Module frame_support::dispatch_context

Provides functions to interact with the dispatch context.

A Dispatch context is created by calling run_in_context and then the given closure will be executed in this dispatch context. Everyting run in this closure will have access to the same dispatch context. This also applies to nested calls of run_in_context. The dispatch context can be used to store and retrieve information locally in this context. The dispatch context can be accessed by using with_context. This function will execute the given closure and give it access to the value stored in the dispatch context.

FRAME integration

The FRAME macros implement UnfilteredDispatchable for each pallet Call enum. Part of this implementation is the call to run_in_context, so that each call to UnfilteredDispatchable::dispatch_bypass_filter or Dispatchable::dispatch will run in a dispatch context.

Example

use frame_support::dispatch_context::{with_context, run_in_context};

// Not executed in a dispatch context, so it should return `None`.
assert!(with_context::<(), _>(|_| println!("Hello")).is_none());

// Run it in a dispatch context and `with_context` returns `Some(_)`.
run_in_context(|| {
    assert!(with_context::<(), _>(|_| println!("Hello")).is_some());
});

#[derive(Default)]
struct CustomContext(i32);

run_in_context(|| {
    with_context::<CustomContext, _>(|v| {
        // Intitialize the value to the default value.
        assert_eq!(0, v.or_default().0);
        v.or_default().0 = 10;
    });

    with_context::<CustomContext, _>(|v| {
        // We are still in the same context and can still access the set value.
        assert_eq!(10, v.or_default().0);
    });

    run_in_context(|| {
        with_context::<CustomContext, _>(|v| {
            // A nested call of `run_in_context` stays in the same dispatch context
            assert_eq!(10, v.or_default().0);
        })
    })
});

run_in_context(|| {
    with_context::<CustomContext, _>(|v| {
        // We left the other context and created a new one, so we should be back
        // to our default value.
        assert_eq!(0, v.or_default().0);
    });
});

In your pallet you will only have to use with_context, because as described above run_in_context will be handled by FRAME for you.

Structs

• Value
  Abstraction over some optional value T that is stored in the dispatch context.

Functions

• run_in_context
  Run the given closure run in a dispatch context.
• with_context
  Runs the given callback in the dispatch context and gives access to some user defined value.