Create basic functions

You now have the scaffolding for the pallet that includes a few custom types and storage items. It's time to start writing the functions that use the custom types and write changes to the storage items you've created. To keep the code modular and flexible, most of the logic is defined in internal helper functions with only one callable function that allow users to interact with the blockchain. One advantage of using internal functions is that you don't need to do any kind of authentication or authorization for the operations performed because the internal functions are only accessible to you as the runtime developer.

The only function you need to expose to users for this workshop is the create_collectible function. This function enables users to create new unique collectibles that are stored in the CollectibleMap and added to the OwnerOfCollectibles map. At a high level, you'll need to write functions to perform the following tasks:

  • Create a unique identifier for each collectible and not allow duplicates.
  • Limit the number of collectibles each account can own to manage the storage each user is allowed to consume.
  • Ensure the total number of collectibles doesn't exceed the maximum allowed by the u64 data type.
  • Allow users to generate new collectibles.

In addition to the basic functionality required to generate new collectibles, your pallet should provide some basic event and error handling. With this in mind, you'll also need to:

  • Create custom error messages to report what happened if something goes wrong.
  • Create a custom event to signal when a new collectible was successfully created.

Errors and events are fairly straightforward, so let's start with those declarations first.

Add custom errors

Here are some potential errors that the create_collectible function should address:

  • DuplicateCollectible - thrown when the collectible item trying to be created already exists.
  • MaximumCollectiblesOwned- thrown when an account exceeds the maximum amount of collectibles a single account can hold.
  • BoundsOverflow - thrown if the supply of collectibles exceeds the u64 limit.

To add the error handling to the runtime:

  1. Open the src/ file for the collectibles pallet in your code editor.
  2. Add the #[pallet::error] macro after the storage macros you previously defined.

  3. Define an enumerated data type with variants for the potential errors that the create_collectibles function might return.

    pub enum Error<T> {
    	/// Each collectible must have a unique identifier
    	/// An account can't exceed the `MaximumOwned` constant
    	/// The total supply of collectibles can't exceed the u64 limit
  4. Save your changes.
  5. Verify that your program compiles by running the following command:

    cargo build --package collectibles

    You can ignore the compiler warnings for now.

Add an event

The runtime can emit events to notify front-end applications about the result of a transaction that executed successfully. Block explorers like Subscan and the Polkadot/Substrate Portal Explorer also display events for completed transactions.

To add a CollectibleCreated event to the runtime:

  1. Open the src/ file for the collectibles pallet in your code editor.
  2. Add the RuntimeEvent from the frame_system configuration to the pallet configuration.

    pub trait Config: frame_system::Config {
    	type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
  3. Add the #[pallet::event] macro after the error macro you previously defined.

  4. Add the event for this pallet to the code.

    #[pallet::generate_deposit(pub(super) fn deposit_event)]
    pub enum Event<T: Config> {
    	 /// A new collectible was successfully created
    	 CollectibleCreated { collectible: [u8; 16], owner: T::AccountId },
  5. Save your changes.
  6. Verify that your program compiles by running the following command:

    cargo build --package collectibles

    You can ignore the compiler warnings for now.

Add internal and callable functions

With errors and events out of the way, it's time to write the core logic for creating collectibles.

  1. Create an internal function that generates the unique_id for new collectibles.

    // Pallet internal functions
    impl<T: Config> Pallet<T> {
    	// Generates and returns the unique_id and color
    	fn gen_unique_id() -> ([u8; 16], Color) {
    		// Create randomness
    		let random = T::CollectionRandomness::random(&b"unique_id"[..]).0;
    		// Create randomness payload. Multiple collectibles can be generated in the same block,
    		// retaining uniqueness.
    		let unique_payload = (
    	// Turns into a byte array
    	let encoded_payload = unique_payload.encode();
    	let hash = frame_support::Hashable::blake2_128(&encoded_payload);
    	// Generate Color 
    	if hash[0] % 2 == 0 {
    			(hash, Color::Red)
    	} else {
    			(hash, Color::Yellow)
  2. Create an internal function that enables minting new collectibles.

        // Function to mint a collectible
    	pub fn mint(
    		owner: &T::AccountId,
    		unique_id: [u8; 16],
    		color: Color,
    	) -> Result<[u8; 16], DispatchError> {
    		// Create a new object
    		let collectible = Collectible::<T> { unique_id, price: None, color, owner: owner.clone() };
    		// Check if the collectible exists in the storage map
    		ensure!(!CollectibleMap::<T>::contains_key(&collectible.unique_id), Error::<T>::DuplicateCollectible);
    		// Check that a new collectible can be created
    		let count = CollectiblesCount::<T>::get();
    		let new_count = count.checked_add(1).ok_or(Error::<T>::BoundsOverflow)?;
    		// Append collectible to OwnerOfCollectibles map
    		OwnerOfCollectibles::<T>::try_append(&owner, collectible.unique_id)
    			.map_err(|_| Error::<T>::MaximumCollectiblesOwned)?;
    		// Write new collectible to storage and update the count
    		CollectibleMap::<T>::insert(collectible.unique_id, collectible);
    		// Deposit the "CollectibleCreated" event.
    		Self::deposit_event(Event::CollectibleCreated { collectible: unique_id, owner: owner.clone() });
    		// Returns the unique_id of the new collectible if this succeeds
  3. Create the callable function that uses the internal functions.

    // Pallet callable functions
    impl<T: Config> Pallet<T> {
    	 /// Create a new unique collectible.
    	 /// The actual collectible creation is done in the `mint()` function.
    	 pub fn create_collectible(origin: OriginFor<T>) -> DispatchResult {
    		  // Make sure the caller is from a signed origin
    			let sender = ensure_signed(origin)?;
    			// Generate the unique_id and color using a helper function
    			let (collectible_gen_unique_id, color) = Self::gen_unique_id();
    			// Write new collectible to storage by calling helper function
    			Self::mint(&sender, collectible_gen_unique_id, color)?;
  4. Save your changes and verify that your program compiles by running the following command:

    cargo build --package collectibles

Your code should now compile without any warnings.