OpenZeppelin ERC721源码分析

ERC721 官方简介是：A standard interface for non-fungible tokens, also known as deeds.也叫非同质代币，或者不可置换代币（NFTs）。提到ERC721，一个好理解的例子就是CryptoKitties 迷恋猫，每一只猫都是独一无二的拥有不同基因，有收藏价值属性。ERC721对于虚拟资产收藏品领域会有很好的应用价值和市场需求。

它和我写的上一篇《OpenZeppelin ERC20源码分析》介绍的ERC20有所不同，ERC721最小的单位为1无法再分割，代表独一无二的，针对不可置换的Token的智能合约标准接口。从 ERC721标准草案中可以看到，兼容ERC20的方法有4个：name，symbol，totalSupply，balanceOf 添加的新方法：ownerOf，takeOwnership ERC721还重写了approve和transfer。

分析OpenZeppelin ERC721源码前同样我画了一个继承和调用关系的思维导图，可以帮助更容易地看源码。

ERC721Basic.sol

pragma solidity ^0.4.23;

/**
 * @title ERC721 标准的基本接口
 * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721Basic {
  event Transfer(
    address indexed _from,
    address indexed _to,
    uint256 _tokenId
  );
  event Approval(
    address indexed _owner,
    address indexed _approved,
    uint256 _tokenId
  );
  event ApprovalForAll(
    address indexed _owner,
    address indexed _operator,
    bool _approved
  );

  function balanceOf(address _owner) public view returns (uint256 _balance);
  function ownerOf(uint256 _tokenId) public view returns (address _owner);
  function exists(uint256 _tokenId) public view returns (bool _exists);

  function approve(address _to, uint256 _tokenId) public;
  function getApproved(uint256 _tokenId)
    public view returns (address _operator);

  function setApprovalForAll(address _operator, bool _approved) public;
  function isApprovedForAll(address _owner, address _operator)
    public view returns (bool);

  function transferFrom(address _from, address _to, uint256 _tokenId) public;
  function safeTransferFrom(address _from, address _to, uint256 _tokenId)
    public;

  function safeTransferFrom(
    address _from,
    address _to,
    uint256 _tokenId,
    bytes _data
  )
    public;
}

ERC721Basic 合约定义了基本的接口方法：

• balanceOf 返回_owner的代币数量
• ownerOf 根据_tokenId返回代币持有者address
• exists _tokenId是否存在
• approve 授权_tokenId给地址to
• getApproved 查询_tokenId的授权人_operator address
• setApprovalForAll 授权_operator具有所有代币的控制权
• isApprovedForAll
• transferFrom 转移代币所有权
• safeTransferFrom 转移代币所有权

同时还定义了Transfer Approval ApprovalForAll 在后面的ERC721实现的代码中再来看事件的触发。

ERC721.sol

pragma solidity ^0.4.23;

import "./ERC721Basic.sol";

/**
 * @title ERC-721 标准的基本接口, 可选的枚举扩展
 * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721Enumerable is ERC721Basic {
  function totalSupply() public view returns (uint256);
  function tokenOfOwnerByIndex(
    address _owner,
    uint256 _index
  )
    public
    view
    returns (uint256 _tokenId);

  function tokenByIndex(uint256 _index) public view returns (uint256);
}


/**
 * @title ERC-721 ERC-721 标准的基本接口, 可选的元数据扩展
 * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721Metadata is ERC721Basic {
  function name() public view returns (string _name);
  function symbol() public view returns (string _symbol);
  function tokenURI(uint256 _tokenId) public view returns (string);
}

/**
 * @title ERC-721 标准的基本接口,完整实现接口
 * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721 is ERC721Basic, ERC721Enumerable, ERC721Metadata {
}

ERC721 合约继承了 ERC721Basic 的基础上，添加枚举和元数据的扩展。

ERC721Enumerable枚举扩展可以使代币更具有可访问性:

• totalSupply 返回代币总量
• tokenOfOwnerByIndex 通过_owner所有者地址和索引值返回所有者代币列表中的_tokenId
• tokenByIndex 通过索引值返回tokenId

ERC721Metadata元数据扩展哦用来描述合约元信息

• name 返回合约名字
• symbol 返回代币符号
• tokenURI 返回_tokenId对应的资源URI

ERC721BasicToken

pragma solidity ^0.4.23;

import "./ERC721Basic.sol";
import "./ERC721Receiver.sol";
import "../../math/SafeMath.sol";
import "../../AddressUtils.sol";

/**
 * @title ERC721 标准基本实现
 * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721BasicToken is ERC721Basic {
  using SafeMath for uint256;
  using AddressUtils for address;

  // Equals to `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`
  // which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
  bytes4 constant ERC721_RECEIVED = 0xf0b9e5ba;

  // token ID 到 持有人owner的映射
  mapping (uint256 => address) internal tokenOwner;

  // token ID 到授权地址address的映射
  mapping (uint256 => address) internal tokenApprovals;

  // 持有人到持有的token数量的映射
  mapping (address => uint256) internal ownedTokensCount;

  // 持有人到操作人授权的映射
  mapping (address => mapping (address => bool)) internal operatorApprovals;

  /**
   * @dev 确保msg.sender是tokenId的持有人
   * @param _tokenId uint256 ID of the token to validate its ownership belongs to msg.sender
   */
  modifier onlyOwnerOf(uint256 _tokenId) {
    require(ownerOf(_tokenId) == msg.sender);
    _;
  }

  /**
   * @dev 通过检查msg.sender是否是代币的持有人，被授权或者操作人来确保msg.sender可以交易一个token
   * @param _tokenId uint256 ID of the token to validate
   */
  modifier canTransfer(uint256 _tokenId) {
    require(isApprovedOrOwner(msg.sender, _tokenId));
    _;
  }

  /**
   * @dev 获取持有者的代币总数
   * @param _owner address to query the balance of
   * @return uint256 representing the amount owned by the passed address
   */
  function balanceOf(address _owner) public view returns (uint256) {
    require(_owner != address(0));
    return ownedTokensCount[_owner];
  }

  /**
   * @dev 根据token ID获取持有者
   * @param _tokenId uint256 ID of the token to query the owner of
   * @return owner address currently marked as the owner of the given token ID
   */
  function ownerOf(uint256 _tokenId) public view returns (address) {
    address owner = tokenOwner[_tokenId];
    require(owner != address(0));
    return owner;
  }

  /**
   * @dev 指定的token是否存在
   * @param _tokenId uint256 ID of the token to query the existence of
   * @return whether the token exists
   */
  function exists(uint256 _tokenId) public view returns (bool) {
    address owner = tokenOwner[_tokenId];
    return owner != address(0);
  }

  /**
   * @dev 批准另一个人address来交易指定的代币
   * @dev 0 address 表示没有授权的地址
   * @dev 给定的时间内，一个token只能有一个批准的地址
   * @dev 只有token的持有者或者授权的操作人才可以调用
   * @param _to address to be approved for the given token ID
   * @param _tokenId uint256 ID of the token to be approved
   */
  function approve(address _to, uint256 _tokenId) public {
    address owner = ownerOf(_tokenId);
    require(_to != owner);
    require(msg.sender == owner || isApprovedForAll(owner, msg.sender));

    if (getApproved(_tokenId) != address(0) || _to != address(0)) {
      tokenApprovals[_tokenId] = _to;
      emit Approval(owner, _to, _tokenId);
    }
  }

  /**
   * @dev 获取token被授权的地址，如果没有设置地址则为0
   * @param _tokenId uint256 ID of the token to query the approval of
   * @return address currently approved for the given token ID
   */
  function getApproved(uint256 _tokenId) public view returns (address) {
    return tokenApprovals[_tokenId];
  }

  /**
   * @dev 设置或者取消对操作人的授权
   * @dev 一个操作人可以代表他们转让发送者的所有token
   * @param _to operator address to set the approval
   * @param _approved representing the status of the approval to be set
   */
  function setApprovalForAll(address _to, bool _approved) public {
    require(_to != msg.sender);
    operatorApprovals[msg.sender][_to] = _approved;
    emit ApprovalForAll(msg.sender, _to, _approved);
  }

  /**
   * @dev 查询是否操作人被指定的持有者授权
   * @param _owner 要查询的授权人地址
   * @param _operator 要查询的授权操作人地址
   * @return bool whether the given operator is approved by the given owner
   */
  function isApprovedForAll(
    address _owner,
    address _operator
  )
    public
    view
    returns (bool)
  {
    return operatorApprovals[_owner][_operator];
  }

  /**
   * @dev 将指定的token所有权转移给另外一个地址
   * @dev 不鼓励使用这个方法，尽量使用`safeTransferFrom` 
   * @dev 要求 msg.sender 必须为所有者，已授权或者操作人
   * @param _from current owner of the token
   * @param _to address to receive the ownership of the given token ID
   * @param _tokenId uint256 ID of the token to be transferred
  */
  function transferFrom(
    address _from,
    address _to,
    uint256 _tokenId
  )
    public
    canTransfer(_tokenId)
  {
    require(_from != address(0));
    require(_to != address(0));

    clearApproval(_from, _tokenId);
    removeTokenFrom(_from, _tokenId);
    addTokenTo(_to, _tokenId);

    emit Transfer(_from, _to, _tokenId);
  }

  /**
   * @dev 更安全的方法，将指定的token所有权转移给另外一个地址
   * @dev 如果目标地址是一个合约，必须实现 `onERC721Received`,这个要求安全交易并返回值
`bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`; 否则交易被还原
   * @dev 要求 msg.sender 必须为所有者，已授权或者操作人
   * @param _from current owner of the token
   * @param _to address to receive the ownership of the given token ID
   * @param _tokenId uint256 ID of the token to be transferred
  */
  function safeTransferFrom(
    address _from,
    address _to,
    uint256 _tokenId
  )
    public
    canTransfer(_tokenId)
  {
    safeTransferFrom(_from, _to, _tokenId, "");
  }

   /**
   * @dev 更安全的方法，将指定的token所有权转移给另外一个地址
   * @dev 如果目标地址是一个合约，必须实现 `onERC721Received`,这个要求安全交易并返回值
`bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`; 否则交易被还原
   * @dev 要求 msg.sender 必须为所有者，已授权或者操作人
   * @param _from current owner of the token
   * @param _to address to receive the ownership of the given token ID
   * @param _tokenId uint256 ID of the token to be transferred
   * @param _data bytes data to send along with a safe transfer check
   */
  function safeTransferFrom(
    address _from,
    address _to,
    uint256 _tokenId,
    bytes _data
  )
    public
    canTransfer(_tokenId)
  {
    transferFrom(_from, _to, _tokenId);
    require(checkAndCallSafeTransfer(_from, _to, _tokenId, _data));
  }

  /**
   * @dev 返回给定的spender是否可以交易一个给定的token
   * @param _spender address of the spender to query
   * @param _tokenId uint256 ID of the token to be transferred
   * @return bool whether the msg.sender is approved for the given token ID,
   *  is an operator of the owner, or is the owner of the token
   */
  function isApprovedOrOwner(
    address _spender,
    uint256 _tokenId
  )
    internal
    view
    returns (bool)
  {
    address owner = ownerOf(_tokenId);
    return (
      _spender == owner ||
      getApproved(_tokenId) == _spender ||
      isApprovedForAll(owner, _spender)
    );
  }

  /**
   * @dev 增发一个新token的内部方法
   * @dev 如果增发的token已经存在则撤销
   * @param _to The address that will own the minted token
   * @param _tokenId uint256 ID of the token to be minted by the msg.sender
   */
  function _mint(address _to, uint256 _tokenId) internal {
    require(_to != address(0));
    addTokenTo(_to, _tokenId);
    emit Transfer(address(0), _to, _tokenId);
  }

  /**
   * @dev 销毁一个token的内部方法
   * @dev 如果token不存在则撤销
   * @param _tokenId uint256 ID of the token being burned by the msg.sender
   */
  function _burn(address _owner, uint256 _tokenId) internal {
    clearApproval(_owner, _tokenId);
    removeTokenFrom(_owner, _tokenId);
    emit Transfer(_owner, address(0), _tokenId);
  }

  /**
   * @dev 清除当前的给定token的授权，内部方法
   * @dev 如果给定地址不是token的持有者则撤销
   * @param _owner owner of the token
   * @param _tokenId uint256 ID of the token to be transferred
   */
  function clearApproval(address _owner, uint256 _tokenId) internal {
    require(ownerOf(_tokenId) == _owner);
    if (tokenApprovals[_tokenId] != address(0)) {
      tokenApprovals[_tokenId] = address(0);
      emit Approval(_owner, address(0), _tokenId);
    }
  }

  /**
   * @dev 内部方法，将给定的token添加到给定地址列表中
   * @param _to address 指定token的新所有者
   * @param _tokenId uint256 ID of the token to be added to the tokens list of the given address
   */
  function addTokenTo(address _to, uint256 _tokenId) internal {
    require(tokenOwner[_tokenId] == address(0));
    tokenOwner[_tokenId] = _to;
    ownedTokensCount[_to] = ownedTokensCount[_to].add(1);
  }

  /**
   * @dev 内部方法，将给定的token从地址列表中移除
   * @param _from address 给定token的之前持有中地址
   * @param _tokenId uint256 ID of the token to be removed from the tokens list of the given address
   */
  function removeTokenFrom(address _from, uint256 _tokenId) internal {
    require(ownerOf(_tokenId) == _from);
    ownedTokensCount[_from] = ownedTokensCount[_from].sub(1);
    tokenOwner[_tokenId] = address(0);
  }

  /**
   * @dev 内部函数，调用目标地址上的 `onERC721Received` 
   * @dev 如果目标地址不是合同则不执行调用
   * @param _from address representing the previous owner of the given token ID
   * @param _to target address that will receive the tokens
   * @param _tokenId uint256 ID of the token to be transferred
   * @param _data bytes optional data to send along with the call
   * @return whether the call correctly returned the expected magic value
   */
  function checkAndCallSafeTransfer(
    address _from,
    address _to,
    uint256 _tokenId,
    bytes _data
  )
    internal
    returns (bool)
  {
    if (!_to.isContract()) {
      return true;
    }
    bytes4 retval = ERC721Receiver(_to).onERC721Received(
      _from, _tokenId, _data);
    return (retval == ERC721_RECEIVED);
  }
}

ERC721BasicToken 实现了ERC721Basic合约定义的接口方法，主要对token的持有人的一个添加和修改，以及授权和交易的管理，实现了基本的非同质化token的业务逻辑。具体方法实现并不难，就是对映射的公有变量的管理，但是对于权限和安全验证值得关注，比如函数修改器还有require。

ERC721Token.sol

pragma solidity ^0.4.23;

import "./ERC721.sol";
import "./ERC721BasicToken.sol";

/**
 * @title 完整 ERC721 Token
 * 该实现包括所有ERC721标准必须的和可选的方法，此外还包括使用操作者批准所有功能
 * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
 */
contract ERC721Token is ERC721, ERC721BasicToken {
  // 代币名称
  string internal name_;

  // 代币符号
  string internal symbol_;

  // 所有者到所有者拥有的代币列表的映射
  mapping(address => uint256[]) internal ownedTokens;

  // 所有者代币列表中代币ID到索引的映射
  mapping(uint256 => uint256) internal ownedTokensIndex;

  // 保存所有代币ID的数组，用于枚举
  uint256[] internal allTokens;

  // allTokens数组中代币ID到索引的映射
  mapping(uint256 => uint256) internal allTokensIndex;

  // 可选的代币资源URIs映射
  mapping(uint256 => string) internal tokenURIs;

  /**
   * @dev Constructor function
   */
  constructor(string _name, string _symbol) public {
    name_ = _name;
    symbol_ = _symbol;
  }

  /**
   * @dev 获取代币名称
   * @return string representing the token name
   */
  function name() public view returns (string) {
    return name_;
  }

  /**
   * @dev 获取代币符号
   * @return string representing the token symbol
   */
  function symbol() public view returns (string) {
    return symbol_;
  }

  /**
   * @dev 根据_tokenId返回对应的资源URI
   * @dev 如果token不存在异常返回空字符串
   * @param _tokenId uint256 ID of the token to query
   */
  function tokenURI(uint256 _tokenId) public view returns (string) {
    require(exists(_tokenId));
    return tokenURIs[_tokenId];
  }

  /**
   * @dev 获取token id 通过给定的token列表中的索引
   * @param _owner address owning the tokens list to be accessed
   * @param _index uint256 representing the index to be accessed of the requested tokens list
   * @return uint256 token ID at the given index of the tokens list owned by the requested address
   */
  function tokenOfOwnerByIndex(
    address _owner,
    uint256 _index
  )
    public
    view
    returns (uint256)
  {
    require(_index < balanceOf(_owner));
    return ownedTokens[_owner][_index];
  }

  /**
   * @dev 获取合约存储的token总数
   * @return uint256 representing the total amount of tokens
   */
  function totalSupply() public view returns (uint256) {
    return allTokens.length;
  }

  /**
   * @dev 根据token 索引值获取合约中token的
   * @dev 如果索引大于等于token总数则撤销
   * @param _index uint256 representing the index to be accessed of the tokens list
   * @return uint256 token ID at the given index of the tokens list
   */
  function tokenByIndex(uint256 _index) public view returns (uint256) {
    require(_index < totalSupply());
    return allTokens[_index];
  }

  /**
   * @dev 内部方法，给存在token添加token URI
   * @dev Reverts if the token ID does not exist
   * @param _tokenId uint256 ID of the token to set its URI
   * @param _uri string URI to assign
   */
  function _setTokenURI(uint256 _tokenId, string _uri) internal {
    require(exists(_tokenId));
    tokenURIs[_tokenId] = _uri;
  }

  /**
   * @dev 内部方法，添加token ID 到给定的地址的列表中
   * @param _to address 给定token ID的新的持有者
   * @param _tokenId uint256 ID of the token to be added to the tokens list of the given address
   */
  function addTokenTo(address _to, uint256 _tokenId) internal {
    // 调用父合约的addTokenTo
    super.addTokenTo(_to, _tokenId);
    uint256 length = ownedTokens[_to].length;
    ownedTokens[_to].push(_tokenId);
    //当前的长度作为索引
    ownedTokensIndex[_tokenId] = length;
  }

  /**
   * @dev 内部方法，从一个给定地址的列表中移除token 
   * @param _from address 给定token ID的之前的持有者address  
   * @param _tokenId uint256 ID of the token to be removed from the tokens list of the given address
   */
  function removeTokenFrom(address _from, uint256 _tokenId) internal {
    // 调用父合约的移除方法
    super.removeTokenFrom(_from, _tokenId);
    // 获取token的索引
    uint256 tokenIndex = ownedTokensIndex[_tokenId];
    // 获取持有人token的最后一个token索引
    uint256 lastTokenIndex = ownedTokens[_from].length.sub(1);
    // 获取最后一个token
    uint256 lastToken = ownedTokens[_from][lastTokenIndex];
    //将最后一个token放到被删除的索引位置，lastTokenIndex置0
    ownedTokens[_from][tokenIndex] = lastToken;
    ownedTokens[_from][lastTokenIndex] = 0; 
    // 注意这里需要处理单元素数组，tokenIndex和lastTokenIndex都将置0.然后可以确保将ownedTokens列表中删除_tokenId,首先将lastToken换到第一个位置，然后删除列表最后位置的元素   
    ownedTokens[_from].length--;
    ownedTokensIndex[_tokenId] = 0;
    ownedTokensIndex[lastToken] = tokenIndex;
  }

  /**
   * @dev 内部方法，增发一个新的token
   * @dev 如果token已经存在了就撤销
   * @param _to address the beneficiary that will own the minted token
   * @param _tokenId uint256 ID of the token to be minted by the msg.sender
   */
  function _mint(address _to, uint256 _tokenId) internal {
    super._mint(_to, _tokenId);

    allTokensIndex[_tokenId] = allTokens.length;
    allTokens.push(_tokenId);
  }

  /**
   * @dev 内部方法，销毁一个指定的token
   * @dev token不存在则撤销
   * @param _owner owner of the token to burn
   * @param _tokenId uint256 ID of the token being burned by the msg.sender
   */
  function _burn(address _owner, uint256 _tokenId) internal {
    super._burn(_owner, _tokenId);

    // 清除资源URI
    if (bytes(tokenURIs[_tokenId]).length != 0) {
      delete tokenURIs[_tokenId];
    }

    // 做所有的token数组后续处理
    uint256 tokenIndex = allTokensIndex[_tokenId];
    uint256 lastTokenIndex = allTokens.length.sub(1);
    uint256 lastToken = allTokens[lastTokenIndex];
    // 可以参考增发removeTokenFrom
    allTokens[tokenIndex] = lastToken;
    allTokens[lastTokenIndex] = 0;

    allTokens.length--;
    allTokensIndex[_tokenId] = 0;
    allTokensIndex[lastToken] = tokenIndex;
  }

}

ERC721Token实现了完整的ERC721标准，在继承了ERC721BasicToken的基础上增加了一些token的操作，主要在包括token的元数据，资源URI，增发销毁，还有就是token索引的映射关系。对于具体实现我们根据实际情况通过继承ERC721BasicToken或者ERC721Token来添加自己的业务逻辑。

OpenZeppelin ERC721源码分析到这里就结束了。

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