ERC721 - NFT 만들기

NFT란?

위키백과

대체 불가능한 토큰 ( NFT )은 블록체인에 저장된 데이터 단위로 고유하면서 상호 교환 할 수 없는 토큰을 뜻한다

NFT는 사진, 비디오, 오디오 및 기타 유형의 디지털 파일을 나타내는데 사용할 수 있다. 사본은 인정되지 않는다. 이러한 디지털 항목의 사본은 누구나 얻을 수 있지만 NFT는 블록 체인에서 추적 되어 소유자에게 저작권 과 소유권 증명을 해야한다.

 

 

NFT 구현

NFT를 구현하기 위해 어떤 인터페이스를 제공하는지 알아보겠습니다.

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);

function balanceOf(address _owner) external view returns (uint256);
function ownerOf(uint256 _tokenId) external view returns (address);
function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes data) external payable;
function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;
function transferFrom(address _from, address _to, uint256 _tokenId) external payable;
function approve(address _approved, uint256 _tokenId) external payable;
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);
function supportsInterface(bytes4 interfaceID) external view returns (bool);

해당 인터페이스를 직접 구현하지 않고 구현된 인터페이스를 기반으로 원하는 기능을 구현합니다.

interface(인터페이스)

library Address {

    function isContract(address account) internal view returns (bool) {
        uint256 size;
        assembly { size := extcodesize(account) }
        return size > 0;
    }
}

library SafeMath {

    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a, "SafeMath: subtraction overflow");
        uint256 c = a - b;

        return c;
    }

    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b > 0, "SafeMath: division by zero");
        uint256 c = a / b;

        return c;
    }

    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b != 0, "SafeMath: modulo by zero");
        return a % b;
    }
}

library Counters {
    using SafeMath for uint256;

    struct Counter {
        uint256 _value; // 기본값: 0
    }

    function current(Counter storage counter) internal view returns (uint256) {
        return counter._value;
    }

    function increment(Counter storage counter) internal {
        counter._value += 1;
    }

    function decrement(Counter storage counter) internal {
        counter._value = counter._value.sub(1);
    }
}

interface IERC165 {
    function supportsInterface(bytes4 interfaceId) external view returns (bool);
}

contract ERC165 is IERC165 {
    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;

    mapping(bytes4 => bool) private _supportedInterfaces;

    constructor () internal {
        _registerInterface(_INTERFACE_ID_ERC165);
    }

    function supportsInterface(bytes4 interfaceId) external view returns (bool) {
        return _supportedInterfaces[interfaceId];
    }

    function _registerInterface(bytes4 interfaceId) internal {
        require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
        _supportedInterfaces[interfaceId] = true;
    }
}

contract IERC721 is IERC165 {
    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);

    function balanceOf(address owner) public view returns (uint256 balance);

    function ownerOf(uint256 tokenId) public view returns (address owner);

    function safeTransferFrom(address from, address to, uint256 tokenId) public;

    function transferFrom(address from, address to, uint256 tokenId) public;
    function approve(address to, uint256 tokenId) public;
    function getApproved(uint256 tokenId) public view returns (address operator);

    function setApprovalForAll(address operator, bool _approved) public;
    function isApprovedForAll(address owner, address operator) public view returns (bool);


    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory data) public;
} 

contract IERC721Receiver {
    function onERC721Received(address operator, address from, uint256 tokenId, bytes memory data)
    public returns (bytes4);
}

contract ERC721 is ERC165, IERC721 {
    using SafeMath for uint256;
    using Address for address;
    using Counters for Counters.Counter;

    bytes4 private constant _ERC721_RECEIVED = 0x150b7a02;

    mapping (uint256 => address) internal _tokenOwner;

    mapping (uint256 => address) internal _tokenApprovals;

    mapping (address => Counters.Counter) internal _ownedTokensCount;

    mapping (address => mapping (address => bool)) private _operatorApprovals;

    bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd;

    constructor () public {
        _registerInterface(_INTERFACE_ID_ERC721);
    }

    function balanceOf(address owner) public view returns (uint256) {
        require(owner != address(0), "ERC721: balance query for the zero address");

        return _ownedTokensCount[owner].current();
    }

    function ownerOf(uint256 tokenId) public view returns (address) {
        address owner = _tokenOwner[tokenId];
        require(owner != address(0), "ERC721: owner query for nonexistent token");

        return owner;
    }

    function approve(address to, uint256 tokenId) public {
        address owner = ownerOf(tokenId);
        require(to != owner, "ERC721: approval to current owner");

        require(msg.sender == owner || isApprovedForAll(owner, msg.sender),
            "ERC721: approve caller is not owner nor approved for all"
        );

        _tokenApprovals[tokenId] = to;
        emit Approval(owner, to, tokenId);
    }

    function getApproved(uint256 tokenId) public view returns (address) {
        require(_exists(tokenId), "ERC721: approved query for nonexistent token");

        return _tokenApprovals[tokenId];
    }

    function setApprovalForAll(address to, bool approved) public {
        require(to != msg.sender, "ERC721: approve to caller");

        _operatorApprovals[msg.sender][to] = approved;
        emit ApprovalForAll(msg.sender, to, approved);
    }

    function isApprovedForAll(address owner, address operator) public view returns (bool) {
        return _operatorApprovals[owner][operator];
    }

    function transferFrom(address from, address to, uint256 tokenId) public {
        //solhint-disable-next-line max-line-length
        require(_isApprovedOrOwner(msg.sender, tokenId), "ERC721: transfer caller is not owner nor approved");

        _transferFrom(from, to, tokenId);
    }

    function safeTransferFrom(address from, address to, uint256 tokenId) public {
        safeTransferFrom(from, to, tokenId, "");
    }

    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) public {
        transferFrom(from, to, tokenId);
        require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
    }

    function _exists(uint256 tokenId) internal view returns (bool) {
        address owner = _tokenOwner[tokenId];
        return owner != address(0);
    }

    function _isApprovedOrOwner(address spender, uint256 tokenId) internal view returns (bool) {
        require(_exists(tokenId), "ERC721: operator query for nonexistent token");
        address owner = ownerOf(tokenId);
        return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
    }

    function _burn(address owner, uint256 tokenId) internal {
        require(ownerOf(tokenId) == owner, "ERC721: burn of token that is not own");

        _clearApproval(tokenId);

        _ownedTokensCount[owner].decrement();
        _tokenOwner[tokenId] = address(0);

        emit Transfer(owner, address(0), tokenId);
    }

    function _burn(uint256 tokenId) internal {
        _burn(ownerOf(tokenId), tokenId);
    }

    function _transferFrom(address from, address to, uint256 tokenId) internal {
        require(ownerOf(tokenId) == from, "ERC721: transfer of token that is not own");
        require(to != address(0), "ERC721: transfer to the zero address");

        _clearApproval(tokenId);

        _ownedTokensCount[from].decrement();
        _ownedTokensCount[to].increment();

        _tokenOwner[tokenId] = to;

        emit Transfer(from, to, tokenId);
    }

    function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data)
        internal returns (bool)
    {
        if (!to.isContract()) {
            return true;
        }

        bytes4 retval = IERC721Receiver(to).onERC721Received(msg.sender, from, tokenId, _data);
        return (retval == _ERC721_RECEIVED);
    }

    function _clearApproval(uint256 tokenId) private {
        if (_tokenApprovals[tokenId] != address(0)) {
            _tokenApprovals[tokenId] = address(0);
        }
    }
}

NFT를 만들기 위해 ERC721을 구현하기 위한 인터페이스가 좀 길지만 전체적인 흐름은 토큰 ID를 주고 받는 코드입니다.

 

기본적으로 ERC721의 인터페이스는 특정 토큰이 생성돼서 소유권을 부여하는 부분이 없습니다.

    mapping (uint256 => address) internal _tokenOwner;
    mapping (address => Counters.Counter) internal _ownedTokensCount;

특정 토큰의 소유  주소를 가지고 있는 정보와, 특정 주소가 몇개의 토큰을 소유하고 있는지에 대한 정보를 가지고 있는 변수입니다.

 

ERC721 컨트랙트에 다음 코드를 추가하여 소유권을 부여하는 _mint를 추가합니다.

contract ERC721 is ERC165, IERC721 {
// ... 중략 ...
        function _mint(address to, uint256 tokenId) internal {
        require(to != address(0), "ERC721: mint to the zero address");
        require(!_exists(tokenId), "ERC721: token already minted");

        _tokenOwner[tokenId] = to; // 소유권 부여 
        _ownedTokensCount[to].increment(); // 토큰 수량 + 1

        emit Transfer(address(0), to, tokenId);
    }
}

여기까지 ERC721 준비는 끝났습니다. 이제 실질적으로 대체 불가능한 무엇인가를 만들어서 tokenid를 만들어 _mint를 호출하여 소유권을 부여해주면 됩니다.

 

대체 불가능한 품목 구현

    contract CharactorByERC721 is ERC721{
      struct Charactor {
          string  name;  // 캐릭터 이름
          uint256 level; // 캐릭터 레벨
      }

      Charactor[] public charactors; // default: [] 
      address public owner;          // 컨트랙트 소유자

      constructor () public {
          owner = msg.sender; 
      }

      modifier isOwner() {
        require(owner == msg.sender);
        _;
      }

      // 캐릭터 생성
      function mint(string memory name, address account) public isOwner {
          uint256 cardId = charactors.length; // 유일한 캐릭터 ID
          charactors.push(Charactor(name, 1));
          _mint(account, cardId); // ERC721 소유권 등록
      }
    }

이름과 레벨 정보를 가진 캐릭터를 생성하여 대체 불가능 토큰화 시키는 코드를 완성했습니다.

ERC721를 관리하는 관리자만 캐릭터를 생성할 수 있는 권한을 부여한 코드는 아래와 같습니다.

      modifier isOwner() {
        require(owner == msg.sender);
        _;
      }

 

동작확인

 

1)Remix 인터페이스를 통한 조회

remix deploy 후 interface를 통한 Trasnaction 결과

2)React를 통한 배포한 Contract 메소드 테스트

소유권이전, 품목 수량 및 목록 조회

배포한 컨트랙트 내부 methods