ERC20解读

  • Confucian
  • 更新于 2022-04-04 19:54
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ERC20代币标准的个人解读

ERC20解读

参考 OpenZepplin文档以太坊官方开发者文档,结合自己的理解。

博客的 Markdown 编辑器暂不支持 Solidity 语法高亮,为了更好阅读代码,可以去 我的GitHub仓库

什么是ERC20

ERC20(Ethereum Request for Comments 20)一种代币标准。EIP-20 中提出。

ERC20 代币合约跟踪同质化(可替代)代币:任何一个代币都完全等同于任何其他代币;没有任何代币具有与之相关的特殊权利或行为。这使得 ERC20 代币可用于交换货币、投票权、质押等媒介。

为什么要遵守ERC20

EIP-20 中的动机:

允许以太坊上的任何代币被其他应用程序(从钱包到去中心化交易所)重新使用的标准接口。

以太坊上的所有应用都默认支持 ERC20 ,如果你想自己发币,那么你的代码必须遵循 ERC20 标准,这样钱包(如MetaMask)等应用才能将你的币显示出来。

代码实现

需要实现以下函数和事件:

function name() public view returns (string)
 function symbol() public view returns (string)
 function decimals() public view returns (uint8)
 function totalSupply() public view returns (uint256)
 function balanceOf(address _owner) public view returns (uint256 balance)
 function transfer(address _to, uint256 _value) public returns (bool success)
 function transferFrom(address _from, address _to, uint256 _value) public returns (bool success)
 function approve(address _spender, uint256 _value) public returns (bool success)
 function allowance(address _owner, address _spender) public view returns (uint256 remaining)
 ​
 event Transfer(address indexed _from, address indexed _to, uint256 _value)
 event Approval(address indexed _owner, address indexed _spender, uint256 _value)

使用 OpenZeppllin 提供的库能够轻松快速地构建 ERC20 Token 。

快速构建

这是一个 GLD token 。

// contracts/GLDToken.sol
 // SPDX-License-Identifier: MIT
 pragma solidity ^0.8.0;
 ​
 import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 ​
 contract GLDToken is ERC20 {
     constructor(uint256 initialSupply) ERC20("Gold", "GLD") {
         _mint(msg.sender, initialSupply);
     }
 }

通常,我们定义代币的发行量和代币名称及符号。

IERC20

先来看下 ERC20 的接口(IERC20),这方便我们在开发中直接定义 ERC20 代币。

同样地,OpenZepplin 为我们提供了相应的库,方便开发者导入即用。

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

EIP 中定义的 ERC20 标准接口:

pragma solidity ^0.8.0;
 ​
 interface IERC20 {
     event Transfer(address indexed from, address indexed to, uint256 value);
     event Approval(address indexed owner, address indexed spender, uint256 value);

     function totalSupply() external view returns (uint256);
     function balanceOf(address account) external view returns (uint256);
     function transfer(address to, uint256 amount) external returns (bool);
     function allowance(address owner, address spender) external view returns (uint256);
     function approve(address spender, uint256 amount) external returns (bool);
     function transferFrom(
         address from,
         address to,
         uint256 amount
     ) external returns (bool);
 }

逐一分析

函数:

  • totalSupply() :返回总共的代币数量。
  • balanceOf(address account) :返回 account 地址拥有的代币数量。
  • transfer(address to, uint256 amount) :将 amount 数量的代币发送给 to 地址,返回布尔值告知是否执行成功。触发 Transfer 事件。
  • allowance(address owner, address spender) :返回授权花费者 spender 通过 transferFrom 代表所有者花费的剩余代币数量。默认情况下为零。当 approvetransferFrom 被调用时,值将改变。
  • approve(address spender, uint256 amount) :授权 spender 可以花费 amount 数量的代币,返回布尔值告知是否执行成功。触发 Approval 事件。
  • transferFrom(address from, address to, uint256 amount) :将 amount 数量的代币从 from 地址发送到 to 地址,返回布尔值告知是否执行成功。触发 Transfer 事件。

事件(定义中的 indexed 便于查找过滤):

  • Transfer(address from, address to, uint256 value) :当代币被一个地址转移到另一个地址时触发。注意:转移的值可能是 0 。
  • Approval(address owner, address spender, uint256 value) :当代币所有者授权别人使用代币时触发,即调用 approve 方法。

元数据

一般除了上述必须实现的函数外,还有一些别的方法:

  • name() :返回代币名称
  • symbol() :返回代币符号
  • decimals() :返回代币小数点后位数

ERC20

来看下 ERC20 代币具体是怎么写的。

同样,OpenZepplin 提供了现成的合约代码:

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

这里贴一个GitHub源码链接 OpenZepplin ERC20

函数概览

constructor(name_, symbol_)
 name()
 symbol()
 decimals()
 totalSupply()
 balanceOf(account)
 transfer(to, amount)
 allowance(owner, spender)
 approve(spender, amount)
 transferFrom(from, to, amount)
 increaseAllowance(spender, addedValue)
 decreaseAllowance(spender, subtractedValue)
 _transfer(from, to, amount)
 _mint(account, amount)
 _burn(account, amount)
 _approve(owner, spender, amount)
 _spendAllowance(owner, spender, amount)
 _beforeTokenTransfer(from, to, amount)
 _afterTokenTransfer(from, to, amount)

事件(同 IERC20)

Transfer(from, to, value)
 Approval(owner, spender, value)

逐一分析

  • constructor(string name, string symbol) :设定代币的名称和符号。decimals 默认是 18 ,要修改成不同的值你应该重载它。这两个值是不变的,只在构造时赋值一次。
  • name() :返回代币的名称。
  • symbol() :返回代币的符号,通常是名称的缩写。
  • decimals() :返回小数点后位数,通常是 18 ,模仿 Ether 和 wei 。要更改就重写它。

totalSupply()、balanceOf(address account)、transfer(address to, uint256 amount)、 allowance(address owner, address spender)、approve(address spender, uint256 amount)、transferFrom(address from, address to, uint256 amount) 都参考 IERC20 。

  • increaseAllowance(address spender, uint256 addedValue) :以原子的方式增加 spender 额度。返回布尔值告知是否执行成功,触发 Approval 事件。
  • _transfer(address from, address to, uint256 amount) :转账。这个内部函数相当于 transfer ,可以用于例如实施自动代币费用,削减机制等。触发 Transfer 事件。
  • _mint(address account, uint256 amount) :铸造 amount 数量的代币给 account 地址,增加总发行量。触发 Transfer 事件,其中参数 from 是零地址。
  • _burn(address account, uint256 amount) :从 account 地址中烧毁 amount 数量的代币,减少总发行量。触发 Transfer 事件,其中参数 to 是零地址。
  • _approve(address owner, uint256 spender, uint256 amount) :设定允许 spender 花费 owner 的代币数量。这个内部函数相当于 approve ,可以用于例如为某些子系统设置自动限额等。
  • spendAllowance(address owner, address spender, uint256 amount) :花费 amount 数量的 owner 授权 spender 的代币。在无限 allowance 的情况下不更新 allowance 金额。如果没有足够的余量,则恢复。可能触发 Approval 事件。
  • _beforeTokenTransfer(address from, address to, uint256 amount) :在任何代币转账前的 Hook 。它包括铸币和烧毁。调用条件:
    • fromto 都不是零地址时,from 手里 amount 数量的代币将发送给 to
    • from 是零地址时,将给 to 铸造 amount 数量的代币。
    • to 是零地址时,from 手里 amount 数量的代币将被烧毁。
    • fromto 不能同时为零地址。
  • _afterTokenTransfer(address from, address to, uint256 amount) :在任何代币转账后的 Hook 。它包括铸币和烧毁。调用条件:
    • fromto 都不是零地址时,from 手里 amount 数量的代币将发送给 to
    • from 是零地址时,将给 to 铸造 amount 数量的代币。
    • to 是零地址时,from 手里 amount 数量的代币将被烧毁。
    • fromto 不能同时为零地址。

小结

ERC20 代码中的 _transfer_mint_burn_approve_spendAllowance_beforeTokenTransfer_afterTokenTransfer 都是 internal 函数(其余为 public ),也就是说它们只能被派生合约调用。

从零开始,自己动手

1.编写IERC20

IERC20.sol

// SPDX-License-Identifier: MIT
 ​
 pragma solidity ^0.8.0;
 ​
 interface IERC20 {
     /// @dev 总发行量
     function totoalSupply() external view returns (uint256);
     /// @dev 查看地址余额
     function balanceOf(address account) external view returns (uint256);
     /// @dev 单地址转账
     function transfer(address account, uint256 amount) external returns (bool);
     /// @dev 查看被授权人代表所有者花费的代币余额
     function allowance(address owner, address spender) external view returns (uint256);
     /// @dev 授权别人花费你拥有的代币
     function approve(address spender, uint256 amount) external returns (bool);
     /// @dev 双地址转账
     function transferFrom(
         address from,
         address to,
         uint256 amount
     ) external returns (bool);
 ​
     /// @dev 发生代币转移时触发
     event Transfer(address indexed from, address indexed to, uint256 value);
     /// @dev 授权时触发
     event Approval(address indexed owner, address indexed spender, uint256 value);
 }

2.加上Metadata

IERC20Metadata.sol

// SPDX-License-Identifier: MIT
 ​
 pragma solidity ^0.8.0;
 ​
 import "IERC20.sol";
 ​
 interface IERC20Metadata is IERC20 {
     /// @dev 代币名称
     function name() external view returns (string memory);
     /// @dev 代币符号
     function symbol() external view returns (string memory);
     /// @dev 小数点后位数
     function decimals() external view returns (uint8);
 }

3.编写ERC20

ERC20.sol

// SPDX-License-Identifier: MIT
 ​
 pragma solidity ^0.8.0;
 ​
 import "./IERC20.sol";
 import "./IERC20Metadata.sol";
 ​
 contract ERC20 is IERC20, IERC30Metadata {
     // 地址余额
     mapping(address => uint256) private _balances;
     // 授权地址余额
     mapping(address => mapping(address => uint256)) private _allowances;
 ​
     uint256 private _totalSupply;
 ​
     string private _name;
     string private _symbol;
 ​
     /// @dev 设定代币名称符号
     constructor(string memory name_, string memory symbol_) {
         _name = name_;
         _symbol = symbol_;
     }
 ​
     function name() public view virtual override returns (string memory) {
         return _name;
     }
 ​
     function symbol() public view virtual override returns (string memory) {
         return _symbol;
     }
 ​
     /// @dev 小数点位数一般为 18
     function decimals() public view virtual override returns (uint8) {
         return 18;
     }
 ​
     function totalSupply() public view virtual  override returns (uint256) {
         return _totalSupply;
     }
 ​
     function balanceOf(address account) public view virtual override returns (uint256) {
         return _balances[account];
     }
 ​
     function transfer(address to, uint256 amount) public virtual override returns (bool) {
         address owner = msg.sender;
         _transfer(owner, to, amount);
         return true;
     }
 ​
     function allowance(address owner, address spender) public view virtual override returns (uint256) {
         return _allowances[owner][spender];
     }
 ​
     function approve(address spender, uint256 amount) public virtual override returns (bool) {
         address owner = msg.sender;
         _approve(owner, spender, amount);
         return true;
     }
 ​
     function transferFrom(
         address from,
         address to,
         uint256 amount
     ) public virtual override returns (bool) {
         address spender = msg.sender;
         _spendAllowance(from, spender, amount);
         _transfer(from, to, amount);
         return true;
     }
 ​
     function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
         address owner = msg.sender;
         _approve(owner, spender, _allowances[owner][spender] + addedValue);
         return true;
     }
 ​
     function decreaseAllowance(address spender, uint256 substractedValue) public virtual returns (bool) {
         address owner = msg.sender;
         uint256 currentAllowance = _allowances[owner][spender];
         require(currentAllowance >= substractedValue, "ERC20: decreased allowance below zero");
         unchecked {
             _approval(owner, spender, currentAllowance - substractedValue);
         }
         return true;
     }
 ​
     function _transfer(
         address from,
         address to,
         uint256 amount
     ) internal virtual {
         require(from != address(0), "ERC20: transfer from the zero address");
         require(to != address(0), "ERC20: transfer to the zero address");
 ​
         _beforeTokenTransfer(from, to, amount);
 ​
         uint256 fromBalance = _balances[from];
         require(fromBalance >= amount, "ERC20: transfer amount exceeds balance");
         unchecked {
             _balances[from] = fromBalance - amount;
         }
         _balances[to] += amount;
 ​
         emit Transfer(from, to, amount);
 ​
         _afterTokenTransfer(from, to, amount);
     }
 ​
     function _mint(address account, uint256 amount) internal virtual {
         require(account != address(0), "ERC20: mint to the zero address");
 ​
         _beforeTokenTransfer(address(0), account, amount);
 ​
         _totalSupply += amount;
         _balances[account] += amount;
         emit Transfer(address(0), account, amount);
 ​
         _afterTokenTransfer(address(0), account, amount);
     }
 ​
     function _burn(address account, uint256 amount) internal virtual {
         require(account != address(0), "ERC20: burn from the zero address");
 ​
         _beforeTokenTransfer(account, address(0), amount);
 ​
         uint256 accountBalance = _balances[account];
         require(accountBalance >= amount, "ERC20: burn amount exceeds balance");
         unchecked {
             _balances[account] = accountBalance - amount;
         }
         _totalSupply -= amount;
 ​
         emit Transfer(account, address(0), amount);
 ​
         _afterTokenTransfer(account, address(0), amount);
     }
 ​
     function _approve(
         address owner,
         address spender,
         uint256 amount
     ) internal virtual {
         require(owner != address(0), "ERC20: approve from the zero address");
         require(spender != address(0), "ERC20: approve  to the zero address");
 ​
         _allowances[owner][spender];
         emit Approval(owner, spender, amount);
     }
 ​
     function _spendAllowance(
         address owner,
         address spender,
         uint256 amount
     ) internal virtual {
         uint256 currentAllowance = allowance(owner, spender);
         if (currentAllowance != type(uint256).max) {
             require(currentAllowance >= amount, "ERC20: insufficient allowance");
             unchecked {
                 _approve(owner, spender, currentAllowance - amount);
             }
         }
     }
 ​
     function _beforeTokenTransfer(
         address from,
         address to,
         uint256 amount
     ) internal virtual {}
 ​
     function _afterTokenTransfer(
         address from,
         address to,
         uint256 amount
     ) internal virtual {}
 }

总结

ERC20 其实就是一种最常见的代币标准,它明确了同质化代币的经典功能并规范了开发者编写 token 时的代码,从而方便各种应用适配。

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