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  OEP: 3
  Title: Non-fungible Token Standard
  Author: tanyuan <tanyuan666@gmail.com>, zhoupw <zhoupw@gmail.com>, tonyclarking<tonyclarking@gmail.com>
  Type: Standard
  Status: Accepted
  Created: 2018-07-26

Table of Contents

Abstract

The OEP-5 Proposal is a standard interface for NFTs. This standard allows for the implementation of a standard API for NFTs within smart contracts.

Motivation

A standard NFT interface which allows NFTs on the DNA blockchain to be conveniently used by other applications.

Specification

Basic Methods

name

def name()

Returns a string, the name of the NFTs - e.g. "My Non-Fungibles".

symbol

def symbol()

Returns a string, the short symbol of the NFTs - e.g. "MNFT".

This symbol should be short (3-8 characters is recommended), with no whitespace characters or new-lines and should be limited to the uppercase latin alphabet (i.e. the 26 letters used in English).

totalSupply

def totalSupply()

Returns the total number of NFTs.

balanceOf

def balanceOf(address)

Returns the number of NFTs assigned to address.

The parameter address must be a 20-byte address. If not, this method must throw an exception.

Ownership Methods

ownerOf

def ownerOf(tokenId)

Returns the address currently marked as the owner of the NFT with id of tokenId. tokenId should be string or a bytearray. If the owner address of tokenId can't be determined, this method must throw an exception.

transfer

def transfer(to, tokenId)

Transfers the NFT from the owner address of tokenId to the to address. tokenId should be string or bytearray. This method can be called by either the token owner, the approved account for the NFT with the id tokenId or an address that has been approved to transfer all tokens for the token owner (see approvalForAll method). If the owner address of tokenId can't be determined, this method must throw an exception. to must be 20-byte address. If not, this method must throw an exception. After a transfer, if an address had been approved by the approve method, it should be cleared automatically.

transferMulti

def TransferMulti(args)

state = {
  to: <TO ADDRESS>,
  tokenId: <TOKEN ID>
}

The transferMulti method allows the transferring of multiple NFTs to multiple to addresses. The parameter is an array of objects, the object is a state struct, which contains two items: the receiver address to (which must be 20-byte address) and the tokenId of the NFT. If any of the transfers fail, all of the transfers must be failed, and the method must throw an exception.

approve

def approve(to, tokenId)

The approve method allows the to address to transfer the tokenId NFT. If this function is called again it overwrites the to address that can transfer the NFT. tokenId should be string or bytearray. The to address must be 20-byte address. If not, this method must throw an exception.

getApproved

def getApproved(tokenId)

Returns the address that is allowed to send the NFT with id tokenId.

Optional Methods

clearApproved

def clearApproved(tokenId)

The clearApproved method removes the address that was set by the approve method for the NFT with the id tokenId. tokenId should be string or bytearray. This method can only be called by the token owner. If not, this method must throw an exception.

approvalForAll

def approvalForAll(owner, to, approval)

The approvalForAll method grants permission to the to address to transfer NFTs on behalf of the owner address. approval should be true to grant permission and false to revoke permission. The owner and to addresses must be 20-byte address. If not, this method must throw an exception.

tokensOf

def tokensOf(address)

Returns a serialized DynamicList of the tokenIds of the NFTs assigned to address.

The parameter address must be a 20-byte address. If not, this method must throw an exception. This method should return the DynamicList of tokens in the serialized hex format.

takeOwnership

def takeOwnership(to, tokenId)

Either the token owner or the approved account can assign the ownership of the NFT with the id tokenId to the to address. tokenId should be string or bytearray. The parameter to must be a 20-byte address. If not, this method must throw an exception.

properties

def properties(tokenId)

Returns a serialized NVM object containing the properties for the given NFT. The NVM object must conform to the Metadata JSON Schema.

The parameter tokenId must correspond to a valid NFT. If not, this method must throw an exception.

Metadata JSON Schema

{
  "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."
    }
  }
}

Events

transfer

TransferEvent = RegisterAction("transfer", "from", "to", "tokenId")

The event must be triggered when NFTs are transferred, including zero value transfers. A token contract which creates new NFTs must trigger a transfer event with the from address set to null. A token contract which burns tokens must trigger a transfer event with the to address set to null when tokens are burned.

approval

ApprovalEvent = RegisterAction("approval", "from", "to", "tokenId")

The event must be triggered on any successful calls to approve.

Implementation

OEP-5 Template: Python Template