calls.md

Implement callable functions

The proof-of-existence pallet exposes two callable functions to users:

• create_claim() allows a user to claim the existence of a file with a hash.

• revoke_claim() allows the owner of a claim to revoke the claim.

These functions use the StorageMap to implement the following logic:

• If a claim is already in storage, then it already has an owner and cannot be claimed again.
• If a claim does not in storage, then it is available to be claimed and written to storage.

To implement this logic in the proof-of-existence pallet:

1. Open the pallets/template/src/lib.rs file in a text editor.

2. Replace the #[pallet::call] line with the following code block:

   // Dispatchable functions allow users to interact with the pallet and invoke state changes.
   // These functions materialize as "extrinsics", which are often compared to transactions.
   // Dispatchable functions must be annotated with a weight and must return a DispatchResult.
   #[pallet::call]
   impl<T: Config> Pallet<T> {
   	#[pallet::call_index(0)]
   	#[pallet::weight(Weight::default())]
   	pub fn create_claim(origin: OriginFor<T>, claim: T::Hash) -> DispatchResult {
   		// Check that the extrinsic was signed and get the signer.
   		// This function will return an error if the extrinsic is not signed.
   		let sender = ensure_signed(origin)?;
   
   		// Verify that the specified claim has not already been stored.
   		ensure!(!Claims::<T>::contains_key(&claim), Error::<T>::AlreadyClaimed);
   
   		// Get the block number from the FRAME System pallet.
   		let current_block = <frame_system::Pallet<T>>::block_number();
   
   		// Store the claim with the sender and block number.
   		Claims::<T>::insert(&claim, (&sender, current_block));
   
   		// Emit an event that the claim was created.
   		Self::deposit_event(Event::ClaimCreated { who: sender, claim });
   
   		Ok(())
   	}
   
   	#[pallet::call_index(1)]
   	#[pallet::weight(Weight::default())]
   	pub fn revoke_claim(origin: OriginFor<T>, claim: T::Hash) -> DispatchResult {
   		// Check that the extrinsic was signed and get the signer.
   		// This function will return an error if the extrinsic is not signed.
   		let sender = ensure_signed(origin)?;
   
   		// Get owner of the claim, if none return an error.
   		let (owner, _) = Claims::<T>::get(&claim).ok_or(Error::<T>::NoSuchClaim)?;
   
   		// Verify that sender of the current call is the claim owner.
   		ensure!(sender == owner, Error::<T>::NotClaimOwner);
   
   		// Remove claim from storage.
   		Claims::<T>::remove(&claim);
   
   		// Emit an event that the claim was erased.
   		Self::deposit_event(Event::ClaimRevoked { who: sender, claim });
   		Ok(())
   	}
   }

3. Save your changes and close the file.

4. Check that your code compiles by running the following command:

   cargo check -p node-template-runtime

You can refer to the node template solution if you get stuck.

Solution

#![cfg_attr(not(feature = "std"), no_std)]

// Re-export pallet items so that they can be accessed from the crate namespace.
pub use pallet::*;

#[frame_support::pallet]
pub mod pallet {
	use super::*;
	use frame_support::pallet_prelude::*;
	use frame_system::pallet_prelude::*;

	#[pallet::pallet]
	pub struct Pallet<T>(_);

	/// Configure the pallet by specifying the parameters and types on which it depends.
	#[pallet::config]
	pub trait Config: frame_system::Config {
		/// Because this pallet emits events, it depends on the runtime's definition of an event.
		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
	}

	// Pallets use events to inform users when important changes are made.
	// Event documentation should end with an array that provides descriptive names for parameters.
	#[pallet::event]
	#[pallet::generate_deposit(pub(super) fn deposit_event)]
	pub enum Event<T: Config> {
		/// Event emitted when a claim has been created.
		ClaimCreated { who: T::AccountId, claim: T::Hash },
		/// Event emitted when a claim is revoked by the owner.
		ClaimRevoked { who: T::AccountId, claim: T::Hash },
	}

	#[pallet::error]
	pub enum Error<T> {
		/// The claim already exists.
		AlreadyClaimed,
		/// The claim does not exist, so it cannot be revoked.
		NoSuchClaim,
		/// The claim is owned by another account, so caller can't revoke it.
		NotClaimOwner,
	}

	#[pallet::storage]
	pub(super) type Claims<T: Config> = StorageMap<_, Blake2_128Concat, T::Hash, (T::AccountId, BlockNumberFor<T>)>;

	// Dispatchable functions allow users to interact with the pallet and invoke state changes.
	// These functions materialize as "extrinsics", which are often compared to transactions.
	// Dispatchable functions must be annotated with a weight and must return a DispatchResult.
	#[pallet::call]
	impl<T: Config> Pallet<T> {
		#[pallet::call_index(0)]
		#[pallet::weight(Weight::default())]
		pub fn create_claim(origin: OriginFor<T>, claim: T::Hash) -> DispatchResult {
			// Check that the extrinsic was signed and get the signer.
			// This function will return an error if the extrinsic is not signed.
			let sender = ensure_signed(origin)?;

			// Verify that the specified claim has not already been stored.
			ensure!(!Claims::<T>::contains_key(&claim), Error::<T>::AlreadyClaimed);

			// Get the block number from the FRAME System pallet.
			let current_block = <frame_system::Pallet<T>>::block_number();

			// Store the claim with the sender and block number.
			Claims::<T>::insert(&claim, (&sender, current_block));

			// Emit an event that the claim was created.
			Self::deposit_event(Event::ClaimCreated { who: sender, claim });

			Ok(())
		}

		#[pallet::call_index(1)]
		#[pallet::weight(Weight::default())]
		pub fn revoke_claim(origin: OriginFor<T>, claim: T::Hash) -> DispatchResult {
			// Check that the extrinsic was signed and get the signer.
			// This function will return an error if the extrinsic is not signed.
			let sender = ensure_signed(origin)?;

			// Get owner of the claim, if none return an error.
			let (owner, _) = Claims::<T>::get(&claim).ok_or(Error::<T>::NoSuchClaim)?;

			// Verify that sender of the current call is the claim owner.
			ensure!(sender == owner, Error::<T>::NotClaimOwner);

			// Remove claim from storage.
			Claims::<T>::remove(&claim);

			// Emit an event that the claim was erased.
			Self::deposit_event(Event::ClaimRevoked { who: sender, claim });
			Ok(())
		}
	}
}