什么是合约事件

Log System Help US:

1. Debug
2. Notify
3. Compliance

Logging on Ethereum

On EVM

Generate a log record during logging operations.

Ethereum Yellowpaper

On Solidity

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.20;
contract EventExample {
    
    event Transfer(address indexed from, address indexed to, uint256 value); // 1️⃣

    mapping(address => uint256) public balances;

    function transfer(address to, uint256 amount) external {
        balances[msg.sender] -= amount;
        balances[to] += amount;
        emit Transfer(msg.sender, to, amount); // 2️⃣
    }

    function mint() external {
        balances[msg.sender] += 100;
        emit Transfer(address(0), msg.sender, 100); //3️⃣
    }
}

• 1️⃣ Define event struct
• 2️⃣ Emit event (exec log2)
• 3️⃣ Emit event (exec log2)

On Node

Log records are stored on the Ethereum blockchain node server.

eth_getlogs、example

Event Record

• address: the address of the contract (intrinsically provided by Ethereum);
• topics[0]: keccak(EVENT_NAME+"("+EVENT_ARGS.map(canonical_type_of).join(",")+")") (canonical_type_of is a function that simply returns the canonical type of a given argument, e.g. for uint indexed foo, it would return uint256). This value is only present in topics[0] if the event is not declared as anonymous;
• topics[n]: abi_encode(EVENT_INDEXED_ARGS[n - 1]) if the event is not declared as anonymous or abi_encode(EVENT_INDEXED_ARGS[n]) if it is (EVENT_INDEXED_ARGS is the series of EVENT_ARGS that are indexed);
• data: ABI encoding of EVENT_NON_INDEXED_ARGS (EVENT_NON_INDEXED_ARGS is the series of EVENT_ARGS that are not indexed, abi_encode is the ABI encoding function used for returning a series of typed values from a function, as described above).

Exercise

What does this log tell us about what happened?

{
  "address":"0x33fd426905f149f8376e227d0c9d3340aad17af1",
  "topics":["0xc3d58168c5ae7397731d063d5bbf3d657854427343f4c083240f7aacaa2d0f62","0x0000000000000000000000001e0049783f008a0085193e00003d00cd54003c71","0x000000000000000000000000111818a51c4177e8980566beea68fe334be7b76a","0x00000000000000000000000091aa2610067019cb9930106d1fae7998ba1e73ee"], "data":"0x000000000000000000000000000000000000000000000000000000000000003c0000000000000000000000000000000000000000000000000000000000000001",
  "blockHash":"...",
  "blockNumber":"...",
  "transactionHash":"...",
  "transactionIndex":"0x7f",
  "logIndex":"0x22e",
  "removed":false
}