初探區塊鏈數字加密資產標准ERC721

ERC721介紹

數字加密貨幣大致可以分為原生幣（coin）和代幣（token）兩大類。前者如BTC、ETH等，擁有自己的區塊鏈。后者如Tether、TRON、ONT等，依附於現有的區塊鏈。市場上流通的基於以太坊的代幣大都遵從ERC20協議。最近出現了一種被稱為ERC721的數字加密資產，例如CryptoKitties。

ERC20可能是其中最廣為人知的標准了。它誕生於2015年，到2017年9月被正式標准化。協議規定了具有可互換性（fungible）代幣的一組基本接口，包括代幣符號、發行量、轉賬、授權等。所謂可互換性（fungibility）指代幣之間無差異，同等數量的兩筆代幣價值相等。交易所里流通的絕大部分代幣都是可互換的，一單位的幣無論在哪兒都價值一單位。

與之相對的則是非互換性（non-fungible）資產。比如CryptoKitties中的寵物貓就是典型的非互換性資產，因為每只貓各有千秋，而且由於不同輩分的稀缺性不同，市場價格也差異巨大。這種非標准化資產很長時間內都沒有標准協議，直到2017年9月才出現ERC721提案，定義了一組常用的接口。ERC721至今仍舊處於草案階段，但已經被不少dApp采用，甚至出現了專門的交易所。

ERC721標准

下面先給出ERC721標准的具體內容，后面會講解。

每個符合ERC721標准的合約必須實現ERC721和ERC165接口。

pragma solidity ^0.4.20;

/// @title ERC-721 Non-Fungible Token Standard
/// @dev See https://eips.ethereum.org/EIPS/eip-721
///  Note: the ERC-165 identifier for this interface is 0x80ac58cd.
interface ERC721 /* is ERC165 */ {
    /// @dev This emits when ownership of any NFT changes by any mechanism.
    ///  This event emits when NFTs are created (`from` == 0) and destroyed
    ///  (`to` == 0). Exception: during contract creation, any number of NFTs
    ///  may be created and assigned without emitting Transfer. At the time of
    ///  any transfer, the approved address for that NFT (if any) is reset to none.
    event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);

    /// @dev This emits when the approved address for an NFT is changed or
    ///  reaffirmed. The zero address indicates there is no approved address.
    ///  When a Transfer event emits, this also indicates that the approved
    ///  address for that NFT (if any) is reset to none.
    event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);

    /// @dev This emits when an operator is enabled or disabled for an owner.
    ///  The operator can manage all NFTs of the owner.
    event ApprovalForAll(address indexed _owner, address indexed _operator, bool _approved);

    /// @notice Count all NFTs assigned to an owner
    /// @dev NFTs assigned to the zero address are considered invalid, and this
    ///  function throws for queries about the zero address.
    /// @param _owner An address for whom to query the balance
    /// @return The number of NFTs owned by `_owner`, possibly zero
    function balanceOf(address _owner) external view returns (uint256);

    /// @notice Find the owner of an NFT
    /// @dev NFTs assigned to zero address are considered invalid, and queries
    ///  about them do throw.
    /// @param _tokenId The identifier for an NFT
    /// @return The address of the owner of the NFT
    function ownerOf(uint256 _tokenId) external view returns (address);

    /// @notice Transfers the ownership of an NFT from one address to another address
    /// @dev Throws unless `msg.sender` is the current owner, an authorized
    ///  operator, or the approved address for this NFT. Throws if `_from` is
    ///  not the current owner. Throws if `_to` is the zero address. Throws if
    ///  `_tokenId` is not a valid NFT. When transfer is complete, this function
    ///  checks if `_to` is a smart contract (code size > 0). If so, it calls
    ///  `onERC721Received` on `_to` and throws if the return value is not
    ///  `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`.
    /// @param _from The current owner of the NFT
    /// @param _to The new owner
    /// @param _tokenId The NFT to transfer
    /// @param data Additional data with no specified format, sent in call to `_to`
    function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes data) external payable;

    /// @notice Transfers the ownership of an NFT from one address to another address
    /// @dev This works identically to the other function with an extra data parameter,
    ///  except this function just sets data to "".
    /// @param _from The current owner of the NFT
    /// @param _to The new owner
    /// @param _tokenId The NFT to transfer
    function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;

    /// @notice Transfer ownership of an NFT -- THE CALLER IS RESPONSIBLE
    ///  TO CONFIRM THAT `_to` IS CAPABLE OF RECEIVING NFTS OR ELSE
    ///  THEY MAY BE PERMANENTLY LOST
    /// @dev Throws unless `msg.sender` is the current owner, an authorized
    ///  operator, or the approved address for this NFT. Throws if `_from` is
    ///  not the current owner. Throws if `_to` is the zero address. Throws if
    ///  `_tokenId` is not a valid NFT.
    /// @param _from The current owner of the NFT
    /// @param _to The new owner
    /// @param _tokenId The NFT to transfer
    function transferFrom(address _from, address _to, uint256 _tokenId) external payable;

    /// @notice Change or reaffirm the approved address for an NFT
    /// @dev The zero address indicates there is no approved address.
    ///  Throws unless `msg.sender` is the current NFT owner, or an authorized
    ///  operator of the current owner.
    /// @param _approved The new approved NFT controller
    /// @param _tokenId The NFT to approve
    function approve(address _approved, uint256 _tokenId) external payable;

    /// @notice Enable or disable approval for a third party ("operator") to manage
    ///  all of `msg.sender`'s assets
    /// @dev Emits the ApprovalForAll event. The contract MUST allow
    ///  multiple operators per owner.
    /// @param _operator Address to add to the set of authorized operators
    /// @param _approved True if the operator is approved, false to revoke approval
    function setApprovalForAll(address _operator, bool _approved) external;

    /// @notice Get the approved address for a single NFT
    /// @dev Throws if `_tokenId` is not a valid NFT.
    /// @param _tokenId The NFT to find the approved address for
    /// @return The approved address for this NFT, or the zero address if there is none
    function getApproved(uint256 _tokenId) external view returns (address);

    /// @notice Query if an address is an authorized operator for another address
    /// @param _owner The address that owns the NFTs
    /// @param _operator The address that acts on behalf of the owner
    /// @return True if `_operator` is an approved operator for `_owner`, false otherwise
    function isApprovedForAll(address _owner, address _operator) external view returns (bool);
}

interface ERC165 {
    /// @notice Query if a contract implements an interface
    /// @param interfaceID The interface identifier, as specified in ERC-165
    /// @dev Interface identification is specified in ERC-165. This function
    ///  uses less than 30,000 gas.
    /// @return `true` if the contract implements `interfaceID` and
    ///  `interfaceID` is not 0xffffffff, `false` otherwise
    function supportsInterface(bytes4 interfaceID) external view returns (bool);
}

如果錢包程序接受安全轉賬，它必須實現錢包接口。

/// @dev Note: the ERC-165 identifier for this interface is 0x150b7a02.
interface ERC721TokenReceiver {
    /// @notice Handle the receipt of an NFT
    /// @dev The ERC721 smart contract calls this function on the recipient
    ///  after a `transfer`. This function MAY throw to revert and reject the
    ///  transfer. Return of other than the magic value MUST result in the
    ///  transaction being reverted.
    ///  Note: the contract address is always the message sender.
    /// @param _operator The address which called `safeTransferFrom` function
    /// @param _from The address which previously owned the token
    /// @param _tokenId The NFT identifier which is being transferred
    /// @param _data Additional data with no specified format
    /// @return `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
    ///  unless throwing
    function onERC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external returns(bytes4);
}

ERC721接口解析

先來一波名詞解釋：

• NFT: 非互換性資產(non-fungible token)
• tokenId: NFT的id，類型為uint256
• owner: NFT的擁有者，類型為address
• balance: 用戶擁有的NFT數量，類型為uint256
• approved: NFT的管理者，只有NFT的owner和approved可以對NFT進行操作，類型為address
• operator：operator擁有一個用戶所有NFT的管理權限，類型為address

接下來解釋下接口內的事件和方法：

event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);

當NFT被轉移時會觸發Transfer事件，在NFT被創建和銷毀時也會觸發此事件。在NFT被轉移時，他的approved會被重置為零地址。

event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);

當NFT的管理者，也就是approved被改變的時候，會觸發Approval事件。如果approved是零地址，說明NFT沒有管理者。

event ApprovalForAll(address indexed _owner, address indexed _operator, bool _approved);

當NFT的owner指定一個用戶擁有他所有NFT的管理權限時，會觸發ApprovalForAll事件。operator擁有owner所有NFT的管理權限。

function function balanceOf(address _owner) external view returns (uint256);

查詢一個用戶擁有的NFT數量。

function ownerOf(uint256 _tokenId) external view returns (address);

查詢一個NFT的擁有者。

function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;

NFT的擁有者或管理者把一個NFT轉移給別人，當to是一個合約的地址時，這個方法會調用onERC721Received方法。

function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes data) external payable;

NFT的擁有者或管理者把一個NFT轉移給別人，這個方法有一個額外的data參數，上面那個safeTransferFrom方法會調用這個方法，然后把data置為""。

function transferFrom(address _from, address _to, uint256 _tokenId) external payable;

NFT的擁有者或管理者把一個NFT轉移給別人，可以看到，這個方法比起上面兩個要少了個safe，說明是不安全的轉移，轉移者要自己驗證to是不是一個正確的地址，如果不是的話，NFT可能會永久丟失。

function approve(address _approved, uint256 _tokenId) external payable;

NFT的擁有者把NFT的管理權限授予一個用戶。

function setApprovalForAll(address _operator, bool _approved) external;

NFT的擁有者授權一個用戶是否擁有自己所有NFT的管理權限。合約必須允許一個用戶有多個operator。

function getApproved(uint256 _tokenId) external view returns (address);

查詢一個NFT的管理者

function isApprovedForAll(address _owner, address _operator) external view returns (bool);

查詢指定的operator是否擁有owner所有NFT的管理權限。

function supportsInterface(bytes4 interfaceID) external view returns (bool);

查詢一個合約是否實現了一個接口。

function onERC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external returns(bytes4);

這個方法處理NFT的轉移，當一筆轉移被發起時，這個方法可能會被調用，目的是拒絕這次轉移。

ERC721的實現

Github上已經有ERC721的實現了，地址：https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/ERC721.sol

不過這是solidity的實現，並且基於公鏈，如果要基於聯盟鏈，可以看一下我的這篇文章基於Hyperledger Fabric實現ERC721

參考

https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/ERC721.sol
https://zhuanlan.zhihu.com/p/112275276