PRC-721

1 PRC-721 Protocol Standard

PRC-721 is a set of standard interfaces designed for issuing non-fungible tokens (NFT) on the Pollux network. PRC-721 is fully compatible with ERC-721, ensuring interoperability and consistency with the widely adopted Ethereum standard. The PRC-721 standard facilitates the creation, transfer, and management of unique and indivisible digital assets on the Pollux blockchain, allowing developers and users to engage with NFTs in a manner consistent with established practices in the blockchain space.

1.1 PRC-721 Smart Contract Interface Implementation

Every contract adhering to the PRC-721 standard is required to incorporate the PRC721 and PRC165 interfaces. Additional extension interfaces can be implemented based on specific business needs.

1.1.1 PRC-721 & PRC-165 Interfaces

pragma solidity ^0.4.20;

interface PRC721 { 
// Returns the number of NFTs owned by the given account 
function balanceOf(address _owner) external view returns (uint256);

// Returns the owner of the given NFT
function ownerOf(uint256 _tokenId) external view returns (address);

// Transfer ownership of NFT
function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes data) external payable;

// Transfer ownership of NFT
function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;

// Transfer ownership of NFT
function transferFrom(address _from, address _to, uint256 _tokenId) external payable;

// Grants address ‘_approved’ the authorization of the NFT ‘_tokenId’
function approve(address _approved, uint256 _tokenId) external payable;

// Grant/revoke all NFTs’ authorization of the ‘_operator’
function setApprovalForAll(address _operator, bool _approved) external;

// Query the authorized address of NFT
function getApproved(uint256 _tokenId) external view returns (address);

// Query whether the ‘_operator’ is the authorized address of the ‘_owner’
function isApprovedForAll(address _owner, address _operator) external view returns (bool);

// The successful ‘transferFrom’ and ‘safeTransferFrom’ will trigger the ‘Transfer’ Event
event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);

// The successful ‘Approval’ will trigger the ‘Approval’ event
event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);

// The successful ‘setApprovalForAll’ will trigger the ‘ApprovalForAll’ event
event ApprovalForAll(address indexed _owner, address indexed _operator, bool _approved);
}
interface PRC165 { // Query whether the interface ‘interfaceID’ is supported function supportsInterface(bytes4 interfaceID) external view returns (bool); }

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

interface PRC721TokenReceiver {
    // This method will be triggered during the ‘safeTransferFrom’ execution if the ‘_to’ is the contract address.
    // The return value must be checked; if it is not bytes4(keccak256("onPRC721Received(address,address,uint256,bytes)")), an exception should be thrown.
    // Smart contracts capable of receiving NFTs must implement the PRC721TokenReceiver interface.
    function onPRC721Received(address _operator, address _from, uint256 _tokenId, bytes _data) external returns (bytes4);
}

1.1.2 OPTIONAL Metadata Extension Interface

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

Solidity

// Metadata extension for PRC-721 smart contracts.

interface PRC721Metadata {

    // Return the token name
    function name() external view returns (string _name);

    // Return the token symbol
    function symbol() external view returns (string _symbol);

    // Returns the URI of the external file corresponding to ‘_tokenId’.
    // External resource files should include names, descriptions, and pictures.
    function tokenURI(uint256 _tokenId) external view returns (string);
}

URI is a URI link describing the _tokenId asset, pointing to a JSON file that conforms to the PRC721 metadata description structure. When tokens are minted, each token needs to be assigned a unique URI:

JSON

{
	"title": "Asset Metadata",
	"type": "object",
	"properties": {
		"name": {
			"type": "string",
			"description": "Identifies the asset to which this NFT represents"
		},
		"description": {
			"type": "string",
			"description": "Describes the asset to which this NFT represents"
		},
		"image": {
			"type": "string",
			"description": "A URI pointing to a resource with mime type image/* representing the asset to which this NFT represents. Consider making any images at a width between 320 and 1080 pixels and aspect ratio between 1.91:1 and 4:5 inclusive."
		}
	}
}

1.1.3 OPTIONAL Enumeration Extension Interface

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

solidity

// Enumerable extension for PRC-721 smart contracts.

interface PRC721Enumerable {

    // Return the total supply of NFT
    function totalSupply() external view returns (uint256);

    // Return the corresponding ‘tokenId’ through ‘_index’
    function tokenByIndex(uint256 _index) external view returns (uint256);

    // Return the ‘tokenId’ corresponding to the index in the NFT list owned by the ‘_owner'
    function tokenOfOwnerByIndex(address _owner, uint256 _index) external view returns (uint256);
}