In Substrate is there a way to use storage and functions from one custom module in another?

Question

I have seen the Substrate Tutorial on creating crates of individual Substrate Runtime modules here in order to re-use the functionality, but I wondered if there is a way for one custom module to access the storage or functions from another custom module?

Something along these lines:

/// In ModuleA

    pub type IndexType = u64;

    decl_storage! {
        trait Store for Module<T: Trait> as ModuleA {
                pub MyIndexCount get(my_index_count): Option<IndexType>;
        }
    }

And then inside ModuleB - what do I need to do to use/include the functionality of ModuleA, and how do I call it?

/// In ModuleB

    decl_module! {
    pub struct Module<T: Trait> for enum Call where origin: T::Origin {
        fn deposit_event<T>() = default;

        pub fn edit_index(origin) -> Result {
            let sender = ensure_signed(origin)?;

            // --->>>> I want to read some storage from ModuleA whilst inside ModuleB
            let c: IndexType = ReadStorageFromModuleA >>> my_index_count().ok_or("Storage Read Error: cannot get index")?;

            // change storage in ModuleA from ModuleB
            WriteToStorageInModuleA <MyIndexCount<T>>::put(&c + 1);

            Ok(())
            }
        }
    }    

Answer 1

If you are building a module (module2) which has a direct dependency on another module (module1), you must inherit module1's trait in module2's trait definition:

pub trait Trait: module1::Trait {
    ...
}

To access public storage items from module1 in module2, you need to do the following:

• Import the appropriate storage trait to access the storage API: StorageValue, StorageMap, etc...
• Access the public storage through module1's storage type
  • <module1::Something<T>>::get()
  • <module1::Something<T>>::put()
  • etc...

To access other public functions from module 1 in module 2, you need to use the Module type:

<module1::Module<T>>::public_function();

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

module1.rs

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

use support::{decl_module, decl_storage, StorageValue};

pub trait Trait: system::Trait {}

decl_storage! {
    trait Store for Module<T: Trait> as TemplateModule {
        pub Something: u32;
    }
}

decl_module! {
    pub struct Module<T: Trait> for enum Call where origin: T::Origin {
    }
}

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

module2.rs

use support::{decl_module, decl_event, StorageValue, dispatch::Result};
use system::ensure_signed;

use crate::module1;

pub trait Trait: module1::Trait {
    type Event: From<Event<Self>> + Into<<Self as system::Trait>::Event>;
}

decl_module! {
    /// The module declaration.
    pub struct Module<T: Trait> for enum Call where origin: T::Origin {
        fn deposit_event<T>() = default;

        pub fn get_value_directly(origin) -> Result {
            let who = ensure_signed(origin)?;
            let value = <module1::Something<T>>::get();
            Self::deposit_event(RawEvent::ValueIs(value, who));
            Ok(())
        }

        pub fn set_value_directly(origin, value: u32) -> Result {
            let _ = ensure_signed(origin)?;
            <module1::Something<T>>::put(value);
            Ok(())
        }

        pub fn get_value_public_function(origin) -> Result {
            let who = ensure_signed(origin)?;
            let value = <module1::Module<T>>::get_value();
            Self::deposit_event(RawEvent::ValueIs(value, who));
            Ok(())
        }
    }
}

decl_event!(
    pub enum Event<T> where <T as system::Trait>::AccountId {
        ValueIs(u32, AccountId),
    }
);