Use a Struct in Storage
Goal
Use a StorageValue struct and use it in on_initialize.
Use Cases
Keep track of different accounts and balances for testing a pallet.
Overview
Creating a struct of similarly grouped storage items is a neat way to keep track of them.
They can be easier to reference than keeping individual StorageValue items separate this way.
In addition, they can be used to ease testing and genesis configuration.
This guide steps through the procedure of creating a struct in storage which:
- Keeps track of an initial amount (
issuance) - Keeps track of the account that receives that amount (
minter) - Keeps track of an account that can burn some amount (
burner) - Is (partially) used in
on_initialize
Steps
1. Create a your struct
Call it MetaData and declare its different types:
#[derive(Clone, Encode, Decode, Eq, PartialEq, RuntimeDebug, Default)]
pub struct MetaData<AccountId, Balance> {
issuance: Balance,
minter: AccountId,
burner: AccountId,
}
2. Declare the struct as a storage item
Use StorageValue to declare the struct as a new single item in storage:
#[pallet::storage]
#[pallet::getter(fn meta_data)]
pub(super) type MetaDataStore<T: Config> = StorageValue<_, MetaData<T::AccountId, T::Balance>, ValueQuery>;
3. Configure GenesisConfig and GenesisBuild
GenesisConfig
Use #[pallet::genesis_config] to declare the admin account that you'll use in your to
initialize values from your MetaData struct:
// Declare `admin` as type `T::AccountId`.
#[pallet::genesis_config]
pub struct GenesisConfig<T: Config> {
pub admin: T::AccountId,
}
// Give it a default value.
#[cfg(feature = "std")]
impl<T: Config> Default for GenesisConfig<T> {
fn default() -> Self {
Self {
admin: Default::default(),
}
}
}
GenesisBuild
Use #[pallet::genesis_build] to initialize the values of your struct, using admin to initialize the values
of type T::AccountId:
#[pallet::genesis_build]
impl<T: Config> GenesisBuild<T> for GenesisConfig<T> {
fn build(&self) {
MetaDataStore::<T>::put(MetaData {
issuance: Zero::zero(),
minter: self.admin.clone(),
burner: self.admin.clone(),
});
}
}
4. Use the struct in on_initialize()
Assign an amount to the issuance field of MetaData to be initialized when the chain is launched:
fn on_initialize(_n: T::BlockNumber) -> Weight {
let mut meta = MetaDataStore::<T>::get();
let value: T::Balance = 50u8.into();
meta.issuance = meta.issuance.saturating_add(value);
// Add the amount to the `minter` account in storage.
Accounts::<T>::mutate(&meta.minter, |bal| {
*bal = bal.saturating_add(value);
});
}
Examples
reward-coinexample pallet