Commodities FRAME Pallet: NFTs for Substrate

This is a FRAME pallet that defines and implements a non-fungible token (NFT) interface as well as an interface for managing a set of such assets, including asset ownership, creation, destruction and transfer.

Interface

This package defines two public traits (Rust interfaces) for working with NFTs: the NFT trait and the UniqueAssets trait.

NFT Trait

The NFT trait uses two types to define a unique asset:

• ID: a URI for the asset
• Info: a set of attributes that uniquely describe the asset

Assets with equivalent attributes (as defined by the Info type) must have an equal ID and assets with different IDs must not have equivalent attributes.

UniqueAssets Trait

This trait is generic with respect to a type that implements the NFT trait; it defines the type abstractions and public functions needed to manage a set of unique assets.

Types

• AccountId: the type used to identify asset owners
• AssetLimit: the maximum number of assets, expressed as an unsigned 128-bit integer, that may exist in this set at once
• UserAssetLimit: the maximum number of assets, expressed as an unsigned 64-bit integer, that any single account may own from this set at once

Functions

• total() -> u128: returns the total number of assets in this set of assets
• burned() -> u128: returns the total number of assets from this set that have been burned
• total_for_account(AccountId) -> u64: returns the total number of asset from this set that are owned by a given account
• assets_for_account(AccountId) -> Vec<NFT>: returns the list of assets from this set that are owned by a given account
• owner_of(NFT::Id) -> AccountId: returns the ID of the account that owns the given asset from this set
• mint(AccountId, NFT::Info) -> Result<AssetID, DispatchError>: use the given attributes to create a new unique asset that belongs to this set and assign ownership of it to the given account
  • Failure cases: asset duplication, asset limit reached for set, asset limit for this set reached for account
• burn(NFT::Id) -> DispatchResult: destroy the given asset
  • Failure cases: asset doesn't exist
• transfer(AccountId, NFT::Id) -> DispatchResult: transfer ownership of the given asset from this set from its current owner to a given target account
  • Failure cases: asset doesn't exist, asset limit for this set reached for target account

Reference Implementation

The reference implementation defined in this project is referred to as a "commodity" - a unique asset that is designed for frequent trading. In order to optimize for this use-case, sorted lists of assets are stored per owner. Although maintaining a sorted list is trivial with Rust vectors, which implement a binary search API that can be used for sorted insertion, it introduces significant overhead when an asset is created because the entire list must be decoded from the backing trie in order to insert the new asset in the correct spot. Maintaining a sorted asset list is desireable for the commodity use case, however, because it allows assets to be efficiently located when destroying or transferring them. An alternative implementation, the Keepsake pallet, is in the works :rocket:

Tests

Refer to the mock runtime and provided tests to see the NFT implementation in action.

Acknowledgements

This project was inspired by works such as the following:

• The ERC-721 specification
• OpenZeppelin's ERC-721 implementation
• the original Substratekitties project, by @shawntabrizi
• Substratekitties from SubstrateCourse, by @xlc

Thanks to the following people who helped me overcome my relatively limited understanding of Rust.

• @JoshOrndoff
• @riusricardo
• @rphmeier
• @thiolliere
• @gnunicorn

Upstream

This project was forked from the Substrate DevHub Pallet Template.