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Non Fungible Tokens

In contrast with fungible tokens, non-fungible tokens (NFT) are unitary and therefore unique. This makes NFTs ideal to represent ownership of assets such as a piece of digital content, or a ticket for an event.

As with fungible tokens, NFTs are not stored in the user's wallet, instead, each NFT lives in a NFT contract. The NFT contract works as a bookkeeper, this is: it is in charge of handling the creation, storage and transfers of NFTs.

In order for a contract to be considered a NFT-contract it has to follow the NEP-171 and NEP-177 standards. The NEP-171 & NEP-177 standards explain the minimum interface required to be implemented, as well as the expected functionality.

Reference Implementation We provide a reference implementation ready to be deployed and use. :::

Be mindful of not confusing an NFT with an NFT-marketplace. NFT simply store information (metadata), while NFT-marketplaces are contracts where NFT can be listed and exchanged for a price. :::


Minting an NFT

In order to create a new NFT (a.k.a. mint it) you need first to deploy an NFT contract and initialize it with an owner. Currently, the owner simply sets an internal variable (Contract.owner_id), meaning it is NOT the default owner of all minted NFTs.

Once deployed and initialized, you can call the nft_mint method. You will need to pass as parameters a unique id, an owner, the token's metadata, and (optionally) royalties. The metadata will include information such as the title, a description, and an URL to associated media.

# 1. Deploy the contract in a testnet account
near dev-deploy --wasmFile non_fungible_token.wasm

# 2. Initialize NFT contract

# 3. Mint an NFT
near call <nft-contract> nft_mint '{"token_id": "<token-unique-id>", "receiver_id": "<nft-owner-account>", "token_metadata": {"title": "<title>", "description": "<description>", "media": "<url>" }, "royalties": {"<account>" : <percentage>, "<account>" : <percentage>}}' --accountId <your-account>

See the metadata standard for the full list of TokenMetadata parameters. :::

Minting Collections

Many times people want to create multiple 100 copies of an NFT (this is called a collection). In such cases, what you actually need to do is to mint 100 different NFTs with the same metadata (but different token-id).

Royalties

You might have noticed that one of the parameters is a structure called royalties. Royalties enable you to create a list of users that should get paid when the token is sell in a marketplace. For example, if anna has 5% of royalties, each time the NFT is sell, anna should get a 5% of the selling price.


Querying Metadata

You can query the NFT's metadata by calling the nft_metadata.

near view <nft-contract> nft_metadata

Approving Users

You can authorize other users to transfer an NFT you own. This is useful, for example, to enable listing your NFT in a marketplace. In such scenario, you trust that the marketplace will only transfer the NFT upon receiving a certain amount of money in exchange.

near call <nft-contract> nft_approve '{
"token_id": "<token-unique-id>",
"account_id": "<authorized-account>",
"msg": "<json-structure>"
}' --accountId <your-account> --depositYocto 1

If the msg parameter is included, then a cross-contract call will be made to <authorized_account>.nft_on_approve(msg). Which in turn will make a callback to nft_resolve_transfer in your NFT contract. :::

Transferring an NFT

Transferring an NFT can happen in two scenarios: (1) you ask to transfer an NFT, and (2) an authorized account asks to transfer the NFT. In both cases, it is necessary to invoke the nft_transfer method, indicating the token id, the receiver, and an (optionally) an approval_id.

near call <nft-contract> nft_transfer '{"receiver_id": "<receiver-account>", "token_id": "<token-unique-id>"}' --accountId <your-account> --depositYocto 1

Attaching NFTs to a Call

Natively, only NEAR tokens (Ⓝ) can be attached to a method calls. However, the NFT standard enables to attach a non-fungible tokens in a call by using the NFT-contract as intermediary. This means that, instead of you attaching tokens directly to the call, you ask the NFT-contract to do both a transfer and a method call in your name.

near call <nft-contract> nft_transfer_call '{"receiver_id": "<receiver-contract>", "token_id": "<token_id>", "msg": "<a-string-message>"}' --accountId <your-account> --depositYocto 1
Optionally, a memo parameter can be passed to provide more information to your contract. :::

How Does it Work?

Assume you want to attach an NFT (🎫) to a call on the receiver contract. The workflow is as follows:

  1. You call nft_transfer_call in the NFT-contract passing: the receiver, a message, and the token-id of 🎫.
  2. The NFT contract transfers the NFT 🎫 to the receiver.
  3. The NFT contract calls receiver.nft_on_transfer(sender, token-owner, token-id, msg).
  4. The NFT contract handles errors in the nft_resolve_transfer callback.
  5. The NFT contract returns true if it succeeded.

The nft_on_transfer method

From the workflow above it follows that the receiver we want to call needs to implement the nft_on_transfer method. When executed, such method will know:

  • Who is sending the NFT, since it is a parameter
  • Who is the current owner, since it is a parameter
  • Which NFT was transferred, since it is a parameter.
  • If there are any parameters encoded as a message

The nft_on_transfer must return true if the NFT has to be returned to the sender.


Events

You can track real time events (such as transfers) by implementing the NFT Event Standards. Events are simple to use, because they are just login messages formatted in a standardize way. Since these logged messages are public, a service can then be built to track them in real time.