以太坊：根据例子学习Solidity-库合约使用

库合约使用

通过在合约中引入模块化方法（），可以帮助我们减少溢出等风险。 在下面的合约例子中，引入 Balances 库 合约使用了 move 方法， 去检查地址余额是否符合预期。

现在大家只需了解下库的作用，后面的文档有 更多关于库的使用。

pragma solidity >=0.4.22 <0.7.0;

library Balances {
    function move(mapping(address => uint256) storage balances, address from, address to, uint amount) internal {
        require(balances[from] >= amount);
        require(balances[to] + amount >= balances[to]);
        balances[from] -= amount;
        balances[to] += amount;
    }
}

contract Token {
    mapping(address => uint256) balances;
    using Balances for *;   // 引入库
    mapping(address => mapping (address => uint256)) allowed;

    event Transfer(address from, address to, uint amount);
    event Approval(address owner, address spender, uint amount);

    function balanceOf(address tokenOwner) public view returns (uint balance) {
        return balances[tokenOwner];
    }
    function transfer(address to, uint amount) public returns (bool success) {
        balances.move(msg.sender, to, amount);
        emit Transfer(msg.sender, to, amount);
        return true;

    }

    function transferFrom(address from, address to, uint amount) public returns (bool success) {
        require(allowed[from][msg.sender] >= amount);
        allowed[from][msg.sender] -= amount;
        balances.move(from, to, amount);   // 使用了库方法
        emit Transfer(from, to, amount);
        return true;
    }

    function approve(address spender, uint tokens) public returns (bool success) {
        require(allowed[msg.sender][spender] == 0, "");
        allowed[msg.sender][spender] = tokens;
        emit Approval(msg.sender, spender, tokens);
        return true;
    }
}