advance-erc721-at-course.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/utils/Counters.sol";

contract XXXERC721 is ERC721 {
    using Counters for Counters.Counter;

    uint256 public mintPrice = 0.005 ether;
    address private owner;
    Counters.Counter private _tokenIds;

    event GiveChange(address _to, uint256 _amount, uint256 _balance);
    event Withdraw(address _owner, uint256 _amount);
    
    constructor() ERC721("XXXNFT", "XXXNFT") {
        owner = msg.sender;
    }
    
    // TODO: 買 NFT
    function mintMultiple(address _to, uint256 _amount) public payable {
        require((mintPrice * _amount) <= msg.value, "Your payment is not enough");

        for (uint256 i = 0; i < _amount; i++) {
            uint256 tokenId = _tokenIds.current();
            safeMint(_to, tokenId);
            _tokenIds.increment();
        }

        uint256 balance = msg.value - (mintPrice * _amount);

        if (balance > 0) {
            payable(msg.sender).transfer(balance);
            emit GiveChange(_to, _amount, balance);
        }
    }

    function safeMint(address _to, uint256 _amount) private {
        _safeMint(_to, _amount);
    }

    // TODO: 轉傳多個 NFT
    function transferNft(uint256[] memory _tokens, address _to) public {
        for (uint256 i = 0; i < _tokens.length; i++) {
            safeTransferFrom(msg.sender, _to, _tokens[i]);
        }
    }

    // 銷毀 Token
    function burn(uint256 _tokenId) public {
      _burn(_tokenId);
    }

    // TODO: 退款
    function refund(uint256 _tokenId) external {
        require(msg.sender == ownerOf(_tokenId), "You are not the owner");
        _burn(_tokenId);
        payable(msg.sender).transfer(mintPrice);
    }

    // TODO: 項目方領錢
    function withdraw () external {
        require(msg.sender == owner, "You are not the owner~XDDD");
        uint256 balance = address(this).balance;
        payable(msg.sender).transfer(balance);

        emit Withdraw(msg.sender, balance);
    }
}