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ERC-721
소개
이 튜토리얼은 Kaia 토큰 표준, 특히 대체 불가능한 토큰 표준(ERC-721)을 준수하는 ERC-721 호환 토큰 예제를 만들 수 있도록 도와줍니다.
ERC-721 대체 불가능한 토큰 표준에서는 아래와 같이 3가지 이벤트와 10가지 메서드를 정의하고 있습니다. ERC-721의 supportsInterface는 ERC-165 표준 인터페이스 감지에서 파생된 것으로, ERC-165는 ERC-721의 일부입니다.
ERC-721 호환 토큰은 아래와 같이 ERC-721과 ERC-165 인터페이스를 구현하는 토큰 컨트랙트입니다.
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);
개발자는 위 인터페이스를 기반으로 새로운 기능과 로직을 추가하여 토큰을 커스터마이징하고 Kaia 네트워크에 배포할 수 있습니다. 자세한 내용은 공식 ERC-721 스펙을 참고하시기 바랍니다.
이 튜토리얼에서는 카드형 대체 불가능한 토큰, 즉 ERC-721 토큰인 MyERC721Card를 구현하는 MyERC721Card.sol을 구현해 보겠습니다.
각 MyERC721Card에는 이름과 레벨이 있습니다(예: 레벨 1의 "King", 레벨 1의 "Queen").
MyERC721Card.sol은 OpenZeppelin의 ERC721 구현을 기반으로 합니다. 이 튜토리얼에 포함된 코드의 주요 부분은 OpenZeppelin 2.3
에서 포크되었습니다.
1. ERC-721 스마트 컨트랙트 작성하기
1.1 MyERC721Card의 전체 구조
MyERC721Card.sol의 전체 소스 코드는 아래와 같습니다.
pragma solidity ^0.5.0;// https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/utils/Address.sol/** * @dev Collection of functions related to the address type, */library Address { /** * @dev Returns true if `account` is a contract. * * This test is non-exhaustive, and there may be false-negatives: during the * execution of a contract's constructor, its address will be reported as * not containing a contract. * * > It is unsafe to assume that an address for which this function returns * false is an externally-owned account (EOA) and not a contract. */ function isContract(address account) internal view returns (bool) { // This method relies in extcodesize, which returns 0 for contracts in // construction, since the code is only stored at the end of the // constructor execution. uint256 size; // solhint-disable-next-line no-inline-assembly assembly { size := extcodesize(account) } return size > 0; }}// https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/math/SafeMath.sol/** * @dev Wrappers over Solidity's arithmetic operations with added overflow * checks. * * Arithmetic operations in Solidity wrap on overflow. This can easily result * in bugs, because programmers usually assume that an overflow raises an * error, which is the standard behavior in high level programming languages. * `SafeMath` restores this intuition by reverting the transaction when an * operation overflows. * * Using this library instead of the unchecked operations eliminates an entire * class of bugs, so it's recommended to use it always. */library SafeMath { /** * @dev Returns the addition of two unsigned integers, reverting on * overflow. * * Counterpart to Solidity's `+` operator. * * Requirements: * - Addition cannot overflow. */ function add(uint256 a, uint256 b) internal pure returns (uint256) { uint256 c = a + b; require(c >= a, "SafeMath: addition overflow"); return c; } /** * @dev Returns the subtraction of two unsigned integers, reverting on * overflow (when the result is negative). * * Counterpart to Solidity's `-` operator. * * Requirements: * - Subtraction cannot overflow. */ function sub(uint256 a, uint256 b) internal pure returns (uint256) { require(b <= a, "SafeMath: subtraction overflow"); uint256 c = a - b; return c; } /** * @dev Returns the multiplication of two unsigned integers, reverting on * overflow. * * Counterpart to Solidity's `*` operator. * * Requirements: * - Multiplication cannot overflow. */ function mul(uint256 a, uint256 b) internal pure returns (uint256) { // Gas optimization: this is cheaper than requiring 'a' not being zero, but the // benefit is lost if 'b' is also tested. // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522 if (a == 0) { return 0; } uint256 c = a * b; require(c / a == b, "SafeMath: multiplication overflow"); return c; } /** * @dev Returns the integer division of two unsigned integers. Reverts on * division by zero. The result is rounded towards zero. * * Counterpart to Solidity's `/` operator. Note: this function uses a * `revert` opcode (which leaves remaining gas untouched) while Solidity * uses an invalid opcode to revert (consuming all remaining gas). * * Requirements: * - The divisor cannot be zero. */ function div(uint256 a, uint256 b) internal pure returns (uint256) { // Solidity only automatically asserts when dividing by 0 require(b > 0, "SafeMath: division by zero"); uint256 c = a / b; // assert(a == b * c + a % b); // There is no case in which this doesn't hold return c; } /** * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo), * Reverts when dividing by zero. * * Counterpart to Solidity's `%` operator. This function uses a `revert` * opcode (which leaves remaining gas untouched) while Solidity uses an * invalid opcode to revert (consuming all remaining gas). * * Requirements: * - The divisor cannot be zero. */ function mod(uint256 a, uint256 b) internal pure returns (uint256) { require(b != 0, "SafeMath: modulo by zero"); return a % b; }}// https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/drafts/Counters.sol/** * @title Counters * @author Matt Condon (@shrugs) * @dev Provides counters that can only be incremented or decremented by one. This can be used e.g. to track the number * of elements in a mapping, issuing ERC721 ids, or counting request ids. * * Include with `using Counters for Counters.Counter;` * Since it is not possible to overflow a 256 bit integer with increments of one, `increment` can skip the SafeMath * overflow check, thereby saving gas. This does assume however correct usage, in that the underlying `_value` is never * directly accessed. */library Counters { using SafeMath for uint256; struct Counter { // This variable should never be directly accessed by users of the library: interactions must be restricted to // the library's function. As of Solidity v0.5.2, this cannot be enforced, though there is a proposal to add // this feature: see https://github.com/ethereum/solidity/issues/4637 uint256 _value; // default: 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); }}/** * @dev Interface of the ERC165 standard, as defined in the * [EIP](https://eips.ethereum.org/EIPS/eip-165). * * Implementers can declare support of contract interfaces, which can then be * queried by others (`ERC165Checker`). * * For an implementation, see `ERC165`. */interface IERC165 { /** * @dev Returns true if this contract implements the interface defined by * `interfaceId`. See the corresponding * [EIP section](https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified) * to learn more about how these ids are created. * * This function call must use less than 30 000 gas. */ function supportsInterface(bytes4 interfaceId) external view returns (bool);}/** * @dev Implementation of the `IERC165` interface. * * Contracts may inherit from this and call `_registerInterface` to declare * their support of an interface. */contract ERC165 is IERC165 { /* * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7 */ bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7; /** * @dev Mapping of interface ids to whether or not it's supported. */ mapping(bytes4 => bool) private _supportedInterfaces; constructor () internal { // Derived contracts need only register support for their own interfaces, // we register support for ERC165 itself here _registerInterface(_INTERFACE_ID_ERC165); } /** * @dev See `IERC165.supportsInterface`. * * Time complexity O(1), guaranteed to always use less than 30 000 gas. */ function supportsInterface(bytes4 interfaceId) external view returns (bool) { return _supportedInterfaces[interfaceId]; } /** * @dev Registers the contract as an implementer of the interface defined by * `interfaceId`. Support of the actual ERC165 interface is automatic and * registering its interface id is not required. * * See `IERC165.supportsInterface`. * * Requirements: * * - `interfaceId` cannot be the ERC165 invalid interface (`0xffffffff`). */ function _registerInterface(bytes4 interfaceId) internal { require(interfaceId != 0xffffffff, "ERC165: invalid interface id"); _supportedInterfaces[interfaceId] = true; }}/** * @dev Required interface of an ERC721 compliant contract. */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); /** * @dev Returns the number of NFTs in `owner`'s account. */ function balanceOf(address owner) public view returns (uint256 balance); /** * @dev Returns the owner of the NFT specified by `tokenId`. */ function ownerOf(uint256 tokenId) public view returns (address owner); /** * @dev Transfers a specific NFT (`tokenId`) from one account (`from`) to * another (`to`). * * * * Requirements: * - `from`, `to` cannot be zero. * - `tokenId` must be owned by `from`. * - If the caller is not `from`, it must be have been allowed to move this * NFT by either `approve` or `setApproveForAll`. */ function safeTransferFrom(address from, address to, uint256 tokenId) public; /** * @dev Transfers a specific NFT (`tokenId`) from one account (`from`) to * another (`to`). * * Requirements: * - If the caller is not `from`, it must be approved to move this NFT by * either `approve` or `setApproveForAll`. */ 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;} // https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/token/ERC721/IERC721Receiver.sol/** * @title ERC721 token receiver interface * @dev Interface for any contract that wants to support safeTransfers * from ERC721 asset contracts. */contract IERC721Receiver { /** * @notice Handle the receipt of an NFT * @dev The ERC721 smart contract calls this function on the recipient * after a `safeTransfer`. This function MUST return the function selector, * otherwise the caller will revert the transaction. The selector to be * returned can be obtained as `this.onERC721Received.selector`. This * function MAY throw to revert and reject the transfer. * Note: the ERC721 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 `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))` */ function onERC721Received(address operator, address from, uint256 tokenId, bytes memory data) public returns (bytes4);}// https://github.com/OpenZeppelin/openzeppelin-solidity/blob/v2.3.0/contracts/token/ERC721/ERC721.solcontract ERC721 is ERC165, IERC721 { using SafeMath for uint256; using Address for address; using Counters for Counters.Counter; // Equals to `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))` // which can be also obtained as `IERC721Receiver(0).onERC721Received.selector` bytes4 private constant _ERC721_RECEIVED = 0x150b7a02; // Mapping from token ID to owner mapping (uint256 => address) private _tokenOwner; // Mapping from token ID to approved address mapping (uint256 => address) private _tokenApprovals; // Mapping from owner to number of owned token mapping (address => Counters.Counter) private _ownedTokensCount; // Mapping from owner to operator approvals mapping (address => mapping (address => bool)) private _operatorApprovals; /* * bytes4(keccak256('balanceOf(address)')) == 0x70a08231 * bytes4(keccak256('ownerOf(uint256)')) == 0x6352211e * bytes4(keccak256('approve(address,uint256)')) == 0x095ea7b3 * bytes4(keccak256('getApproved(uint256)')) == 0x081812fc * bytes4(keccak256('setApprovalForAll(address,bool)')) == 0xa22cb465 * bytes4(keccak256('isApprovedForAll(address,address)')) == 0xe985e9c * bytes4(keccak256('transferFrom(address,address,uint256)')) == 0x23b872dd * bytes4(keccak256('safeTransferFrom(address,address,uint256)')) == 0x42842e0e * bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)')) == 0xb88d4fde * * => 0x70a08231 ^ 0x6352211e ^ 0x095ea7b3 ^ 0x081812fc ^ * 0xa22cb465 ^ 0xe985e9c ^ 0x23b872dd ^ 0x42842e0e ^ 0xb88d4fde == 0x80ac58cd */ bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd; constructor () public { // register the supported interfaces to conform to ERC721 via ERC165 _registerInterface(_INTERFACE_ID_ERC721); } /** * @dev Gets the balance of the specified address. * @param owner address to query the balance of * @return uint256 representing the amount owned by the passed address */ function balanceOf(address owner) public view returns (uint256) { require(owner != address(0), "ERC721: balance query for the zero address"); return _ownedTokensCount[owner].current(); } /** * @dev Gets the owner of the specified token ID. * @param tokenId uint256 ID of the token to query the owner of * @return address currently marked as the owner of the given token ID */ function ownerOf(uint256 tokenId) public view returns (address) { address owner = _tokenOwner[tokenId]; require(owner != address(0), "ERC721: owner query for nonexistent token"); return owner; } /** * @dev Approves another address to transfer the given token ID * The zero address indicates there is no approved address. * There can only be one approved address per token at a given time. * Can only be called by the token owner or an approved operator. * @param to address to be approved for the given token ID * @param tokenId uint256 ID of the token to be approved */ 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); } /** * @dev Gets the approved address for a token ID, or zero if no address set * Reverts if the token ID does not exist. * @param tokenId uint256 ID of the token to query the approval of * @return address currently approved for the given token ID */ function getApproved(uint256 tokenId) public view returns (address) { require(_exists(tokenId), "ERC721: approved query for nonexistent token"); return _tokenApprovals[tokenId]; } /** * @dev Sets or unsets the approval of a given operator * An operator is allowed to transfer all tokens of the sender on their behalf. * @param to operator address to set the approval * @param approved representing the status of the approval to be set */ 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); } /** * @dev Tells whether an operator is approved by a given owner. * @param owner owner address which you want to query the approval of * @param operator operator address which you want to query the approval of * @return bool whether the given operator is approved by the given owner */ function isApprovedForAll(address owner, address operator) public view returns (bool) { return _operatorApprovals[owner][operator]; } /** * @dev Transfers the ownership of a given token ID to another address. * Usage of this method is discouraged, use `safeTransferFrom` whenever possible. * Requires the msg.sender to be the owner, approved, or operator. * @param from current owner of the token * @param to address to receive the ownership of the given token ID * @param tokenId uint256 ID of the token to be transferred */ 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); } /** * @dev Safely transfers the ownership of a given token ID to another address * If the target address is a contract, it must implement `onERC721Received`, * which is called upon a safe transfer, and return the magic value * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise, * the transfer is reverted. * Requires the msg.sender to be the owner, approved, or operator * @param from current owner of the token * @param to address to receive the ownership of the given token ID * @param tokenId uint256 ID of the token to be transferred */ function safeTransferFrom(address from, address to, uint256 tokenId) public { safeTransferFrom(from, to, tokenId, ""); } /** * @dev Safely transfers the ownership of a given token ID to another address * If the target address is a contract, it must implement `onERC721Received`, * which is called upon a safe transfer, and return the magic value * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise, * the transfer is reverted. * Requires the msg.sender to be the owner, approved, or operator * @param from current owner of the token * @param to address to receive the ownership of the given token ID * @param tokenId uint256 ID of the token to be transferred * @param _data bytes data to send along with a safe transfer check */ 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"); } /** * @dev Returns whether the specified token exists. * @param tokenId uint256 ID of the token to query the existence of * @return bool whether the token exists */ function _exists(uint256 tokenId) internal view returns (bool) { address owner = _tokenOwner[tokenId]; return owner != address(0); } /** * @dev Returns whether the given spender can transfer a given token ID. * @param spender address of the spender to query * @param tokenId uint256 ID of the token to be transferred * @return bool whether the msg.sender is approved for the given token ID, * is an operator of the owner, or is the owner of the token */ 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)); } /** * @dev Internal function to mint a new token. * Reverts if the given token ID already exists. * @param to The address that will own the minted token * @param tokenId uint256 ID of the token to be minted */ 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; _ownedTokensCount[to].increment(); emit Transfer(address(0), to, tokenId); } /** * @dev Internal function to burn a specific token. * Reverts if the token does not exist. * Deprecated, use _burn(uint256) instead. * @param owner owner of the token to burn * @param tokenId uint256 ID of the token being burned */ 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); } /** * @dev Internal function to burn a specific token. * Reverts if the token does not exist. * @param tokenId uint256 ID of the token being burned */ function _burn(uint256 tokenId) internal { _burn(ownerOf(tokenId), tokenId); } /** * @dev Internal function to transfer ownership of a given token ID to another address. * As opposed to transferFrom, this imposes no restrictions on msg.sender. * @param from current owner of the token * @param to address to receive the ownership of the given token ID * @param tokenId uint256 ID of the token to be transferred */ 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); } /** * @dev Internal function to invoke `onERC721Received` on a target address. * The call is not executed if the target address is not a contract. * * This function is deprecated. * @param from address representing the previous owner of the given token ID * @param to target address that will receive the tokens * @param tokenId uint256 ID of the token to be transferred * @param _data bytes optional data to send along with the call * @return bool whether the call correctly returned the expected magic value */ 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); } /** * @dev Private function to clear current approval of a given token ID. * @param tokenId uint256 ID of the token to be transferred */ function _clearApproval(uint256 tokenId) private { if (_tokenApprovals[tokenId] != address(0)) { _tokenApprovals[tokenId] = address(0); } }}contract MyERC721Card is ERC721{ struct Card { string name; // Name of the Card uint256 level; // Level of the Card } Card[] public cards; // First Item has Index 0 address public owner; constructor () public { owner = msg.sender; // owner of MyERC721Card contract who can create a new card } function mintCard(string memory name, address account) public { require(owner == msg.sender); // Only the Owner can create Items uint256 cardId = cards.length; // Unique card ID cards.push(Card(name, 1)); _mint(account, cardId); // Mint a new card }}
MyERC721Card.sol은 하나의 인터페이스(IERC165), 3개의 라이브러리(Address, SafeMath, Counters), 4개의 컨트랙트(ERC165, IERC721, IERC721Receiver, MyERC721Card)로 구성되어 있습니다.
IERC165인터페이스는 ERC-165 사양에 설명된 인터페이스를 정의합니다.Address라이브러리는account가 컨트랙트인지 아닌지를 테스트하는isContract메서드를 정의합니다.SafeMath라이브러리는 Solidity의 산술 연산에 대한 래퍼를 정의하며,uint256유형의 Solidity를 안전하게 계산하기 위해 오버플로우 검사를 추가했습니다.Counters라이브러리는 한 개씩만 증가 또는 감소할 수 있는 카운터를 정의합니다. 이는 ERC721 ID를 발행할 때 요소의 개수를 추적하는 데 사용됩니다.ERC165는IERC165인터페이스를 구현합니다.IERC721은 ERC-721 명세서에 설명된 인터페이스를 정의하며, 여기에는 ERC-165도 포함되어 있습니다.IERC721Receiver는MyERC721Card컨트랙트에서 사용되는onERC721Received를 정의합니다.ERC721은IERC721과ERC165를 구현합니다.MyERC721Card는ERC721을 사용하여 이름과 레벨이 있는 카드형 대체 불가능한 토큰을 구현하며,MyERC721Card컨트랙트의 소유자만 새로운 카드를 발행할 수 있습니다.
1.2 중요한 방법 살펴보기
몇 가지 중요한 방법을 자세히 살펴보겠습니다.
(1) ERC721의 constructor 및 _INTERFACE_ID_ERC721의 constructor
constructor는 아래와 같이 ERC-721 인터페이스에서 파생된 4바이트 해시인 _INTERFACE_ID_ERC721을 등록합니다.
/* * bytes4(keccak256('balanceOf(address)')) == 0x70a08231 * bytes4(keccak256('ownerOf(uint256)')) == 0x6352211e * bytes4(keccak256('approve(address,uint256)')) == 0x095ea7b3 * bytes4(keccak256('getApproved(uint256)')) == 0x081812fc * bytes4(keccak256('setApprovalForAll(address,bool)')) == 0xa22cb465 * bytes4(keccak256('isApprovedForAll(address,address)')) == 0xe985e9c * bytes4(keccak256('transferFrom(address,address,uint256)')) == 0x23b872dd * bytes4(keccak256('safeTransferFrom(address,address,uint256)')) == 0x42842e0e * bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)')) == 0xb88d4fde * * => 0x70a08231 ^ 0x6352211e ^ 0x095ea7b3 ^ 0x081812fc ^ * 0xa22cb465 ^ 0xe985e9c ^ 0x23b872dd ^ 0x42842e0e ^ 0xb88d4fde == 0x80ac58cd */ bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd; constructor () public { // register the supported interfaces to conform to ERC721 via ERC165 _registerInterface(_INTERFACE_ID_ERC721); }
등록 후, ERC-721 및 ERC-165의 supportsInterface 인터페이스는 _INTERFACE_ID_ERC721에 대해 호출될 때 true를 반환하며 이 컨트랙트가 ERC-721 인터페이스를 구현하고 있음을 알려줍니다.
(2) function balanceOf(address owner) public view returns (uint256 balance);
balanceOf는 ERC-721의 필수 메서드입니다. balanceOf는 owner의 계정에 있는 NFT의 수를 반환합니다.
function balanceOf(address owner) public view returns (uint256) { require(owner != address(0), "ERC721: balance query for the zero address"); return _ownedTokensCount[owner].current(); }
balanceOf는 owner가 _ownedTokensCount에 유지 관리하는 Counter 객체에서 현재 카운트를 반환할 뿐입니다.
// Mapping from owner to number of owned token mapping (address => Counters.Counter) private _ownedTokensCount;
(3) safeTransferFrom 및 transferFrom
이 함수는 주어진 토큰 ID의 소유권을 다른 주소로 전송합니다. ERC-721에서 요구하는 두 가지 safeTransferFrom 메서드가 있는데, 하나는 data가 있고 다른 하나는 data가 없습니다. 두 메서드 모두 data가 없는 메서드는 data를 ""로 설정한다는 점을 제외하면 서로 동일하게 작동합니다. safeTransferFrom는 아래와 같이 더 많은 검사가 포함된 transferFrom을 호출하며, safeTransferFrom은 ERC-721의 필수 메서드이기도 한 transferFrom보다 선호됩니다.
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 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); }
safeTransferFrom은 to 주소가 토큰을 받을 수 있는지 확인합니다. checkOnERC721Received에는 검증 로직이 있습니다. to 주소가 컨트랙트인 경우, 컨트랙트는 ERC-721의 onERC721Received 인터페이스를 구현하고 아래와 같이 정확한 4바이트 해시를 반환해야만 ERC-721 토큰을 수신할 수 있습니다.
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); }
_transferFrom은 실제로 아래와 같이 주어진 tokenId의 소유권을 이전합니다.
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); }
(4) function _mint(address to, uint256 tokenId) internal
_mint는 ERC-721의 일부가 아닙니다. 하지만 새로운 ERC-721 토큰을 생성하는 방법이 필요했고, 이번 구현에서는 아래와 같이 새로운 토큰을 생성하기 위해 _mint를 도입했습니다.
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; _ownedTokensCount[to].increment(); emit Transfer(address(0), to, tokenId); }
_mint는 내부 메서드이며 이 컨트랙트 내부에서 호출할 수 있습니다. MyERC721Card.sol에서 _mint는 MyERC721Card 컨트랙트의 mintCard 메 서드에서만 호출됩니다. 스마트 컨트랙트의 소유자만 mintCard를 호출할 수 있습니다.
function mintCard(string name, address account) public { require(owner == msg.sender); // Only the Owner can create Items uint256 cardId = cards.length; // Unique card ID cards.push(Card(name, 1)); _mint(account, cardId); // Mint a new card }
2. 스마트 컨트랙트 배포
Remix 온라인 IDE를 사용하거나 Truffle을 사용하여 MyERC721Card 스마트 컨트랙트 위에 배포할 수 있습니다.
2.1 Remix Online IDE를 사용하여 스마트 컨트랙트 배포하기
- Please visit Kaia Plugin for Remix and create a
MyERC721Cardcontract. 전체 소스코드는 ERC-721 스마트 컨트랙트 작성하기에서 확인하실 수 있습니다. - 컨트랙트를 배포할 계정을 생성합니다.
- If you do not have an account yet, create one at https://toolkit.kaia.io/account/accountKeyLegacy.
- Get some test KAIA from the faucet - https://faucet.kaia.io
- 아래와 같이
MyERC721Card.sol을 배포합니다.
이제 MyERC721Card가 출시되었습니다! ERC-721 호환 대체불가토큰인 카드를 발행하고 전송할 수 있습니다.
계정 0x2645BA5Be42FfEe907ca8e9d88f6Ee6dAd8c1410에 대해 아래와 같이 King 카드와 Queen 카드 2장을 발행하겠습니다.
이제 두 장의 카드를 발행했으며, 이 대체 불가능한 토큰인 MyERC721Card의 상태를 확인해 보겠습니다.
balanceOf는 계정0x2645BA5Be42FfEe907ca8e9d88f6Ee6dAd8c1410에 두 개의 카드가 있음을 보여줍니다.- 매개변수
1이 있는cards는 토큰 ID1의MyERC721Card가 레벨 1의Queen임을 보여줍니다. - 매개변수
0이 있는ownerOf는 토큰 ID0을 가진MyERC721Card의 소유자가0x2645BA5Be42FfEe907ca8e9d88f6Ee6dAd8c1410임을 나타냅니다.