初探ERC-721

ERC-721协议是以太坊开发中最常使用的第二大协议，第一大协议当然是我们的ERC-20。

ERC-721官方简要解释是Non-Fungible Tokens，简写为NFTs，多翻译为非同质代币。

ERC-721最早进入大家的视野都是因为 《CryptoKitties》 加密猫的功劳。所以我下面会结合加密猫游戏的案例来说说我们的ERC-721。

ERC-721特点:

1.无法分割，ERC-20Token可以无限细分为10^18份，而ERC721的Token最小的单位为1，无法再分割。

2.独一无二，每一个Token完全不同，并且每个Token对不同的用户都有不同的价值。

回想一下，加密猫最小单位为 1只，并且每一只猫的基因长相也完全不同。

下面我会附上以太坊EIP721接口标准。来自：https://eips.ethereum.org/EIPS/eip-721

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);
}

 

 

接口说明：

• balanceOf(): 返回由_owner 持有的NFTs的数量。

• ownerOf(): 返回tokenId代币持有者的地址。

• approve(): 授予地址_to具有_tokenId的控制权，方法成功后需触发Approval 事件。

• setApprovalForAll(): 授予地址_operator具有所有NFTs的控制权，成功后需触发ApprovalForAll事件。

• getApproved()、isApprovedForAll(): 用来查询授权。

• safeTransferFrom(): 转移NFT所有权，一次成功的转移操作必须发起 Transer 事件。安全是指如果目标地址为合约地址，则执行onERC721Received进行资产转移，否则的话交易会回滚；如果目标地址为外部账号地址，则正常转移。

细心的朋友应该有发现 ERC-721协议继承自 ERC-165,ERC-165同样也是一个合约标准，这个标准要求合约提供其实现了哪些接口，这样在与合约进行交互的时候可以先调用此接口进行查询。

下面我会截取一些加密猫的源码分析给大家看。

你也可以在Etherscan上查看完整源码：https://etherscan.io/address/0x06012c8cf97bead5deae237070f9587f8e7a266d#contracts

struct Kitty {

        uint256 genes;

        uint64 birthTime;

        uint64 cooldownEndBlock;

        uint32 matronId;

        uint32 sireId;

        uint32 siringWithId;

        uint16 cooldownIndex;

        uint16 generation;
    }

首先，声明了Kitty结构体，其中包含

genes — 是256位的整数，主要作为猫的遗传密码，是决定猫外貌的核心数据；
birthTime — 猫出生时所打包的区块的时间戳；
cooldownEndBlock — 猫可以再次繁殖的最小时间；
matronId 和 sireId — 分辨是猫母亲的ID号和猫父亲的ID号；
siringWithId — 如果猫怀孕了，则设置为父亲的ID，否则为零；
cooldownIndex — 猫繁殖所需的冷却时间（猫需要等待多久才能繁殖）；
generation — 猫的“世代号”（指明这是第几代猫）。合约创造的第一只猫是0代，新一代猫的“世代号”是其父母中较大的一代再加1。

Kitty[] kitties;

Kitty数组，存在“链上”所有猫咪数据。可通过ID号获取猫咪具体信息，也可通过kitties.Length获取猫咪数量。

    mapping (uint256 => address) public kittyIndexToOwner;

    mapping (address => uint256) ownershipTokenCount;

kittyIndexToOwner  所有猫咪对应的用户地址。

ownershipTokenCount  用户地址下的猫咪数量。

 

    function _transfer(address _from, address _to, uint256 _tokenId) internal {
        // Since the number of kittens is capped to 2^32 we can't overflow this
        ownershipTokenCount[_to]++;
        // transfer ownership
        kittyIndexToOwner[_tokenId] = _to;
        // When creating new kittens _from is 0x0, but we can't account that address.
        if (_from != address(0)) {
            ownershipTokenCount[_from]--;
            // once the kitten is transferred also clear sire allowances
            delete sireAllowedToAddress[_tokenId];
            // clear any previously approved ownership exchange
            delete kittyIndexToApproved[_tokenId];
        }
        // Emit the transfer event.
        Transfer(_from, _to, _tokenId);
    }

这是交易函数

 

 function _createKitty(
        uint256 _matronId,
        uint256 _sireId,
        uint256 _generation,
        uint256 _genes,
        address _owner
    )
        internal
        returns (uint)
    {
        require(_matronId == uint256(uint32(_matronId)));
        require(_sireId == uint256(uint32(_sireId)));
        require(_generation == uint256(uint16(_generation)));

        // New kitty starts with the same cooldown as parent gen/2
        uint16 cooldownIndex = uint16(_generation / 2);
        if (cooldownIndex > 13) {
            cooldownIndex = 13;
        }

        Kitty memory _kitty = Kitty({
            genes: _genes,
            birthTime: uint64(now),
            cooldownEndBlock: 0,
            matronId: uint32(_matronId),
            sireId: uint32(_sireId),
            siringWithId: 0,
            cooldownIndex: cooldownIndex,
            generation: uint16(_generation)
        });
        uint256 newKittenId = kitties.push(_kitty) - 1;

        require(newKittenId == uint256(uint32(newKittenId)));

        // emit the birth event
        Birth(
            _owner,
            newKittenId,
            uint256(_kitty.matronId),
            uint256(_kitty.sireId),
            _kitty.genes
        );

        // This will assign ownership, and also emit the Transfer event as
        // per ERC721 draft
        _transfer(0, _owner, newKittenId);

        return newKittenId;
    }

新的猫咪的产生函数

 

总结

    随着区块链技术的发展，初创公司和开发者开始对ERC721越来越感兴趣，这是一种让加密数字资产更容易普及的方式。

    ERC721可能会比ERC20应用的更加广泛。虚拟资产预计将来会有几万亿的市场，届时，将是ERC721标准大放异彩的时候。

 

下一篇文章我会为大家详细解析一个非常冷门的Token协议，ERC918