304. 花別人的錢 + dry the code - ERC20 (ep.20)

1. 從第三方帳戶轉帳的函式

transferFrom(address from, address to, uint256 amount)

1. 呼叫者msg.sender 從代幣持有者from 轉帳給接收者amount數量的代幣
2. 需檢查呼叫者是否有足夠的額度可用
3. 轉帳時要檢查持有者是否有足夠的餘額
4. 轉帳時需要同時減去額度

2. Dry the code

1. 畫面更簡潔, 也更不容易犯錯

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.23;

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 allowance(address owner, address spender) external view returns (uint256);

    function approve(address spender, uint256 amount) external returns (bool);
    function transfer(address to, uint256 amount) external returns (bool);
    function transferFrom(address from, address to, uint256 amount) external returns (bool);
}

contract ERC20 is IERC20 { 
    uint256 _totalSupply;
    mapping(address => uint256) _balance;
    mapping(address => mapping(address => uint256)) _allowance; 
    
    constructor(){
        _balance[msg.sender] = 10000;
        _totalSupply = 10000;
    }

    function totalSupply() public view returns (uint256){
        return _totalSupply;
    }

    function balanceOf(address accmount) public view returns (uint256) {
        return _totalSupply;
    }

    function _transfer(address from, address to, uint256 amount) internal {
        uint256 myBalance = _balance[from];
        require(myBalance >= amount, "No money to transfer");
        require(to != address(0), "Transfer to address 0");

        _balance[from] = myBalance - amount;
        _balance[to] = _balance[to] + amount;
        emit Transfer(from, to, amount);
    }
    function transfer(address to, uint256 amount) public returns (bool){
        _transfer(msg.sender, to, amount);
        return true;
    }
    
    function allowance(address owner, address spender) public view returns (uint256){
        return _allowance[owner][spender];
        //_allowance => (mapping(address => mapping(address => uint256)))
        //_allowance[owner] => (mapping(address => uint256))
        //(_allowance[owner])[spender] => uint256
    }
    
    function _approve(address owner, address spender, uint256 amount) internal {
        _allowance[owner][spender] = amount;
        emit Approval(msg.sender, spender, amount);
    }
    function approve(address spender, uint256 amount) public returns (bool){
        _approve(msg.sender, spender, amount);
        return true;
    }
    
    function transferFrom(address from, address to, uint256 amount) public returns (bool){
        uint256 myAllowance = _allowance[from][msg.sender];
        require(myAllowance >= amount, "ERROR: myAllowance < amount");
        _approve(from, msg.sender, myAllowance - amount); 
        _transfer(from, to, amount);
        return true; 
    }
}