Skeleton and Rust Architecture

In this article, you'll learn about the basic architecture behind the NFT contract that you'll develop while following this "Zero to Hero" series.

You'll discover the contract's layout and you'll see how the Rust files are structured in order to build a feature-complete smart contract.

Skeleton Contract

You can find the skeleton contract in our GitHub repository

New to Rust?

If you are new to Rust and want to dive into smart contract development, our Quick-start guide is a great place to start.

Introduction

This tutorial presents the code skeleton for the NFT smart contract and its file structure.

Once every file and functions have been covered, we will guide you through the process of building the mock-up contract to confirm that your Rust setup works.

File structure

Following a regular Rust project, the file structure for this smart contract has:

nft-contract
├── Cargo.lock
├── Cargo.toml
├── README.md
└── src
    ├── approval.rs
    ├── enumeration.rs
    ├── lib.rs
    ├── metadata.rs
    ├── mint.rs
    ├── nft_core.rs
    ├── events.rs
    └── royalty.rs

The file Cargo.toml defines the code dependencies
The src folder contains all the Rust source files

Source files

Here is a brief description of what each source file is responsible for:

File	Description
approval.rs	Has the functions that controls the access and transfers of non-fungible tokens
enumeration.rs	Contains the methods to list NFT tokens and their owners
lib.rs	Holds the smart contract initialization functions
metadata.rs	Defines the token and metadata structure
mint.rs	Contains token minting logic
nft_core.rs	Core logic that allows you to transfer NFTs between users.
royalty.rs	Contains payout-related functions
events.rs	Contains events related structures

tip

Explore the code in our GitHub repository.

`approval.rs`

This allows people to approve other accounts to transfer NFTs on their behalf.

This file contains the logic that complies with the standard's approvals management extension. Here is a breakdown of the methods and their functions:

Method	Description
nft_approve	Approves an account ID to transfer a token on your behalf.
nft_is_approved	Checks if the input account has access to approve the token ID.
nft_revoke	Revokes a specific account from transferring the token on your behalf.
nft_revoke_all	Revokes all accounts from transferring the token on your behalf.
nft_on_approve	This callback function, initiated during `nft_approve`, is a cross contract call to an external contract.

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See full example on GitHub

You'll learn more about these functions in the approvals section of the Zero to Hero series.

`enumeration.rs`

This file provides the functions needed to view information about NFTs, and follows the standard's enumeration extension.

Method	Description
nft_total_supply	Returns the total amount of NFTs stored on the contract
nft_tokens	Returns a paginated list of NFTs stored on the contract regardless of their owner.
nft_supply_for_owner	Allows you view the total number of NFTs owned by any given user
nft_tokens_for_owner	Returns a paginated list of NFTs owned by any given user

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You'll learn more about these functions in the enumeration section of the tutorial series.

`lib.rs`

This file outlines what information the contract stores and keeps track of.

Method	Description
new_default_meta	Initializes the contract with default `metadata` so the user doesn't have to provide any input.
new	Initializes the contract with the user-provided `metadata`.

Keep in mind

The initialization functions (new, new_default_meta) can only be called once.

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See full example on GitHub

You'll learn more about these functions in the minting section of the tutorial series.

`metadata.rs`

This file is used to keep track of the information to be stored for tokens, and metadata. In addition, you can define a function to view the contract's metadata which is part of the standard's metadata extension.

Name	Description
TokenMetadata	This structure defines the metadata that can be stored for each token (title, description, media, etc.).
Token	This structure outlines what information will be stored on the contract for each token.
JsonToken	When querying information about NFTs through view calls, the return information is stored in this JSON token.
nft_metadata	This function allows users to query for the contact's internal metadata.

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You'll learn more about these functions in the minting section of the tutorial series.

`mint.rs`

Contains the logic to mint the non-fungible tokens

Method	Description
nft_mint	This function mints a non-fungible token.

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`nft_core.rs`

Core logic that allows to transfer NFTs between users.

Method	Description
nft_transfer	Transfers an NFT to a receiver ID.
nft_transfer_call	Transfers an NFT to a receiver and calls a function on the receiver ID's contract. The function returns `true` if the token was transferred from the sender's account.
nft_token	Allows users to query for the information about a specific NFT.
nft_on_transfer	Called by other contracts when an NFT is transferred to your contract account via the `nft_transfer_call` method. It returns `true` if the token should be returned back to the sender.
nft_resolve_transfer	When you start the `nft_transfer_call` and transfer an NFT, the standard also calls a method on the receiver's contract. If the receiver needs you to return the NFT to the sender (as per the return value of the `nft_on_transfer` method), this function allows you to execute that logic.

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You'll learn more about these functions in the core section of the tutorial series.

`royalty.rs`

Contains payout-related functions.

Method	Description
nft_payout	This view method calculates the payout for a given token.
nft_transfer_payout	Transfers the token to the receiver ID and returns the payout object that should be paid for a given balance.

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You'll learn more about these functions in the royalty section of the tutorial series.

`events.rs`

Contains events-related structures.

Method	Description
EventLogVariant	This enum represents the data type of the EventLog.
EventLog	Interface to capture data about an event.
NftMintLog	An event log to capture token minting.
NftTransferLog	An event log to capture token transfer.

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You'll learn more about these functions in the events section of the tutorial series.

Building the skeleton

If you haven't cloned the main repository yet, open a terminal and run:

git clone https://github.com/near-examples/nft-tutorial/

Next, go to the nft-contract-skeleton/ folder and build the contract with cargo-near:

cd nft-tutorial
cd nft-contract-skeleton/
cargo near build

Since this source is just a skeleton you'll get many warnings about unused code, such as:

   Compiling nft_contract_skeleton v0.1.0 (/Users/near-examples/Documents/my/projects/near/examples/nft-tutorial/nft-contract-basic)
 │ warning: unused imports: `LazyOption`, `LookupMap`, `UnorderedMap`, `UnorderedSet`
 │  --> src/lib.rs:3:29
 │   |
 │ 3 | use near_sdk::collections::{LazyOption, LookupMap, UnorderedMap, UnorderedSet};
 │   |                             ^^^^^^^^^^  ^^^^^^^^^  ^^^^^^^^^^^^  ^^^^^^^^^^^^
 │   |
 │   = note: `#[warn(unused_imports)]` on by default
 │
 │ warning: unused import: `Base64VecU8`
 │  --> src/lib.rs:4:28
 │   |
 │ 4 | use near_sdk::json_types::{Base64VecU8, U128};
 │   |

 │ warning: `nft_contract_skeleton` (lib) generated 48 warnings (run `cargo fix --lib -p nft_contract_skeleton` to apply 45 suggestions)
 │     Finished release [optimized] target(s) in 11.01s
 ✓ Contract successfully built!

Don't worry about these warnings, you're not going to deploy this contract yet. Building the skeleton is useful to validate that your Rust toolchain works properly and that you'll be able to compile improved versions of this NFT contract in the upcoming tutorials.

Conclusion

You've seen the layout of this NFT smart contract, and how all the functions are laid out across the different source files. Using yarn, you've been able to compile the contract, and you'll start fleshing out this skeleton in the next Minting tutorial.

Versioning for this article

At the time of this writing, this example works with the following versions:

rustc: 1.76.0
near-sdk-rs: 5.1.0
cargo-near: 0.6.1
NFT standard: NEP171, version 1.0.0

Skeleton and Rust Architecture

Introduction

File structure

Source files

`approval.rs`

`enumeration.rs`

`lib.rs`

`metadata.rs`

`mint.rs`

`nft_core.rs`

`royalty.rs`

`events.rs`

Building the skeleton

Conclusion

Concepts

Build

Tutorials

RPC

Tools

Resources

Introduction​

File structure​

Source files​

approval.rs​

enumeration.rs​

lib.rs​

metadata.rs​

mint.rs​

nft_core.rs​

royalty.rs​

events.rs​

Building the skeleton​

Conclusion​

Introduction

File structure

Source files

`approval.rs`

`enumeration.rs`

`lib.rs`

`metadata.rs`

`mint.rs`

`nft_core.rs`

`royalty.rs`

`events.rs`

Building the skeleton

Conclusion