委托转账原理分析：

假设：【A账号】有10000个token代币，【B账号】没有token代币，【C账号】也没有token代币

那么：【A账号】委托【B账号】转给【C账号】100个token代币，怎么来实现呢？

首先：【A账号】和【B账号】建立一种委托关联，登录【A账户】执行 approve(B, 100) 方法。

结果：_allowances[A][B] = 100token

然后：登录【B账户】执行 transferFrom(A, C, 100)，这里的B就是委托账号发送者，gas从B扣,必须确保token数量小于_allowances[A][B]

总结：其实就是A转入C，但是要经过B的账号来发送交易

contract ERC20 {
    using SafeMath for uint256;
    mapping (address => uint256) internal _balances;
    mapping (address => mapping (address => uint256)) internal _allowances;
 
    // 委托转账，执行这个方法之前必须先执行approve建立委托关联数据加入_allowances中
    function transferFrom(address from, address to, uint256 amount) public returns (bool) {
        _transfer(from, to, amount);

        uint256 currentAllowance = _allowances[from][msg.sender];
        require(currentAllowance >= amount, "实际转账数量超过了委托数量");
        _approve(from, msg.sender, currentAllowance - amount);
        return true;
    }

    // 建立一种委托关联(这个方法也就是在maping对象_allowances中)
    // 添加【需要发送token的人】和委托者【实际转账的人】之间的关系(后续的委托操作会查询这个maping中的数据记录)
    function approve(address spender, uint256 value) public returns (bool) {
        _approve(msg.sender, spender, value);
        return true;
    }
 
    // 委托关联执行的逻辑
    function _approve(address owner, address spender, uint256 value) internal {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = value;
        emit Approval(owner, spender, value);
    }
 
    // 转账交易执行的逻辑
    function _transfer(address from, address to, uint256 value) internal {
        require(from != address(0), "ERC20: transfer from the zero address");
        require(to != address(0), "ERC20: transfer to the zero address");
        uint256 senderBalance = _balances[from];
        require(senderBalance >= amount, "ERC20: transfer amount exceeds balance");
        _balances[from] = senderBalance - value;
        _balances[to] += value;
        emit Transfer(from, to, value);
    }
}