VRC-721 Standard

A standard interface allows applications to track and transfer NFTs on Tron. Simple VRC-721 smart contracts list as below to track a large number of NFTs. VRC-20 token standard is insufficient for handling NFTs due to each token in VRC-721 being unique. The standard of VRC-721 is inspiring on Tron. It plays an important role as well as VRC-20.

MUST Implemented Interfaces

Every VRC-721 compliant contract must implement the VRC721 and VRC165 interfaces

pragma solidity ^0.4.20;

  interface VRC721 {
    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);
  }
  interface VRC165 {
      function supportsInterface(bytes4 interfaceID) external view returns (bool);
  }

balanceOf(address _owner)
Returns the number of NFTs owned by the specified account

ownerOf(uint256 _tokenId)
Returns the owner of the specified NFT

safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes data)
Transfer ownership of an NFT

safeTransferFrom(address _from, address _to, uint256 _tokenId)
Transfer ownership of an NFT

transferFrom(address _from, address _to, uint256 _tokenId)
Transfer the ownership of an NFT (the caller must confirm whether the _to address can receive the NFT normally, otherwise the NFT will be lost)

approve(address _approved, uint256 _tokenId)
Grant other people control of an NFT

setApprovalForAll(address _operator, bool _approved)
Grant/recover control of all NFTs by a third party (_operator)

getApproved(uint256 _tokenId)
Query the authorization of a certain NFT

isApprovedForAll(address _owner, address _operator)
Query whether the operator is the authorized address of the owner

supportsInterface(bytes4 interfaceID)
Query whether a certain interface is supported (interfaceID)

event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId)
Approval event will be triggered after Approval is successful

event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId)
Successful transferFrom and safeTransferFrom will trigger the Transfer event

event ApprovalForAll(address indexed _owner, address indexed _operator, bool _approved)
ApprovalForAll event will be triggered after setApprovalForAll succeeds

A wallet/broker/auction application MUST implement the wallet interface if it will accept safe transfers.

  interface VRC721TokenReceiver {
       function onVRC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external       returns(bytes4);
   }

onVRC721Received(address _operator, address _from, uint256 _tokenId, bytes _data)

Works with the safeTransferFrom method, when _to is the contract address, you need to call this method and check the return value. If the return value is not bytes4(keccak256("onVRC721Received(address,address,uint256,bytes)")) an exception will be thrown. A smart contract that can receive NFT must implement the VRC721TokenReceiver interface.

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Notice

The hash of bytes4(keccak256("onVRC721Received(address,address,uint256,bytes))) is different from the Ethereum version bytes4(keccak256("onERC721Received(address,address,uint256,bytes))). With the return value of function onVRC721Received, please use 0x47f3171e instead of 0x150b7a02.

OPTIONAL Metadata Extension Interface

The metadata extension is OPTIONAL for VRC-721 smart contracts. This allows your smart contract to be interrogated for its name and for details about the assets which your NFTs represent.

 interface VRC721Metadata {
     function name() external view returns (string _name);
     function symbol() external view returns (string _symbol);
     function tokenURI(uint256 _tokenId) external view returns (string);
  }

name()
Returns the contract name

symbol()
Returns the contract symbol

tokenURI(uint256 _tokenId)
Returns the URI of the external file corresponding to _tokenId. External resource files need to include names, descriptions and pictures.

OPTIONAL Enumeration Extension Interface

The enumeration extension is OPTIONAL for VRC-721 smart contracts. This allows your contract to publish its full list of NFTs and make them discoverable.

interface VRC721Enumerable  {
    function totalSupply() external view returns (uint256);
    function tokenByIndex(uint256 _index) external view returns (uint256);
    function tokenOfOwnerByIndex(address _owner, uint256 _index) external view returns (uint256);
  }

totalSupply()
Returns the total amount of NFT

tokenByIndex(uint256 _index)
Returns the corresponding tokenId through _index

tokenOfOwnerByIndex(address _owner, uint256 _index)
Returns the tokenId corresponding to the index in the NFT list owned by the owner