How do I use storage and functions from one custom pallet in another?

Question

Is there a way for one custom pallet to access the storage or functions from another custom pallet?

For Example:

Pallet 1

#[pallet::storage]
pub(super) type MyValue<T: Config> = StorageValue<_, u64, ValueQuery>;

Pallet 2

pub fn do_something(origin: OriginFor<T>, new_value: u64) -> DispatchResult {
    let _sender = ensure_signed(origin)?;

    let value: u64 = how_to_read_my_value();

    if value < new_value {
        how_to_set_my_value(new_value);
    }

    Ok(())
}    

How could I implement how_to_read_my_value and how_to_set_my_value?

Answer 1

If you are building a pallet (pallet2) which has a direct dependency on another pallet (pallet1), you can you "tight coupling" inherit pallet1's Config in pallet2's Config definition:

pub trait Config: pallet1::Config {
    ...
}

To access public storage items from pallet1 in pallet2, you then just need to call the appropriate function on that storage item:

let value: u64 = pallet1::MyValue::<T>::get();

if value < new_value {
    pallet1::MyValue::<T>set(new_value);
}

To access other public functions from pallet1 in pallet2, you need to use the Pallet type where these functions are implmented:

pallet1::Pallet::<T>::public_function();

Here is a simple example of two pallets interacting in this way:

pallet1.rs

Note that all the things in this pallet are marked public (pub)

use frame_support::pallet_prelude::*;

#[frame_support::pallet]
mod pallet {
    pub trait Config: system::Config {}

    #[pallet::storage]
    pub(super) type MyValue<T: Config> = StorageValue<_, u64, ValueQuery>;

    // -- snip --
}

impl<T: Config> Pallet<T> {
    pub fn get_value() -> u32 {
        <Something<T>>::get()
    }
}

pallet2.rs

use frame_support::pallet_prelude::*;
use crate::pallet1;

mod pallet {
    pub trait Config: pallet1::Config {
        // -- snip --
    }

    #[pallet::call]
    impl<T: Config> Pallet<T> {
        pub fn get_value_directly(_origin: OriginFor<T>) -> Result {
            let value = pallet1::MyValue::<T>::get();
            Ok(())
        }

        pub fn set_value_directly(_origin: OriginFor<T>, value: u32) -> Result {
            pallet1::MyValue::<T>::put(value);
            Ok(())
        }

        pub fn get_value_public_function(_origin: OriginFor<T>) -> Result {
            let value = pallet1::Pallet::<T>::get_value();
            Ok(())
        }
    }
}