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Blockchain Solidity Modifier的使用underscore

refer to:
https://ethereum.stackexchange.com/questions/5861/are-underscores-in-modifiers-code-or-are-they-just-meant-to-look-cool

modifier 就是一个function before hook,

1.  _ 下划线表示被hook的方法内容。

2.  _ 可以出现多次

3.  modifier的执行顺序和出现顺序是一致的(上面的stackexchange中的的例子, 把各种内容都放到 message中,是特别厉害的)

4. modifier 也可以出现多次

下面是一个例子:

pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
import "@openzeppelin/contracts/utils/Counters.sol";

contract VeryGoodNftWithMaxSupply is ERC721URIStorage {

  uint256 private _currentId = 0;
  address public rootAddress = address(0);
  uint256 public _maxSupply = 10;

  event Minted(address to, uint256 nftId, address minter);

  modifier shouldLessThanMaxSupply(){
    require(
      _currentId <= _maxSupply, "Reached max supply, no available nft left"
    );
    // 下划线表示被hook方法的内容。 会在编译、执行的时候做个替换
    _;
  }

  constructor() ERC721("VeryGoodNftWithMaxSupply", "VGNWM") {
    rootAddress = msg.sender;
  }

  function mint(address to) external shouldLessThanMaxSupply{
    uint256 nftId = _currentId + 1;
    _mint(to, nftId);
    _currentId = nftId;
    address from = msg.sender;
    emit Minted(to, nftId, from);
  }
}

一个function带有多个modifier:

   //  执行顺序就是 checkOne,  checkTwo, checkThree, 然后才是方法体
    function test() checkOne checkTwo checkThree returns (uint256) {
        messages.push("test - 1");
        testVariable = 345;
        messages.push("test - 2");
        return testVariable;
    }

可以把函数的参数传给modifier,

例如:

  // 这个modifier 有参数。
modifier shouldInWhiteList(address to){ string memory message = string.concat("This address( ", Strings.toString(to), " ) is not in white list"); require( whiteList.include(to), message); _; }
// 这里包含多个modifier , 注意把函数的参数to 传递给了modifier function mint(address to) external shouldLessThanMaxSupply shouldInWhiteList(to) { uint256 nftId = _currentId + 1; _mint(to, nftId); _currentId = nftId; address from = msg.sender; emit Minted(to, nftId, from); }

modifier 需要在function的returns前面,

例如:

// 这是个external 方法
function doSomeOperation(a,b) external modifier1 modifier2(b) returns(uint) {
   return 10;
}
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