BS721 Spec: Non Fungible Tokens

BS721 is a specification for non-fungible tokens based on CosmWasm 721. The name and design is based on Ethereum's ERC721 standard, with some enhancements. The types in here can be imported by contracts that wish to implement this spec, or by contracts that call to any standard bs721 contract.

The specification is split into multiple sections, a contract may only implement some of this functionality, but must implement the base.

Base

This handles ownership, transfers, and allowances. These must be supported as is by all BS721 contracts. Note that all tokens must have an owner, as well as an ID. The ID is an arbitrary string, unique within the contract.

Messages

TransferNft{recipient, token_id} - This transfers ownership of the token to recipient account. This is designed to send to an address controlled by a private key and does not trigger any actions on the recipient if it is a contract.

Requires token_id to point to a valid token, and env.sender to be the owner of it, or have an allowance to transfer it.

SendNft{contract, token_id, msg} - This transfers ownership of the token to contract account. contract must be an address controlled by a smart contract, which implements the Bs721Receiver interface. The msg will be passed to the recipient contract, along with the token_id.

Requires token_id to point to a valid token, and env.sender to be the owner of it, or have an allowance to transfer it.

Approve{spender, token_id, expires} - Grants permission to spender to transfer or send the given token. This can only be performed when env.sender is the owner of the given token_id or an operator. There can be multiple spender accounts per token, and they are cleared once the token is transferred or sent.

Revoke{spender, token_id} - This revokes a previously granted permission to transfer the given token_id. This can only be granted when env.sender is the owner of the given token_id or an operator.

ApproveAll{operator, expires} - Grant operator permission to transfer or send all tokens owned by env.sender. This approval is tied to the owner, not the tokens and applies to any future token that the owner receives as well.

RevokeAll{operator} - Revoke a previous ApproveAll permission granted to the given operator.

Queries

OwnerOf{token_id, include_expired} - Returns the owner of the given token, as well as anyone with approval on this particular token. If the token is unknown, returns an error. Return type is OwnerOfResponse. If include_expired is set, show expired owners in the results, otherwise, ignore them.

Approval{token_id, spender, include_expired} - Return an approval of spender about the given token_id. Return type is ApprovalResponse. If include_expired is set, show expired owners in the results, otherwise, ignore them.

Approvals{token_id, include_expired} - Return all approvals that owner given access to. Return type is ApprovalsResponse. If include_expired is set, show expired owners in the results, otherwise, ignore them.

AllOperators{owner, include_expired, start_after, limit} - List all operators that can access all of the owner's tokens. Return type is OperatorsResponse. If include_expired is set, show expired owners in the results, otherwise, ignore them. If start_after is set, then it returns the first limit operators after the given one.

NumTokens{} - Total number of tokens issued

Receiver

The counter-part to SendNft is ReceiveNft, which must be implemented by any contract that wishes to manage BS721 tokens. This is generally not implemented by any BS721 contract.

ReceiveNft{sender, token_id, msg} - This is designed to handle SendNft messages. The address of the contract is stored in env.sender so it cannot be faked. The contract should ensure the sender matches the token contract it expects to handle, and not allow arbitrary addresses.

The sender is the original account requesting to move the token and msg is a Binary data that can be decoded into a contract-specific message. This can be empty if we have only one default action, or it may be a ReceiveMsg variant to clarify the intention. For example, if I send to an exchange, I can specify the price I want to list the token for.

Metadata

Queries

ContractInfo{} - This returns top-level metadata about the contract. Namely, name, symbol and uri.

NftInfo{token_id} - This returns metadata about one particular token. The return value is based on ERC721 Metadata JSON Schema, but directly from the contract, not as a Uri. Only the image link is a Uri.

AllNftInfo{token_id} - This returns the result of both NftInfo and OwnerOf as one query as an optimization for clients, which may want both info to display one NFT.

Enumerable

Queries

Pagination is achieved via start_after and limit. Limit is a request set by the client, if unset, the contract will automatically set it to DefaultLimit (suggested 10). If set, it will be used up to a MaxLimit value (suggested 30). Contracts can define other DefaultLimit and MaxLimit values without violating the BS721 spec, and clients should not rely on any particular values.

If start_after is unset, the query returns the first results, ordered lexicographically by token_id. If start_after is set, then it returns the first limit tokens after the given one. This allows straightforward pagination by taking the last result returned (a token_id) and using it as the start_after value in a future query.

Tokens{owner, start_after, limit} - List all token_ids that belong to a given owner. Return type is TokensResponse{tokens: Vec<token_id>}.

AllTokens{start_after, limit} - Requires pagination. Lists all token_ids controlled by the contract.