Solidity极简入门: 34. ERC721

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/WTFSolidity

BTC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。

IERC721

IERC721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

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;
        }
    }

IERC721Metadata

IERC721Metadata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：

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）。

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/WTFSolidity

ERC165

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。

IERC721

/**
 * @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

IERC721Receiver

// 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;
        }
    }

IERC721Metadata

IERC721Metadata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：

写一个免费铸造的APE

// 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

// 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 "";
    }
}

// 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 "";
    }
}

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Solidity极简入门: 34. ERC721

Solidity极简入门: 34. ERC721

ERC165

IERC721

IERC721事件

IERC721函数

IERC721Receiver

IERC721Metadata

ERC721主合约

写一个免费铸造的APE

总结

ERC165

IERC721

IERC721事件

IERC721函数

IERC721Receiver

IERC721Metadata

ERC721主合约

写一个免费铸造的APE

总结