Implement lockable currency

This guide shows you how to write a pallet that allows users to lock funds for staking and voting. The LockableCurrency trait is useful in the context of economic systems that enforce accountability by collateralizing fungible resources. You can use the Substrate staking pallet to manage locked funds in time-based increments.

In this guide, we will implement the set_lock, extend_lock and remove_lock methods in our own custom pallet.

Before you begin

You will need to have a pallet already integrated in a runtime to follow this guide. This guide assumes you are using a runtime that contains the Balances pallet to handle the accounts and balances for your chain. You can use the template pallet in the node template to follow.

Declare the necessary dependencies

The methods from LockableCurrency require us to import a few traits from frame_support.

  1. Ensure you have the following traits imported in the top section of your pallet:

    use frame_support::{
    	dispatch::DispatchResult,
    	traits::{Currency, LockIdentifier, LockableCurrency, WithdrawReasons},
    };
  2. Declare the LockIdentifier constant.

    To use LockableCurrency, you must declare a LockIdentifier. Given that it requires a [u8; 8] type, this identifier must be eight characters long.

    const EXAMPLE_ID: LockIdentifier = *b"example ";

    You will need this to declare the methods we'll be including later on.

Declare your custom pallet types

Defining your custom configuration type will allow your pallet to inherit the methods we want to implement.

  1. Define the lockable currency type, let's call it StakeCurrency:

    	type StakeCurrency: LockableCurrency<Self::AccountId, Moment = Self::BlockNumber>;
  2. Define a type to satisfy the associated type Balance from the Currency trait we're using:

    	type BalanceOf<T> =
    		<<T as Config>::StakeCurrency as Currency<<T as frame_system::Config>::AccountId>>::Balance;

    This ensures that our pallet has a type to handle the amount field in the methods we'll be implementing.

  3. Specify the new type for your runtime:

    	impl pallet_template::Config for Runtime {
    		type Event = Event;
    		type StakeCurrency = Balances; // <- add this line
    	}

    Passing in Balances ensures that your pallet's LockableCurrency methods have the same understanding of Balance than the pallet that handles accounts balances of your blockchain.

Create dispatchable functions using the required methods

The required methods are:

  • fn lock_capital: Locks the specified amount of tokens from the caller.
  • fn extend_lock: Extends the lock period.
  • fn unlock_all: Releases all locked tokens.
  • Write lock_capital, using T::StakeCurrency::set_lock:

    	#[pallet::weight(10_000 + T::DbWeight::get().writes(1))]
    	pub(super) fn lock_capital(
    		origin: OriginFor<T>,
    		#[pallet::compact] amount: BalanceOf<T>
    	) -> DispatchResultWithPostInfo {
    
    		let user = ensure_signed(origin)?;
    
    		T::StakeCurrency::set_lock(
    			EXAMPLE_ID,
    			&user,
    			amount,
    			WithdrawReasons::all(),
    		);
    
    		Self::deposit_event(Event::Locked(user, amount));
    		Ok(().into())
    	}
  • Write extend_lock, using T::StakeCurrency::extend_lock:

    	#[pallet::weight(1_000)]
    	pub(super) fn extend_lock(
    		origin: OriginFor<T>,
    		#[pallet::compact] amount: BalanceOf<T>,
    	) -> DispatchResultWithPostInfo {
    		let user = ensure_signed(origin)?;
    
    		T::StakeCurrency::extend_lock(
    			EXAMPLE_ID,
    			&user,
    			amount,
    			WithdrawReasons::all(),
    		);
    
    		Self::deposit_event(Event::ExtendedLock(user, amount));
    		Ok(().into())
    	}
  • Write unlock_all, using T::StakeCurrency::remove_lock:

    	#[pallet::weight(1_000)]
    	pub(super) fn unlock_all(
    		origin: OriginFor<T>,
    	) -> DispatchResultWithPostInfo {
    		let user = ensure_signed(origin)?;
    
    		T::StakeCurrency::remove_lock(EXAMPLE_ID, &user);
    
    		Self::deposit_event(Event::Unlocked(user));
    		Ok(().into())
    	}
  • Write the events for all three dispatchables:

    	#[pallet::event]
    	#[pallet::generate_deposit(pub(super) fn deposit_event)]
    	pub enum Event<T: Config> {
    		Locked(T::AccountId, BalanceOf<T>),
    		Unlocked(T::AccountId),
    		LockExtended(T::AccountId, BalanceOf<T>),
    	}

Examples