函数修饰符(modifier)

函数修饰符用于修改函数的行为。例如，向函数添加条件限制。

contract Owner {

   // 定义修饰符 onlyOwner 不带参数
   modifier onlyOwner {
      require(msg.sender == owner);
      _;
   }

   // 定义修饰符 costs 带参数
   modifier costs(uint price) {
      if (msg.value >= price) {
         _;
      }
   }
}

修饰符定义中出现特殊符号_的地方，用于插入函数体。如果在调用此函数时，满足了修饰符的条件，则执行该函数，否则将抛出异常

pragma solidity ^0.5.0;

contract Owner {
   address owner;

   constructor() public {
      owner = msg.sender;
   }

   // 定义修饰符 onlyOwner 不带参数
   modifier onlyOwner {
      require(msg.sender == owner);
      _;
   }

   // 定义修饰符 costs 带参数
   modifier costs(uint price) {
      if (msg.value >= price) {
         _;
      }
   }
}

contract Register is Owner {
   mapping (address => bool) registeredAddresses;
   uint price;

   constructor(uint initialPrice) public { price = initialPrice; }

   // 使用修饰符 costs
   function register() public payable costs(price) {
      registeredAddresses[msg.sender] = true;
   }

   // 使用修饰符 onlyOwner
   function changePrice(uint _price) public onlyOwner {
      price = _price;
   }
}

限制调用者,时间,金额

pragma solidity ^0.5.0;

contract Test {
   address public owner = msg.sender;
   uint public creationTime = now;

   modifier onlyBy(address _account) {
      require(
         msg.sender == _account,
         "Sender not authorized."
      );
      _;
   }
   function changeOwner(address _newOwner) public onlyBy(owner) {
      owner = _newOwner;
   }
   modifier onlyAfter(uint _time) {
      require(
         now >= _time,
         "Function called too early."
      );
      _;
   }
   function disown() public onlyBy(owner) onlyAfter(creationTime + 6 weeks) {
      delete owner;
   }
   modifier costs(uint _amount) {
      require(
         msg.value >= _amount,
         "Not enough Ether provided."
      );
      _;
      if (msg.value > _amount)
         msg.sender.transfer(msg.value - _amount);
   }
   function forceOwnerChange(address _newOwner) public payable costs(200 ether) {
      owner = _newOwner;
      if (uint(owner) & 0 == 1) return;        
   }
}