初探區塊鏈數字加密資產標準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。後者如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準了。它誕生於2015年,到2017年9月被正式標準化。協議規定了具有可互換性(fungible)代幣的一組基本介面,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。交易所裡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,因為每隻貓各有千秋,而且由於不同輩分的稀缺性不同,市場價格也差異巨大。這種非標準化資產很長時間內都沒有標準協議,直到2017年9月才出現ERC721提案,定義了一組常用的介面。ERC721至今仍舊處於草案階段,但已經被不少dApp採用,甚至出現了專門的交易所。
ERC721標準
下面先給出ERC721標準的具體內容,後面會講解。
每個符合ERC721標準的合約必須實現ERC721
和ERC165
介面。
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); }
如果錢包程式接受安全轉賬,它必須實現錢包介面。
/// @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介面解析
先來一波名詞解釋:
- NFT: 非互換性資產(non-fungible token)
- tokenId: NFT的id,型別為uint256
- owner: NFT的擁有者,型別為address
- balance: 使用者擁有的NFT數量,型別為uint256
- approved: NFT的管理者,只有NFT的owner和approved可以對NFT進行操作,型別為address
- operator:operator擁有一個使用者所有NFT的管理許可權,型別為address
接下來解釋下介面內的事件和方法:
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被建立和銷燬時也會觸發此事件。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指定一個使用者擁有他所有NFT的管理許可權時,會觸發ApprovalForAll事件。operator擁有owner所有NFT的管理許可權。
function function balanceOf(address _owner) external view returns (uint256);
查詢一個使用者擁有的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,這個方法會呼叫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引數,上面那個safeTransferFrom方法會呼叫這個方法,然後把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的管理許可權授予一個使用者。
function setApprovalForAll(address _operator, bool _approved) external;
NFT的擁有者授權一個使用者是否擁有自己所有NFT的管理許可權。合約必須允許一個使用者有多個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的管理許可權。
function supportsInterface(bytes4 interfaceID) external view returns (bool);
查詢一個合約是否實現了一個介面。
function onERC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external returns(bytes4);
這個方法處理NFT的轉移,當一筆轉移被髮起時,這個方法可能會被呼叫,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