ERC721 "automatic" token URI

🧐 Motivation Current ERC721 minting with token URI requires manually specifying the URI per token.

To reduce gas cost, the OpenSea example uses concatenation of a base token URI and the tokenID. (TradeableERC721Token.sol)

📝 Details

We want to provide an ERC721 extension ERC721DefaultURI with a base URI parameter, which overrides ERC721Metadata’s tokenURI() with the concatenation. To discuss the API please comment on the forum thread about this.

[nachomazzara]

To put in here what we thought (omit non-changed code):

pragma solidity ^0.5.0;

import "../../GSN/Context.sol";
import "./ERC721.sol";
import "./IERC721Metadata.sol";
import "../../introspection/ERC165.sol";

contract ERC721Metadata is Context, ERC165, ERC721, IERC721Metadata {
    //...
    
    // Base Token URI
    string private _baseTokenURI;

    //...
    
    // **PROPOSED CHANGE**: 
    // - `external` to `public` (Allow 'super')
    // - Concat baseURI with tokenURI
    function tokenURI(uint256 tokenId) public view returns (string memory) { 
        return string(abi.encodePacked(_baseTokenURI, _tokenURI(tokenId)));
    }
    

    /**
     * @dev Internal returns an URI for a given token ID.
     * Throws if the token ID does not exist. May return an empty string.
     * @param tokenId uint256 ID of the token to query
     */
    function _tokenURI(uint256 tokenId) internal view returns (string memory) { 
        require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");
        return _tokenURIs[tokenId];
    }
    
    /**
     * @dev Internal function to set the base token URI.
     * @param uri string base URI to assign
     */
    function _setBaseTokenURI(string memory uri) internal {
        _baseTokenURI = uri;
    }
}

It continues following the _INTERFACE_ID_ERC721_METADATA = 0x5b5e139f;.

The idea is not only to use the tokenId as the path for the URI as used here but just to have full control of what to save.

_setBaseTokenURI should be exposed to authorized users.

I’m wondering if we should emit an event once the baseTokenURI is changed. Indexers can be reactive to this, updating all the metadata references.

[nventuro]

@nachomazzara from Decentraland commented that it is currently not possible for users to easily implement this feature (which they need) due to tokenURI() being external, which makes it impossible to call the base implementation via super. The workaround they’ve found is to override _setTokenURI and use a separate mapping, which is far from ideal and will probably be disallowed once Solidity 0.6.0 comes out.

If @logeekal will not end up tackling this, someone else should do it: we already have a concrete design proposal, and many users would be benefited by this feature.