Flow Non-Fungible Token Standard

This standard defines the minimum functionality required to implement a safe, secure, and easy-to-use non-fungible token contract on the Flow blockchain

The version of the contracts in the master branch is the Cadence 1.0 version of the contracts and is not the same as the ones that are currently deployed to testnet and mainnet. See the cadence-0.42 branch for the currently deployed versions.

What is Cadence?

Cadence is the resource-oriented programming language for developing smart contracts on Flow.

Before reading this standard, we recommend completing the Cadence tutorials to build a basic understanding of the programming language.

Resource-oriented programming, and by extension Cadence, provides an ideal programming model for non-fungible tokens (NFTs). Users are able to store their NFT objects directly in their accounts and transact peer-to-peer. Learn more in this blog post about resources.

Import Addresses

The NonFungibleToken, ViewResolver, and MetadataViews contracts are already deployed on various networks. You can import them in your contracts from these addresses. There is no need to deploy them yourself.

Note: With the emulator, you must use the -contracts flag to deploy these contracts.

Core Types

Contracts that implement the NonFungibleToken interface are expected to utilize two resource interfaces:

• NFT - A resource interface that describes the structure of a single NFT.

• Collection - A resource interface that describes an object that can hold multiple NFTs of the same type and defines ways to deposit, withdraw, and query information about the stored NFTs.

  Users typically store one collection per NFT type, saved at a well-known location in their account storage.

  For example, all NBA Top Shot Moments owned by a single user are held in a TopShot.Collection stored in their account at the path /storage/MomentCollection.

Core Features

The NonFungibleToken contract defines the following set of functionality that should be included in each implementation:

Create a new NFT collection

Create a new collection using the Token.createEmptyCollection(nftType: Type) function.

This function MUST return an empty collection that contains no NFTs.

Users typically save new collections to a contract-defined location in their account and public a capability to their collection.

Withdraw an NFT

Withdraw an NFT from a Collection using the withdraw() function. This function emits the NonFungibleToken.Withdrawn event automatically.

Deposit an NFT

Deposit an NFT into a Collection using the deposit() function. This function emits the NonFungibleToken.Deposited event automatically.

⚠️ Important

In order to comply with the deposit function in the interface, an implementation MUST take a @{NonFungibleToken.NFT} resource as an argument. This means that anyone can send a resource object that conforms to {NonFungibleToken.NFT} to a deposit function. In an implementation, you MUST cast the token as your specific token type before depositing it or you will deposit another token type into your collection. For example:

/// `ExampleNFT` much be changed to the name of your contract
let token <- token as! @ExampleNFT.NFT

List NFTs in an account

Return a list of NFTs in a Collection using the getIDs function.

Return the NFT type that a collection can accept in a deposit

Return types of NFTs that a Collection can accept in a deposit using the getSupportedNFTTypes functions.

Get Available SubNFTs, if any

Some NFTs can own other NFTs, the standard provides a function that projects can optionally implement to return information the owned NFTs.

NFT Metadata

The primary documentation for metadata views is on the Flow developer portal. Please refer to that for the most thorough exploration of the views with examples.

NFT metadata is represented in a flexible and modular way using the standard proposed in FLIP-0636.

When writing an NFT contract, you should implement the MetadataViews.Resolver interface, which allows your NFT to utilize one or more metadata types called views.

Each view represents a different type of metadata, such as an on-chain creator biography or an off-chain video clip. Views do not specify or require how to store your metadata, they only specify the format to query and return them, so projects can still be flexible with how they store their data.

How to read metadata

This example shows how to read basic information about an NFT including the name, description, image and owner.

Source: get_nft_metadata.cdc

How to implement metadata

The example NFT contract shows a basic example for how to implement metadata views.

List of views

The views marked as Core views are considered the minimum required views to provide a full picture of any NFT. If you want your NFT to be able to be easily accessed and understood by third-party apps such as the Flow NFT Catalog it should implement all of them as a pre-requisite.

How to propose a new view

Please open a issue or a pull request to propose a new metadata view or changes to an existing view.

Feedback

We'd love to hear from anyone who has feedback. For example:

• Are there any features that are missing from the standard?
• Are the current features defined in the best way possible?
• Are there any pre and post conditions that are missing?
• Are the pre and post conditions defined well enough? Error messages?
• Are there any other actions that need an event defined for them?
• Are the current event definitions clear enough and do they provide enough information?
• Are there any openings for bugs or vulnerabilities that we are not noticing?

Please create an issue in this repository if there is a feature that you believe needs discussing or changing.

Comparison to other standards on Ethereum

This standard covers much of the same ground as ERC-721 and ERC-1155, but without most of the downsides.

• Tokens cannot be sent to contracts that don't understand how to use them, because an account needs to have a Receiver or Collection in its storage to receive tokens.
• If the recipient is a contract that has a stored Collection, the tokens can just be deposited to that Collection without having to do a clunky approve, transferFrom.
• Events are defined in the contract for withdrawing and depositing, so a recipient will always be notified that someone has sent them tokens with their own deposit event.
• This version can support batch transfers of NFTs. Even though it isn't explicitly defined in the contract, a batch transfer can be done within a transaction by just withdrawing all the tokens to transfer, then depositing them wherever they need to be, all atomically.
• Transfers can trigger actions because users can define custom Receivers to execute certain code when a token is sent.
• Easy ownership indexing: rather than iterating through all tokens to find which ones you own, you have them all stored in your account's collection and can get the list of the ones you own instantly.

How to test the standard

If you want to test out these contracts, we recommend either testing them with the Flow Playground or with the Visual Studio Code Extension.

If you are not making/testing any modifications to the standard contracts, they are already deployed to the addresses listed above and you can just import from those directly instead of deploying them yourself.

If you want to test changes to the standards, the steps to follow are:

1. Deploy ViewResolver.cdc
2. Deploy NonFungibleToken.cdc, importing ViewResolver.
3. Deploy ExampleNFT.cdc, importing NonFungibleToken.

Then you can experiment with some of the other transactions and scripts in transactions/ or even write your own. You'll need to replace some of the import address placeholders with addresses that you deploy to, as well as some of the transaction arguments.

Running automated tests

You can find automated tests written in the Cadence testing framework in the tests/ directory. Use flow test tests/test_example_nft.cdc to run these tests.

More tests, written in Go, are in the lib/go/test/ directory. They use the transaction templates package that is contained in the lib/go/templates/ directory. To run the Go tests, you can run make test from the repository root. Contract and transaction assets must be generated before individual tests can be run, so if you are wanting to run the tests individually via go test, you must run make generate from within the lib/go/ directory after every revision you make to the contract or transaction files.

License

The works in these files:

• ExampleNFT.cdc
• NonFungibleToken.cdc
• MetadataViews.cdc
• ViewResolver.cdc

are under the Unlicense.