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ERC-721 标准

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ERC-721 标准 是 CoddyKit 上的免费 Web3 & DApp Development Fundamentals 课时。这是第 1 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Web3 & DApp Development Fundamentals 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Web3 & DApp Development Fundamentals 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

What Is ERC-721?

ERC-721 is the standard for non-fungible tokens (NFTs). Unlike ERC-20, each token is unique and identified by a distinct tokenId.

NFTs represent ownership of one-of-a-kind items: art, collectibles, in-game assets, and more.

Fungible vs Non-Fungible

The key contrast:

ERC-20 - all tokens identical and interchangeable, tracked by balance
ERC-721 - each token unique, tracked by owner per tokenId

You own specific token #42, not a fraction of a pool.

Core Ownership Functions

ERC-721 tracks ownership with two key views:

ownerOf(tokenId) - who owns a specific token
balanceOf(owner) - how many tokens an address owns

<code>function ownerOf(uint256 tokenId) external view returns (address);
function balanceOf(address owner) external view returns (uint256);</code>

Transfer Functions

ERC-721 provides transferFrom and the safer safeTransferFrom, which checks that a receiving contract can handle NFTs.

<code>function transferFrom(address from, address to, uint256 tokenId) external;
function safeTransferFrom(address from, address to, uint256 tokenId) external;</code>

Approval Functions

Owners can authorize others to transfer specific tokens with approve, or grant blanket permission over all their tokens with setApprovalForAll.

<code>function approve(address to, uint256 tokenId) external;
function setApprovalForAll(address operator, bool approved) external;
function getApproved(uint256 tokenId) external view returns (address);
function isApprovedForAll(address owner, address operator) external view returns (bool);</code>

Required Events

ERC-721 defines three events: Transfer, Approval, and ApprovalForAll. Note the tokenId is indexed so transfers of a specific NFT are easy to track.

<code>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);</code>

safeTransferFrom and onERC721Received

safeTransferFrom protects against NFTs getting stuck. If the recipient is a contract, it must implement onERC721Received and return the correct magic value, otherwise the transfer reverts.

<code>// receiving contract must implement:
function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data)
    external returns (bytes4);</code>

The Metadata Extension

Most NFTs implement the optional metadata extension adding name(), symbol(), and crucially tokenURI(tokenId), which points to the token's off-chain JSON metadata.

<code>function name() external view returns (string memory);
function symbol() external view returns (string memory);
function tokenURI(uint256 tokenId) external view returns (string memory);</code>

ERC-165 Interface Detection

ERC-721 builds on ERC-165, which lets contracts advertise which interfaces they support via supportsInterface. Marketplaces use it to detect NFTs.

<code>function supportsInterface(bytes4 interfaceId) external view returns (bool);</code>

Using OpenZeppelin ERC721

As with ERC-20, the practical approach is to inherit OpenZeppelin's audited ERC721 implementation and add your minting logic.

<code>// import '@openzeppelin/contracts/token/ERC721/ERC721.sol';
// contract MyNFT is ERC721 {
//     constructor() ERC721('MyNFT', 'MNFT') {}
// }</code>

Where the Asset Lives

An NFT on-chain is just an id and an owner. The actual image or media usually lives off-chain (often on IPFS), referenced by the tokenURI. The chain proves ownership, not the file itself.

Quick Check

Test your understanding of the ERC-721 standard.

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Recap

You learned the ERC-721 standard:

NFTs are unique, identified by tokenId, tracked via ownerOf
Transfers use transferFrom and safer safeTransferFrom
Approvals: per-token approve and blanket setApprovalForAll
Metadata extension adds tokenURI; ERC-165 enables interface detection
Assets usually live off-chain, referenced by the URI

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