Type Definition redux_rs::Middleware

type Middleware<State, Action> = fn(_: &mut Store<State, Action>, _: Action) -> Option<Action>;

Function signature for a middleware.

Middleware provides the possibility to intercept actions dispatched before they reach the reducer.

It receives a mutable reference to the store and the action currently dispatching. The return type is an Option to indicate whether or not to proceed in the dispatching chain. Some(Action) indicates to proceed with the specified action (might be changed to trigger further changes), None halts the complete chain, including the reducer and subscriptions.

Example

The following will decrement before incrementing, never actually incrementing.

type State = i8;

#[derive(Clone, Copy)]
enum Action {
    Increment,
    Decrement
}

fn shall_not_increment_middleware(store: &mut Store<State, Action>, action: Action) -> Option<Action> {
    match action {
        Action::Increment => Some(Action::Decrement),
        Action::Decrement => None
    }
}

fn reducer(state: &State, action: &Action) -> State {
    match action {
        Action::Increment => state + 1,
        Action::Decrement => state - 1
    }
}

let mut store = Store::new(reducer, 0);
store.add_middleware(shall_not_increment_middleware);