ERC721

pragma solidity ^0.6.2;
contract NFT{
    string public name = "Cocktail NFT";    // token名字
    string public symbol = "CNFT";      // token简称    
    uint256 public totalSupply =  1000000;  // 允许的最大发行量
    uint256 public maxTokenId;  // 已经发行的token数量
    mapping(address => uint256) public balances;    // 余额
    mapping(uint256 => bool) public tokenExists;    // token是否存在
    mapping(uint256 => address) public tokenOwners; // token拥有者
    mapping(address => mapping(address => uint256)) allowed;    // 代理
    address public owner;   // 铸币人
    event Approval(address,address,uint256);    
    event Transfer(address,address,uint256);
    constructor() public{
        owner = msg.sender; // 设置铸币人
    }
    // 查询token的拥有者
    function ownerOf(uint256 _tokenId) public view returns (address) {
        require(tokenExists[_tokenId]);
        return tokenOwners[_tokenId];
    }
    // 授权
    function approve(address _to,uint256 _tokenId) public{
        require(msg.sender ==ownerOf(_tokenId));    // 证明我是该token的拥有者
        require(msg.sender != _to);         
        allowed[msg.sender][_to] = _tokenId;    // 授权指定地址可以动用我的指定token
        emit Approval(msg.sender, _to, _tokenId);
    }
    // token代理人提币
    function takeOwnership(uint256 _tokenId) public{
        require(tokenExists[_tokenId]);     // 该token是存在的
        address oldOwner = ownerOf(_tokenId);   // 该token的拥有者地址
        require(msg.sender != oldOwner);
        require(allowed[oldOwner][msg.sender] == _tokenId);   // 该token的拥有者已对我授权可以动用该资产
        balances[oldOwner] -= 1;        
        tokenOwners[_tokenId] = msg.sender;   // 转账
        balances[msg.sender] += 1;
        emit Transfer(oldOwner, msg.sender,_tokenId);
    }
    // 转账
    function transfer(address _to, uint256 _tokenId) public{
        require(tokenExists[_tokenId]);     // 该token是存在的
        require(msg.sender == ownerOf(_tokenId));   // 该token的拥有者为我
        require(msg.sender != _to);     // 不允许自己给自己转账
        require(_to != address(0));     // 接收人不能为空
        balances[msg.sender] -= 1;
        tokenOwners[_tokenId] = _to;    // 转账
        balances[_to] += 1;
        emit Transfer(msg.sender, _to, _tokenId);
    }
    // 铸币
    function mint() public{
        maxTokenId += 1;
        require(maxTokenId <= totalSupply);
        balances[msg.sender] += 1;
        tokenExists[maxTokenId] = true;
        tokenOwners[maxTokenId] = msg.sender;
    }
    //  仅限管理员操作
    modifier onlyOwner(){
        require(msg.sender == owner, "No authority");
        _;
    }
}