Unique ERC721-compatible collections specification.

Collection and token properties.

We don't specify any special requirements for common Unique collections. But ERC721-compatible collections MUST meet these requirements:

1. Collection's flag erc721metadata MUST be true // getting flags not available in solidity interfaces right now

2. Collection MAY have property {key: "baseURI", value: "<base URI of the tokens>"}

3. Collection MUST have such tokenPropertyPermission:
   {key: "URI", permission: {mutable: true, collectionAdmin: true, tokenOwner: false}} - token full URI

4. Collection MAY have such tokenPropertyPermission:
   {key: "URISuffix", permission: {mutable: true, collectionAdmin: true, tokenOwner: false}} - token URI suffix

Functions

UniqueNFT stub interface has to contain such functions:

supportsInterface (ERC165)

function supportsInterface(bytes4 interfaceID) external view returns (bool);

When called with interfaceID 0x5b5e139f (ERC721Metadata), it MUST return true if collection's flag erc721metadata is true, otherwise false.

Other interfaceID values are assumed to be processed as usual.

tokenURI

function tokenURI(uint256 tokenId) public view virtual override returns (string memory);

Definitions:

• "baseURI is non-empty" means that there is a property in the collection with key "baseURI" and value which IS NOT empty string.
• "URI is non-empty" means that there is a property in the token with key "URI" and value which IS NOT empty string.
• "URISuffix is non-empty" means that there is a property in the token with key "URISuffix" and value which IS NOT empty string. In other words, when baseURI and URI and s is empty string, tokenURI function MUST consider it as absence of this property at all.

In other words, reading of token properties URI and URISuffix should be safe and don't throw if there is no such tokenPropertyPermission.

Desired behaviour:

if the collections has flag erc721metadata value not true, throw an error.

Otherwise,

• if token property URI (safe property read) is non-empty, return it, token property URI
• otherwise, if the collection property baseURI is empty or absent, return "" (empty string)
• otherwise, if token property URISuffix (safe property read) is non-empty, return concatenation of collection property baseURI and token property URISuffix
• otherwise, return collection property baseURI

Example in pseudocode:

let isCollectionERC721MetadataCompatible = collection.flags.erc721metadata === true

let baseURI = collectionProperty("baseURI")
let URI = tokenProperty(tokenId, "URI")
let URISuffix = tokenProperty(tokenId, "URISuffix")

let baseURIIsOk = !!baseURI && baseURI.length > 0
let URIIsOk = !!URI && URI.length > 0
let suffixIsOk = !!URISuffix && URISuffix.length > 0

if (!isCollectionERC721MetadataCompatible) {
  throw;
} else if (URIIsOk) {
  return URI;
} else if (!baseURIIsOk) {
  return "";
} else if (suffixIsOk) {
  return baseURI + URISuffix;
} else {
  return baseURI;
}

A bit simpler implementation can be found here: ERC721URIStorage tokenURI implementation