
import "forge-std/Script.sol";
import "../src/Counter.sol";
contract CounterEmitScript is Script {
function run() external {
// 开始广播交易（写入链上）
vm.startBroadcast();
// 使用部署后的合约地址
address counterAddr = address(0xYourDeployedCounterAddress);
Counter counter = Counter(counterAddr);
// 发起交易
counter.increment();
vm.stopBroadcast();
}
}这个脚本只是发起交易。要获取事件日志，我们需要读取交易 receipt，这部分要用外部工具（如 cast 或 ethers.js）处理，因为 Solidity 脚本本身无法访问 receipt。
我们可以在执行 forge script 时，加上 --json 选项并从输出中解析出交易哈希，然后用 cast 查看交易 receipt 和事件日志，步骤如下：
- 执行脚本并保存 JSON 输出：
# 配置私钥
$ export PRIVATE_KEY=0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80
$ forge script script/Counter.s.sol:CounterEmitScript \
--rpc-url http://127.0.0.1:8545 \
--private-key $PRIVATE_KEY \
--broadcast \
--json \
-vv > result.jsonresult.json内容如下：
{"logs":[],"returns":{},"success":true,"raw_logs":[{"address":"0x5fbdb2315678afecb367f032d93f642f64180aa3","topics":["0x20d8a6f5a693f9d1d627a598e8820f7a55ee74c183aa8f1a30e8d4e8dd9a8d84"],"data":"0x0000000000000000000000000000000000000000000000000000000000000001"}],"traces":[["Deployment",{"arena":[{"parent":null,"children":[],"idx":0,"trace":{"depth":0,"success":true,"caller":"0x1804c8ab1f12e6bbf3894d4083f33e07309d1f38","address":"0x5b73c5498c1e3b4dba84de0f1833c4a029d90519","maybe_precompile":false,"selfdestruct_address":null,"selfdestruct_refund_target":null,"selfdestruct_transferred_value":null,"kind":"CREATE","value":"0x0","data":"0x...","output":"0x...","gas_used":259762,"gas_limit":1073669316,"status":"Return","steps":[],"decoded":{"label":null,"return_data":null,"call_data":null}},"logs":[],"ordering":[]}]}],["Execution",{"arena":[{"parent":null,"children":[1,2,3,4],"idx":0,"trace":{"depth":0,"success":true,"caller":"0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266","address":"0x5b73c5498c1e3b4dba84de0f1833c4a029d90519","maybe_precompile":null,"selfdestruct_address":null,"selfdestruct_refund_target":null,"selfdestruct_transferred_value":null,"kind":"CALL","value":"0x0","data":"0xc0406226","output":"0x","gas_used":173200,"gas_limit":1073720760,"status":"Stop","steps":[],"decoded":{"label":null,"return_data":null,"call_data":null}},"logs":[],"ordering":[{"Call":0},{"Call":1},{"Call":2},{"Call":3}]},{"parent":0,"children":[],"idx":1,"trace":{"depth":1,"success":true,"caller":"0x5b73c5498c1e3b4dba84de0f1833c4a029d90519","address":"0x7109709ecfa91a80626ff3989d68f67f5b1dd12d","maybe_precompile":null,"selfdestruct_address":null,"selfdestruct_refund_target":null,"selfdestruct_transferred_value":null,"kind":"CALL","value":"0x0","data":"0x7fb5297f","output":"0x","gas_used":0,"gas_limit":1056941001,"status":"Return","steps":[],"decoded":{"label":null,"return_data":null,"call_data":null}},"logs":[],"ordering":[]},{"parent":0,"children":[],"idx":2,"trace":{"depth":1,"success":true,"caller":"0x5b73c5498c1e3b4dba84de0f1833c4a029d90519","address":"0x5fbdb2315678afecb367f032d93f642f64180aa3","maybe_precompile":false,"selfdestruct_address":null,"selfdestruct_refund_target":null,"selfdestruct_transferred_value":null,"kind":"CREATE","value":"0x0","data":"0x6...","output":"0x6...","gas_used":113557,"gas_limit":1056909248,"status":"Return","steps":[],"decoded":{"label":null,"return_data":null,"call_data":null}},"logs":[],"ordering":[]},{"parent":0,"children":[],"idx":3,"trace":{"depth":1,"success":true,"caller":"0x5b73c5498c1e3b4dba84de0f1833c4a029d90519","address":"0x5fbdb2315678afecb367f032d93f642f64180aa3","maybe_precompile":null,"selfdestruct_address":null,"selfdestruct_refund_target":null,"selfdestruct_transferred_value":null,"kind":"CALL","value":"0x0","data":"0xd09de08a","output":"0x","gas_used":23762,"gas_limit":1056797138,"status":"Stop","steps":[],"decoded":{"label":null,"return_data":null,"call_data":null}},"logs":[{"raw_log":{"topics":["0x20d8a6f5a693f9d1d627a598e8820f7a55ee74c183aa8f1a30e8d4e8dd9a8d84"],"data":"0x0000000000000000000000000000000000000000000000000000000000000001"},"decoded":{"name":null,"params":null},"position":0}],"ordering":[{"Log":0}]},{"parent":0,"children":[],"idx":4,"trace":{"depth":1,"success":true,"caller":"0x5b73c5498c1e3b4dba84de0f1833c4a029d90519","address":"0x7109709ecfa91a80626ff3989d68f67f5b1dd12d","maybe_precompile":null,"selfdestruct_address":null,"selfdestruct_refund_target":null,"selfdestruct_transferred_value":null,"kind":"CALL","value":"0x0","data":"0x76eadd36","output":"0x","gas_used":0,"gas_limit":1056773421,"status":"Return","steps":[],"decoded":{"label":null,"return_data":null,"call_data":null}},"logs":[],"ordering":[]}]}]],"gas_used":194264,"labeled_addresses":{},"returned":"0x","address":null}
{"chain":31337,"estimated_gas_price":"2.000000001","estimated_total_gas_used":293726,"estimated_amount_required":"0.000587452000293726","token_symbol":"ETH"}
{"chain":"anvil-hardhat","status":"success","tx_hash":"0xe680bfb48e6bff6767d19270ae66105aa2cfe8fd871f1c8895ad569cb1bfc07e","contract_address":null,"block_number":2,"gas_used":44826,"gas_price":876462626}
{"chain":"anvil-hardhat","status":"success","tx_hash":"0x175be95259405d886063011e8f80fddfb53c02fc2f13ac8e68cbab010ae69624","contract_address":"0x5FbDB2315678afecb367f032d93F642f64180aa3","block_number":1,"gas_used":175515,"gas_price":1000000001}
{"status":"success","transactions":"/path/to/counter/broadcast/Counter.s.sol/31337/run-latest.json","sensitive":"/Users/neo/vscode/mengbin92/tsp/counter/cache/Counter.s.sol/31337/run-latest.json"}- 从
run-latest.json中提取交易哈希（或使用 jq），从之前的result.json中我们可以知道交易相关的内容都存储在run-latest.json中，运行如下命令可以获取哈希：
$ jq -r '.transactions[1].hash' $(pwd)counter/broadcast/Counter.s.sol/31337/run-latest.json
# 我这里显示的是这个
0xe680bfb48e6bff6767d19270ae66105aa2cfe8fd871f1c8895ad569cb1bfc07e为什么是
[1]？ 因为在我们的场景里，[0]是部署合约的交易，[1]是increment()的调用交易。
- 用
cast获取交易 receipt 和事件日志：
$ cast receipt 0xe680bfb48e6bff6767d19270ae66105aa2cfe8fd871f1c8895ad569cb1bfc07e --rpc-url http://127.0.0.1:8545输出示例：
接下来我们可以用 cast 解码 data：
$ cast decode-event --sig "Incremented(uint256)" 0x0000000000000000000000000000000000000000000000000000000000000001
# 因为我们合约部署后只调用了一次 increment()，所以这里输出是这样的
1--sig 是 Foundry 的 cast decode-event 命令中用于指定 事件签名（event signature） 的参数，用来告诉 cast 要按照哪个事件的格式去解码日志数据。 在我们的例子里，事件的定义为 event Incremented(uint256 newCount)，对应的签名就是 Incremented(uint256)
六、小练习建议
- 编写测试验证事件是否被正确触发
- 编写监听脚本或前端页面响应事件
下一课预告
第 4 课：Solidity 合约中的错误处理机制（require、revert、assert）和自定义错误。
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