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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 supportsInterfaceERC-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.sol
contract 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.

  • IERC165 인터페이스는 ERC-165 사양에 설명된 인터페이스를 정의합니다.
  • Address 라이브러리는 account가 컨트랙트인지 아닌지를 테스트하는 isContract 메서드를 정의합니다.
  • SafeMath 라이브러리는 Solidity의 산술 연산에 대한 래퍼를 정의하며, uint256 유형의 Solidity를 안전하게 계산하기 위해 오버플로우 검사를 추가했습니다.
  • Counters 라이브러리는 한 개씩만 증가 또는 감소할 수 있는 카운터를 정의합니다. 이는 ERC721 ID를 발행할 때 요소의 개수를 추적하는 데 사용됩니다.
  • ERC165IERC165 인터페이스를 구현합니다.
  • IERC721ERC-721 명세서에 설명된 인터페이스를 정의하며, 여기에는 ERC-165도 포함되어 있습니다.
  • IERC721ReceiverMyERC721Card 컨트랙트에서 사용되는 onERC721Received를 정의합니다.
  • ERC721IERC721ERC165를 구현합니다.
  • MyERC721CardERC721을 사용하여 이름과 레벨이 있는 카드형 대체 불가능한 토큰을 구현하며, MyERC721Card 컨트랙트의 소유자만 새로운 카드를 발행할 수 있습니다.

1.2 중요한 방법 살펴보기

몇 가지 중요한 방법을 자세히 살펴보겠습니다.

(1) ERC721의 constructor_INTERFACE_ID_ERC721constructor

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. balanceOfowner의 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();
}

balanceOfowner_ownedTokensCount에 유지 관리하는 Counter 객체에서 현재 카운트를 반환할 뿐입니다.


// Mapping from owner to number of owned token
mapping (address => Counters.Counter) private _ownedTokensCount;

(3) safeTransferFromtransferFrom

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 data""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);
}

safeTransferFromto 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 _mintMyERC721Card 컨트랙트의 mintCard 메서드에서만 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

ERC721-1-deploy

이제 MyERC721Card가 출시되었습니다! ERC-721 호환 대체불가토큰인 카드를 발행하고 전송할 수 있습니다.

계정 0x2645BA5Be42FfEe907ca8e9d88f6Ee6dAd8c1410에 대해 아래와 같이 King 카드와 Queen 카드 2장을 발행하겠습니다.

ERC721-2-mint-king ERC721-3-mint-queen

이제 두 장의 카드를 발행했으며, 이 대체 불가능한 토큰인 MyERC721Card의 상태를 확인해 보겠습니다.

ERC721-4-cards-status

  • balanceOf는 계정 0x2645BA5Be42FfEe907ca8e9d88f6Ee6dAd8c1410에 두 개의 카드가 있음을 보여줍니다.
  • 매개변수 1이 있는 cards는 토큰 ID 1MyERC721Card가 레벨 1의 Queen임을 보여줍니다.
  • 매개변수 0이 있는 ownerOf는 토큰 ID 0을 가진 MyERC721Card의 소유자가 0x2645BA5Be42FfEe907ca8e9d88f6Ee6dAd8c1410임을 나타냅니다.
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