一個Solidity Event的定義如下：
event Deposit(
address indexed _from,
bytes32 indexed _id,
uint _value
);
- 最多 3 indexed參數.
- 如果一個 indexed 參數的類型是大於32 bytes (比如 string 和 bytes), 就不存實際數據, 而是存數據的KECCAK256摘要（Digest）.
EVM Log Primitives
先來看log0, log1, ..., log4 EVM 指令.
EVM 日誌功能使用不同的術語:
- “topics”: 最多 4 個topics. 每個topic 32 bytes.
- “data”: 數據是event的Payload. 可以是任意長度的bytes.
一個Solidity event 如何映射到一個log 原語?
- 所有“non-indexed 參數” 被存爲data.
- 每個“indexed 參數” 被存爲一個32 bytes的topic.
The log0 Primitive
Log0生成一個只有data的日誌項目, 沒有topic. data可以任意長度的 bytes.
下面看一個例子
pragma solidity ^0.4.18;
contract Logger {
function Logger() public {
log0(0xc0fefe);
}
}
編譯後
0x40指針是內存的空閒指針。第一部分將數據導入內存，第二部分將數據的大小在棧上準備好
memory: { 0x40 => 0x60 }
tag_1:
// copy data into memory
0xc0fefe
[0xc0fefe]
mload(0x40)
[0x60 0xc0fefe]
swap1
[0xc0fefe 0x60]
dup2
[0x60 0xc0fefe 0x60]
mstore
[0x60]
memory: {
0x40 => 0x60
0x60 => 0xc0fefe
}
// calculate data start position and size
0x20
[0x20 0x60]
add
[0x80]
mload(0x40)
[0x60 0x80]
dup1
[0x60 0x60 0x80]
swap2
[0x60 0x80 0x60]
sub
[0x20 0x60]
swap1
[0x60 0x20]
log0
在執行log0前, 在棧上有2個參數: [0x60 0x20].
start: 0x60 是用來存放數據的內存指針.size: 0x20 (或者32) 指定了載入數據的大小.
Logging With Topics
下面的例子使用 log2 原語. 第一個參數是數據（可以任意長字節），其後跟着 2個 topics (每個32 bytes ):
// log-2.sol
pragma solidity ^0.4.18;
contract Logger {
function Logger() public {
log2(0xc0fefe, 0xaaaa1111, 0xbbbb2222);
}
}
彙編代碼是非常相似的。唯一的區別是2個topics (0xbbbb2222, 0xaaaa1111)被推倒了棧上:
tag_1: // push topics 0xbbbb2222 0xaaaa1111 // copy data into memory 0xc0fefe mload(0x40) swap1 dup2 mstore 0x20 add mload(0x40) dup1 swap2 sub swap1 // create log log2
數據還是0xc0fefe, 被拷貝到內存. 執行 log2前，狀態如下:
stack: [0x60 0x20 0xaaaa1111 0xbbbb2222]
memory: {
0x60: 0xc0fefe
}
log2
頭兩個參數指定日誌數據的內存領域，2個新增的棧上元素是2個32 bytes 的topics.
All EVM Logging Primitives
EVM支持5個日誌的原語:
0xa0 LOG0 0xa1 LOG1 0xa2 LOG2 0xa3 LOG3 0xa4 LOG4
Logging Testnet Demo
pragma solidity ^0.4.18;
contract Logger {
function Logger() public {
log0(0x0);
log1(0x1, 0xa);
log2(0x2, 0xa, 0xb);
log3(0x3, 0xa, 0xb, 0xc);
log4(0x4, 0xa, 0xb, 0xc, 0xd);
}
}
這個合約被部署到Rinkeby測試網絡. 在： https://rinkeby.etherscan.io/tx/0x0e88c5281bb38290ae2e9cd8588cd979bc92755605021e78550fbc4d130053d1
Solidity Events
下面是一個Log事件，帶着3個uint256的參數(non-indexed):
pragma solidity ^0.4.18;
contract Logger {
event Log(uint256 a, uint256 b, uint256 c);
function log(uint256 a, uint256 b, uint256 c) public {
Log(a, b, c);
}
}
生成的原始日誌在:
https://rinkeby.etherscan.io/tx/0x9d3d394867330ae75d7153def724d062b474b0feb1f824fe1ff79e772393d395
數據是事件參數, ABI編碼:
0000000000000000000000000000000000000000000000000000000000000001 0000000000000000000000000000000000000000000000000000000000000002 0000000000000000000000000000000000000000000000000000000000000003
有一個topic, 1個 32 bytes hash:
0x00032a912636b05d31af43f00b91359ddcfddebcffa7c15470a13ba1992e10f0
這是事件類型的簽名的SHA3哈希:
# Install pyethereum # https://github.com/ethereum/pyethereum/#installation > from ethereum.utils import sha3 > sha3("Log(uint256,uint256,uint256)").hex() '00032a912636b05d31af43f00b91359ddcfddebcffa7c15470a13ba1992e10f0'
因爲Solidity事件爲事件簽名用掉了一個topic, 留給indexed 參數的只有3個topic.
Solidity Event With Indexed Arguments
下面是有一個 indexed uint256 參數的事件:
pragma solidity ^0.4.18;
contract Logger {
event Log(uint256 a, uint256 indexed b, uint256 c);
function log(uint256 a, uint256 b, uint256 c) public {
Log(a, b, c);
}
}
有2個topic:
0x00032a912636b05d31af43f00b91359ddcfddebcffa7c15470a13ba1992e10f0 0x0000000000000000000000000000000000000000000000000000000000000002
- 第一個topic是方法的簽名.
- 第二個topic是indexed參數的值.
除了indexed 參數，數據是ABI編碼:
0000000000000000000000000000000000000000000000000000000000000001 0000000000000000000000000000000000000000000000000000000000000003
String/Bytes Event Parameter
將事件的參數設爲字符串:
pragma solidity ^0.4.18;
contract Logger {
event Log(string a, string indexed b, string c);
function log(string a, string b, string c) public {
Log(a, b, c);
}
}
交易在: https://rinkeby.etherscan.io/tx/0x21221c2924bbf1860db9e098ab98b3fd7a5de24dd68bab1ea9ce19ae9c303b56
有2個topics:
0xb857d3ea78d03217f929ae616bf22aea6a354b78e5027773679b7b4a6f66e86b 0xb5553de315e0edf504d9150af82dafa5c4667fa618ed0a6f19c69b41166c5510
- 第一個topic是方法的簽名.
- 第二個topic是字符串參數的SHA256摘要.
驗證“b”的哈希值和第二個topic是一樣的:
>>> sha3("b").hex()
'b5553de315e0edf504d9150af82dafa5c4667fa618ed0a6f19c69b41166c5510'
日誌數據是2個non-indexed 字符串 “a” 和 “c”, ABI編碼:
0000000000000000000000000000000000000000000000000000000000000040 0000000000000000000000000000000000000000000000000000000000000080 0000000000000000000000000000000000000000000000000000000000000001 6100000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000000000000000000000000000000000001 6300000000000000000000000000000000000000000000000000000000000000
indexed 字符串參數沒有被存儲，所以DApp客戶無法恢復它.
如果你確實需要最初的字符串, 那就記錄2次, indexed 和 non-indexed:
event Log(string a, string indexed indexedB, string b);
Log("a", "b", "b");