区块链学堂（29）：Modifiers-蒲公英云

区块链学堂（29）：Modifiers

Modifiers can be used to easily change the behaviour of functions, for example to automatically check a condition prior to executing the function. They are inheritable properties of contracts and may be overridden by derived contracts. 引用自here

一个使用modifier的例子

pragma solidity 0.4.10;
contract demo {
    uint public deadline;
    function demo() {
        deadline = now;
        deadline += 10;
    }
    modifier afterDeadline {
        if (now <= deadline) throw;
        _;
    }
    function f() afterDeadline returns (string s) {
        return "afterDeadline";
    }
    function g() afterDeadline returns (uint n) {
        n = 100;
    }
}

从上面的例子中可以看到，我们先在构造函数中，给deadline赋予一个值。在当前时间+10s
然后modifier afterDeadline, 必须要当前时间过了10s，才不抛出异常
然后我们可以在很多方法中使用modifier afterDeadline(). 从而使得代码非常简洁和美观。

另一个例子，给上一节中的代币合约中添加Modifier

Step 1: 首先定义一个modifier

modifier onlyMinter {
    if (msg.sender != minter) throw;
    _;
  }

Step 2: 新增一个新的 function mint2()

function changeMinter(address _to) onlyMinter {
```
  minter = _to;
}
```

对比一下之前的function mint()

function mint(address receiver, uint amount) {
        if (msg.sender != minter) throw;
        balances[receiver] += amount;
    }

很明显可以看到，通过modifier，在执行主函数之前，先进行一个条件检查。通过modifier,可以使代码变得很美。

Step 3: 添加个function changeMinter()

function changeMinter(address _to) onlyMinter {
    minter = _to;
  }

上一节代码加上Step 1-3后的完整代码如下：

pragma solidity 0.4.7;
contract Coin {
    address public minter;
    mapping (address => uint) public balances;
    event Sent(address from, address to, uint amount);
    function Coin() {
        minter = msg.sender;
    }
    function mint(address receiver, uint amount) {
        if (msg.sender != minter) throw;
        balances[receiver] += amount;
    }
    function send(address receiver, uint amount) {
        if (balances[msg.sender] < amount) return;
        balances[msg.sender] -= amount;
        balances[receiver] += amount;
        Sent(msg.sender, receiver, amount);
    }
    //对挖矿使用onlyOwner
    modifier onlyMinter {
      if (msg.sender != minter) throw;
      _;
    }
    function mint2(address receiver, uint amount) onlyMinter {
      balances[receiver] += amount;
    }
    function changeMinter(address _to) onlyMinter {
      minter = _to;
    }
}