# Solidity极简入门 ERC721专题：3. ERC721主合约

**Published by:** [0xAA](https://paragraph.com/@wtfacademy/)
**Published on:** 2022-04-25
**URL:** https://paragraph.com/@wtfacademy/solidity-erc721-3-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 WTF技术社群discord，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 不知不觉我已经完成了Solidity极简教程的前13讲（基础），内容包括：Helloworld.sol，变量类型，存储位置，函数，控制流，构造函数，修饰器，事件，继承，抽象合约，接口，库，异常。在进阶内容之前，我决定做一个ERC721的专题，把之前的内容综合运用，帮助大家更好的复习基础知识，并且更深刻的理解ERC721合约。希望在学习完这个专题之后，每个人都能发行自己的NFT。ERC721主合约我在ERC721前两讲介绍了它的相关库和接口，终于这讲可以介绍主合约了。ERC721主合约包含6个状态变量和28个函数，我们将会一一介绍。并且，我给ERC721代码增加了中文注释，方便大家使用。状态变量 // 代币名称 string private _name; // 代币代号 string private _symbol; // tokenId到owner地址Mapping mapping(uint256 => address) private _owners; // owner地址到持币数量Mapping mapping(address => uint256) private _balances; // tokenId到授权地址Mapping mapping(uint256 => address) private _tokenApprovals; // owner地址到是否批量批准Mapping mapping(address => mapping(address => bool)) private _operatorApprovals; _name和_symbol是两个string，存储代币的名称和代号。_owners是tokenId到owner地址的Mapping，存储每个代币的持有人。_balances是owner地址到持币数量的Mapping，存储每个地址的持仓量。_tokenApprovals是tokenId到授权地址的Mapping，存储每个token的授权信息。_operatorApprovals是owner地址到是否批量批准的Mapping，存储每个owner的批量授权信息。注意，批量授权会把你钱包持有这个系列的所有nft都授权给另一个地址，别人可以随意支配。函数constructor：构造函数，设定ERC721代币的名字和代号（_name和_symbol变量）。 constructor(string memory name_, string memory symbol_) { _name = name_; _symbol = symbol_; } supportsInterface：实现IERC165接口supportsInterface，详见ERC721专题第二讲 function supportsInterface(bytes4 interfaceId) public view virtual override(ERC165, IERC165) returns (bool) { return interfaceId == type(IERC721).interfaceId || interfaceId == type(IERC721Metadata).interfaceId || super.supportsInterface(interfaceId); } balanceOf：实现IERC721的balanceOf，利用_balances变量查询owner地址的balance。 function balanceOf(address owner) public view virtual override returns (uint256) { require(owner != address(0), "ERC721: balance query for the zero address"); return _balances[owner]; } ownerOf：实现IERC721的ownerOf，利用_owners变量查询tokenId的owner。 function ownerOf(uint256 tokenId) public view virtual override returns (address) { address owner = _owners[tokenId]; require(owner != address(0), "ERC721: owner query for nonexistent token"); return owner; } name：实现IERC721Metadata的name，查询代币名称。 function name() public view virtual override returns (string memory) { return _name; } symbol：实现IERC721Metadata的symbol，查询代币代号。 function symbol() public view virtual override returns (string memory) { return _symbol; } tokenURI：实现IERC721Metadata的tokenURI，查询代币metadata存放的网址。Opensea还有小狐狸钱包显示你NFT的图片，调用的就是这个函数。 function tokenURI(uint256 tokenId) public view virtual override returns (string memory) { require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token"); string memory baseURI = _baseURI(); return bytes(baseURI).length > 0 ? string(abi.encodePacked(baseURI, tokenId.toString())) : ""; } _baseURI：基URI，会被tokenURI()调用，跟tokenId拼成tokenURI，默认为空，需要子合约重写这个函数。 function _baseURI() internal view virtual returns (string memory) { return ""; } approve：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, uint256 tokenId) public virtual override { address owner = ERC721.ownerOf(tokenId); require(to != owner, "ERC721: approval to current owner"); require( _msgSender() == owner || isApprovedForAll(owner, _msgSender()), "ERC721: approve caller is not owner nor approved for all" ); _approve(to, tokenId); } getApproved：实现IERC721的getApproved，利用_tokenApprovals变量查询tokenId的授权地址。 function getApproved(uint256 tokenId) public view virtual override returns (address) { require(_exists(tokenId), "ERC721: approved query for nonexistent token"); return _tokenApprovals[tokenId]; } setApprovalForAll：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) public virtual override { _setApprovalForAll(_msgSender(), operator, approved); } isApprovedForAll：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) public view virtual override returns (bool) { return _operatorApprovals[owner][operator]; } transferFrom：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, uint256 tokenId ) public virtual override { //solhint-disable-next-line max-line-length require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved"); _transfer(from, to, tokenId); } safeTransferFrom：实现IERC721的safeTransferFrom，安全转账，调用了_safeTransfer函数。 function safeTransferFrom( address from, address to, uint256 tokenId, bytes memory _data ) public virtual override { require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved"); _safeTransfer(from, to, tokenId, _data); } _safeTransfer：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 from, address to, uint256 tokenId, bytes memory _data ) internal virtual { _transfer(from, to, tokenId); require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer"); } _exists：查询 tokenId是否存在（等价于查询他的owner是否为非0地址）。 function _exists(uint256 tokenId) internal view virtual returns (bool) { return _owners[tokenId] != address(0); } _isApprovedOrOwner：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 spender, uint256 tokenId) internal view virtual returns (bool) { require(_exists(tokenId), "ERC721: operator query for nonexistent token"); address owner = ERC721.ownerOf(tokenId); return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender)); } _safeMint：安全mint函数，铸造tokenId并转账给 to地址。 条件:tokenId尚不存在,若to为智能合约，他要支持IERC721Receiver-onERC721Received接口。 function _safeMint(address to, uint256 tokenId) internal virtual { _safeMint(to, tokenId, ""); } _safeMint的实现，调用了_checkOnERC721Received函数和_mint函数。 function _safeMint( address to, uint256 tokenId, bytes memory _data ) internal virtual { _mint(to, tokenId); require( _checkOnERC721Received(address(0), to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer" ); } _mint：internal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。条件:tokenId尚不存在。to不是0地址. function _mint(address to, uint256 tokenId) internal virtual { require(to != address(0), "ERC721: mint to the zero address"); require(!_exists(tokenId), "ERC721: token already minted"); _beforeTokenTransfer(address(0), to, tokenId); _balances[to] += 1; _owners[tokenId] = to; emit Transfer(address(0), to, tokenId); _afterTokenTransfer(address(0), to, tokenId); } _burn：internal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(uint256 tokenId) internal virtual { address owner = ERC721.ownerOf(tokenId); _beforeTokenTransfer(owner, address(0), tokenId); // 清空授权 _approve(address(0), tokenId); _balances[owner] -= 1; delete _owners[tokenId]; emit Transfer(owner, address(0), tokenId); _afterTokenTransfer(owner, address(0), tokenId); } _transfer：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:tokenId 被 from 拥有to 不是0地址 function _transfer( address from, address to, uint256 tokenId ) internal virtual { require(ERC721.ownerOf(tokenId) == from, "ERC721: transfer from incorrect owner"); require(to != address(0), "ERC721: transfer to the zero address"); _beforeTokenTransfer(from, to, tokenId); // 清空授权 _approve(address(0), tokenId); _balances[from] -= 1; _balances[to] += 1; _owners[tokenId] = to; emit Transfer(from, to, tokenId); _afterTokenTransfer(from, to, tokenId); } _approve：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 to, uint256 tokenId) internal virtual { _tokenApprovals[tokenId] = to; emit Approval(ERC721.ownerOf(tokenId), to, tokenId); } _setApprovalForAll：批量授权函数。通过调整_operatorApprovals变量，批量授权 to 来操作 owner全部代币，同时释放ApprovalForAll事件。 function _setApprovalForAll( address owner, address operator, bool approved ) internal virtual { require(owner != operator, "ERC721: approve to caller"); _operatorApprovals[owner][operator] = approved; emit ApprovalForAll(owner, operator, approved); } _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, uint256 tokenId, bytes memory _data ) private returns (bool) { if (to.isContract()) { try IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data) returns (bytes4 retval) { return retval == IERC721Receiver.onERC721Received.selector; } catch (bytes memory reason) { if (reason.length == 0) { revert("ERC721: transfer to non ERC721Receiver implementer"); } else { assembly { revert(add(32, reason), mload(reason)) } } } } else { return true; } } _beforeTokenTransfer：这个函数在转账之前会被调用（包括mint和burn）。默认为空，子合约可以选择重写。 function _beforeTokenTransfer( address from, address to, uint256 tokenId ) internal virtual {} _afterTokenTransfer：这个函数在转账之后会被调用（包括mint和burn）。默认为空，子合约可以选择重写。 function _afterTokenTransfer( address from, address to, uint256 tokenId ) internal virtual {} 总结本文是ERC721专题的第三讲，我介绍了ERC721主合约的全部变量和函数，并给出了合约的中文注释。有了ERC721标准，NFT项目方只需要把mint函数包装一下，就可以发行NFT了。下一讲，我们会介绍无聊猿BAYC的合约，了解一下最火NFT在标准ERC721合约上做了什么改动。

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