402. 查詢 - ERC721 (ep.24)

1. 查詢該擁有者持有的NFT數量 function balanceOf(address account) external view returns(uint256 balance);

1. 給定一個位址owner, 回傳該位址所持有的NFT數量uint256
2. 由於每個位址address, 對應到持有數量uint256
3. 使用mapping(address => uint256) _balance 來儲存
4. 特殊檢查

• address 0 不能為NFT的持有者
• 如一個NFT的擁有者為0, 通常代表該NFT已經被銷毀, 或者尚未被發行

2. 查詢該NFT的持有者 function ownerOf(uint256 tokenId) external view returns (address owner);

1. 給定一個TokenID (uint256), 回傳該NFT的持有者address
2. 由於是每個Token ID(uint256)對應到持有者位址address
3. 使用 mapping(uint256 => address) _owners來儲存

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.23;

interface IERC20 {
    // Event
    // event Transfer(address indexed from, address indexed to, uint256 tokenId);
    // event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);
    // event ApprovalForAll(address indexed owner, address indexed operator, bool approved);
    
    //Query
    function balanceOf(address account) external view returns(uint256 balance);
    function ownerOf(uint256 tokenId) external view returns (address owner);

    //Transfer
    // function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) external;
    // function safeTransferFrom(address from, address to, uint256 tokenId) external;
    // function transferFrom(address from, address to, uint256 tokenId) external;

    // Approve
    // function approve(address to, uint256 tokenId) external;
    // function setApprovalforAll(address operator, bool _approved) external;
    // function getApproved(uint256 tokenId) external view returns (address operator);
    // function isApprovedForAll(address owner, address operator) external view returns(bool);
}

contract ERC721 is IERC721 {
    mapping(address => uint256) _balances;
    mapping(uint256 => address) _owners;

    function balanceOf(address owner) public view returns(uint256){
        require(owner != address(0), "ERROR: address 0 cannot be owner");
        return _balances[owner];
    }

    function ownerOf(uint256 tokenId) public view returns (address){
        address owner = _owners[tokenId];
        require(owner != address(0), "ERROR: tokenId is not valid Id");
        return owner;
    }
}

在 2022 年，我們該如何寫智能合約 - 24 - ERC721 實作 查詢篇