# Solidity极简入门: 34. ERC721

**Published by:** [0xAA](https://paragraph.com/@wtfacademy/)
**Published on:** 2022-07-03
**URL:** https://paragraph.com/@wtfacademy/solidity-34-erc721

## Content

我最近在重新学solidity，巩固一下细节，也写一个“Solidity极简入门”，供小白们使用（编程大佬可以另找教程），每周更新1-3讲。 欢迎关注我的推特：@0xAA_Science 欢迎加入WTF科学家社区，内有加微信群方法：链接 所有代码和教程开源在github（1024个star发课程认证，2048个star发社群NFT）: github.com/AmazingAng/WTFSolidityBTC和ETH这类代币都属于同质化代币，矿工挖出的第1枚BTC与第10000枚BTC并没有不同，是等价的。但世界中很多物品是不同质的，其中包括房产、古董、虚拟艺术品等等，这类物品无法用同质化代币抽象。因此，以太坊EIP721提出了ERC721标准，来抽象非同质化的物品。这一讲，我们将介绍ERC721标准，并基于它发行一款NFT。ERC165通过ERC165标准，智能合约可以声明它支持的接口，供其他合约检查。IERC165接口合约只声明了一个supportsInterface函数，输入要查询的interfaceId接口id，若合约实现了该接口id，则返回true：interface IERC165 { /** * @dev 如果合约实现了查询的`interfaceId`，则返回true * 规则详见：https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified[EIP section] * */ function supportsInterface(bytes4 interfaceId) external view returns (bool); } 我们可以看下ERC721是如何实现supportsInterface()函数的： function supportsInterface(bytes4 interfaceId) external pure override returns (bool) { return interfaceId == type(IERC721).interfaceId || interfaceId == type(IERC165).interfaceId; } 当查询的是IERC721或IERC165的接口id时，返回true；反之返回false。IERC721IERC721是ERC721标准的接口合约，规定了ERC721要实现的基本函数。它利用tokenId来表示特定的非同质化代币，授权或转账都要明确tokenId；而ERC20只需要明确转账的数额即可。/** * @dev ERC721标准接口. */ interface IERC721 is IERC165 { event Transfer(address indexed from, address indexed to, uint256 indexed tokenId); event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId); event ApprovalForAll(address indexed owner, address indexed operator, bool approved); function balanceOf(address owner) external view returns (uint256 balance); function ownerOf(uint256 tokenId) external view returns (address owner); function safeTransferFrom( address from, address to, uint256 tokenId, bytes calldata data ) external; function safeTransferFrom( address from, address to, uint256 tokenId ) external; function transferFrom( address from, address to, uint256 tokenId ) external; function approve(address to, uint256 tokenId) external; function setApprovalForAll(address operator, bool _approved) external; function getApproved(uint256 tokenId) external view returns (address operator); function isApprovedForAll(address owner, address operator) external view returns (bool); } IERC721事件IERC721包含3个事件，其中Transfer和Approval事件在ERC20中也有。Transfer事件：在转账时被释放，记录代币的发出地址from，接收地址to和tokenid。Approval事件：在授权时释放，记录授权地址owner，被授权地址approved和tokenid`。ApprovalForAll事件：在批量授权时释放，记录批量授权的发出地址owner，被授权地址operator和授权与否的approved。IERC721函数balanceOf：返回某地址的NFT持有量balance。ownerOf：返回某tokenId的主人owner。transferFrom：普通转账，参数为转出地址from，接收地址to和tokenId。safeTransferFrom：安全转账（如果接收方是合约地址，会要求实现ERC721Receiver接口）。参数为转出地址from，接收地址to和tokenId。approve：授权另一个地址使用你的NFT。参数为被授权地址approve和tokenId。getApproved：查询tokenId被批准给了哪个地址。setApprovalForAll：将自己持有的该系列NFT批量授权给某个地址operator。isApprovedForAll：查询某地址的NFT是否批量授权给了另一个operator地址。safeTransferFrom：安全转账的重载函数，参数里面包含了data。IERC721Receiver如果一个合约没有实现ERC721的相关函数，转入的NFT就进了黑洞，永远转不出来了。为了防止误转账，ERC721实现了safeTransferFrom()安全转账函数，目标合约必须实现了IERC721Receiver接口才能接收ERC721代币，不然会revert。IERC721Receiver接口只包含一个onERC721Received()函数。// ERC721接收者接口：合约必须实现这个接口来通过安全转账接收ERC721 interface IERC721Receiver { function onERC721Received( address operator, address from, uint tokenId, bytes calldata data ) external returns (bytes4); } 我们看下ERC721利用_checkOnERC721Received来确保目标合约实现了onERC721Received()函数（返回onERC721Received的selector）： function _checkOnERC721Received( address from, address to, uint tokenId, bytes memory _data ) private returns (bool) { if (to.isContract()) { return IERC721Receiver(to).onERC721Received( msg.sender, from, tokenId, _data ) == IERC721Receiver.onERC721Received.selector; } else { return true; } } IERC721MetadataIERC721Metadata是ERC721的拓展接口，实现了3个查询metadata元数据的常用函数：name()：返回代币名称。symbol()：返回代币代号。tokenURI()：通过tokenId查询metadata的链接url，ERC721特有的函数。interface IERC721Metadata is IERC721 { function name() external view returns (string memory); function symbol() external view returns (string memory); function tokenURI(uint256 tokenId) external view returns (string memory); } ERC721主合约ERC721主合约实现了IERC721，IERC165和IERC721Metadata定义的所有功能，包含4个状态变量和17个函数。实现都比较简单，每个函数的功能见代码注释：// SPDX-License-Identifier: MIT // by 0xAA pragma solidity ^0.8.4; import "./IERC165.sol"; import "./IERC721.sol"; import "./IERC721Receiver.sol"; import "./IERC721Metadata.sol"; import "./Address.sol"; import "./String.sol"; contract ERC721 is IERC721, IERC721Metadata{ using Address for address; // 使用Address库，用isContract来判断地址是否为合约 using Strings for uint256; // 使用String库， // Token名称 string public override name; // Token代号 string public override symbol; // tokenId 到 owner address 的持有人映射 mapping(uint => address) private _owners; // address 到 持仓数量 的持仓量映射 mapping(address => uint) private _balances; // tokenID 到 授权地址 的授权映射 mapping(uint => address) private _tokenApprovals; // owner地址。到operator地址 的批量授权映射 mapping(address => mapping(address => bool)) private _operatorApprovals; /** * 构造函数，初始化`name` 和`symbol` . */ constructor(string memory name_, string memory symbol_) { name = name_; symbol = symbol_; } // 实现IERC165接口supportsInterface function supportsInterface(bytes4 interfaceId) external pure override returns (bool) { return interfaceId == type(IERC721).interfaceId || interfaceId == type(IERC165).interfaceId || interfaceId == type(IERC721Metadata).interfaceId; } // 实现IERC721的balanceOf，利用_balances变量查询owner地址的balance。 function balanceOf(address owner) external view override returns (uint) { require(owner != address(0), "owner = zero address"); return _balances[owner]; } // 实现IERC721的ownerOf，利用_owners变量查询tokenId的owner。 function ownerOf(uint tokenId) public view override returns (address owner) { owner = _owners[tokenId]; require(owner != address(0), "token doesn't exist"); } // 实现IERC721的isApprovedForAll，利用_operatorApprovals变量查询owner地址是否将所持NFT批量授权给了operator地址。 function isApprovedForAll(address owner, address operator) external view override returns (bool) { return _operatorApprovals[owner][operator]; } // 实现IERC721的setApprovalForAll，将持有代币全部授权给operator地址。调用_setApprovalForAll函数。 function setApprovalForAll(address operator, bool approved) external override { _operatorApprovals[msg.sender][operator] = approved; emit ApprovalForAll(msg.sender, operator, approved); } // 实现IERC721的getApproved，利用_tokenApprovals变量查询tokenId的授权地址。 function getApproved(uint tokenId) external view override returns (address) { require(_owners[tokenId] != address(0), "token doesn't exist"); return _tokenApprovals[tokenId]; } // 授权函数。通过调整_tokenApprovals来，授权 to 地址操作 tokenId，同时释放Approval事件。 function _approve( address owner, address to, uint tokenId ) private { _tokenApprovals[tokenId] = to; emit Approval(owner, to, tokenId); } // 实现IERC721的approve，将tokenId授权给 to 地址。条件：to不是owner，且msg.sender是owner或授权地址。调用_approve函数。 function approve(address to, uint tokenId) external override { address owner = _owners[tokenId]; require( msg.sender == owner || _operatorApprovals[owner][msg.sender], "not owner nor approved for all" ); _approve(owner, to, tokenId); } // 查询 spender地址是否被可以使用tokenId（他是owner或被授权地址）。 function _isApprovedOrOwner( address owner, address spender, uint tokenId ) private view returns (bool) { return (spender == owner || _tokenApprovals[tokenId] == spender || _operatorApprovals[owner][spender]); } /* * 转账函数。通过调整_balances和_owner变量将 tokenId 从 from 转账给 to，同时释放Tranfer事件。 * 条件: * 1. tokenId 被 from 拥有 * 2. to 不是0地址 */ function _transfer( address owner, address from, address to, uint tokenId ) private { require(from == owner, "not owner"); require(to != address(0), "transfer to the zero address"); _approve(owner, address(0), tokenId); _balances[from] -= 1; _balances[to] += 1; _owners[tokenId] = to; emit Transfer(from, to, tokenId); } // 实现IERC721的transferFrom，非安全转账，不建议使用。调用_transfer函数 function transferFrom( address from, address to, uint tokenId ) external override { address owner = ownerOf(tokenId); require( _isApprovedOrOwner(owner, msg.sender, tokenId), "not owner nor approved" ); _transfer(owner, from, to, tokenId); } /** * 安全转账，安全地将 tokenId 代币从 from 转移到 to，会检查合约接收者是否了解 ERC721 协议，以防止代币被永久锁定。调用了_transfer函数和_checkOnERC721Received函数。条件： * from 不能是0地址. * to 不能是0地址. * tokenId 代币必须存在，并且被 from拥有. * 如果 to 是智能合约, 他必须支持 IERC721Receiver-onERC721Received. */ function _safeTransfer( address owner, address from, address to, uint tokenId, bytes memory _data ) private { _transfer(owner, from, to, tokenId); require(_checkOnERC721Received(from, to, tokenId, _data), "not ERC721Receiver"); } /** * 实现IERC721的safeTransferFrom，安全转账，调用了_safeTransfer函数。 */ function safeTransferFrom( address from, address to, uint tokenId, bytes memory _data ) public override { address owner = ownerOf(tokenId); require( _isApprovedOrOwner(owner, msg.sender, tokenId), "not owner nor approved" ); _safeTransfer(owner, from, to, tokenId, _data); } // safeTransferFrom重载函数 function safeTransferFrom( address from, address to, uint tokenId ) external override { safeTransferFrom(from, to, tokenId, ""); } /** * 铸造函数。通过调整_balances和_owners变量来铸造tokenId并转账给 to，同时释放Tranfer事件。铸造函数。通过调整_balances和_owners变量来铸造tokenId并转账给 to，同时释放Tranfer事件。 * 这个mint函数所有人都能调用，实际使用需要开发人员重写，加上一些条件。 * 条件: * 1. tokenId尚不存在。 * 2. to不是0地址. */ function _mint(address to, uint tokenId) internal virtual { require(to != address(0), "mint to zero address"); require(_owners[tokenId] == address(0), "token already minted"); _balances[to] += 1; _owners[tokenId] = to; emit Transfer(address(0), to, tokenId); } // 销毁函数，通过调整_balances和_owners变量来销毁tokenId，同时释放Tranfer事件。条件：tokenId存在。 function _burn(uint tokenId) internal virtual { address owner = ownerOf(tokenId); require(msg.sender == owner, "not owner of token"); _approve(owner, address(0), tokenId); _balances[owner] -= 1; delete _owners[tokenId]; emit Transfer(owner, address(0), tokenId); } // _checkOnERC721Received：函数，用于在 to 为合约的时候调用IERC721Receiver-onERC721Received, 以防 tokenId 被不小心转入黑洞。 function _checkOnERC721Received( address from, address to, uint tokenId, bytes memory _data ) private returns (bool) { if (to.isContract()) { return IERC721Receiver(to).onERC721Received( msg.sender, from, tokenId, _data ) == IERC721Receiver.onERC721Received.selector; } else { return true; } } /** * 实现IERC721Metadata的tokenURI函数，查询metadata。 */ function tokenURI(uint256 tokenId) public view virtual override returns (string memory) { require(_owners[tokenId] != address(0), "Token Not Exist"); string memory baseURI = _baseURI(); return bytes(baseURI).length > 0 ? string(abi.encodePacked(baseURI, tokenId.toString())) : ""; } /** * 计算{tokenURI}的BaseURI，tokenURI就是把baseURI和tokenId拼接在一起，需要开发重写。 * BAYC的baseURI为ipfs://QmeSjSinHpPnmXmspMjwiXyN6zS4E9zccariGR3jxcaWtq/ */ function _baseURI() internal view virtual returns (string memory) { return ""; } } 写一个免费铸造的APE我们来利用ERC721来写一个免费铸造的WTF APE，总量设置为10000，只需要重写一下mint()和baseURI()函数即可。由于baseURI()设置的和BAYC一样，元数据会直接获取无聊猿的，类似RRBAYC：// SPDX-License-Identifier: MIT // by 0xAA pragma solidity ^0.8.4; import "./ERC721.sol"; contract WTFApe is ERC721{ uint public MAX_APES = 10000; // 总量 // 构造函数 constructor(string memory name_, string memory symbol_) ERC721(name_, symbol_){ } //BAYC的baseURI为ipfs://QmeSjSinHpPnmXmspMjwiXyN6zS4E9zccariGR3jxcaWtq/ function _baseURI() internal pure override returns (string memory) { return "ipfs://QmeSjSinHpPnmXmspMjwiXyN6zS4E9zccariGR3jxcaWtq/"; } // 铸造函数 function mint(address to, uint tokenId) external { require(tokenId >= 0 && tokenId < MAX_APES, "tokenId out of range"); _mint(to, tokenId); } } 总结这一讲，我介绍了ERC721标准、接口及其实现，并在合约代码进行了中文注释。并且我们利用ERC721做了一个免费铸造的WTF APE NFT，元数据直接调用于BAYC。ERC721标准仍在不断发展中，目前比较流行的版本为ERC721Enumerable（提高NFT可访问性）和ERC721A（节约铸造gas）。

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