How to create a delayed loading indicator when working with ngrx/store?

This heavily relies on this question which deals with another matter.

In order to delay the loader, there must be a clear difference between the start of loaded from the caller's perspective and when the loader is actually displayed (i.e. isLoading = true).

The following code does the trick:

The reducer

export const loadingReducerInner = createReducer(
	initialLoadingState,
	on(LoadingSetVisibleAction, (state) => {
			return {
				isLoading: true,
				loaderMessage: state.loaderMessage
			};
	}),
	on(LoadingEndedAction, _ => {
			return {
				isLoading: false,
				loaderMessage: ""
			};
	})
);

export function loadingReducer(state: LoadingState, action: Action): LoadingState {
	return loadingReducerInner(state, action);
}

The effect

There are two possible things that might happen:

• the load is fast enough (<500ms in this case) and LoadingSetVisibleAction is not called because a LoadingEndedAction will be dispatched

• the load is slower and the loader is set to true before LoadingEndedAction is dispatched

  @Injectable()
  export class LoadingEffects {
  
    constructor(private actions$: Actions) { }
  
    startDelayTimer$ = createEffect(() => this.actions$.pipe(
    		ofType(LoadingStartedAction),
    		switchMap(_ => {
    			const ret$ = timer(500).pipe(
    				takeUntil( this.actions$.pipe(ofType(LoadingEndedAction))),
    				mapTo(LoadingSetVisibleAction())
    			);
    			return ret$;
    		})
    	));
  

  }

Of course, the caller must ensure that LoadingStartedAction is always paired with a LoadingEndedAction dispatch both on positive and negative (an exception is raised during the AJAX call) flows.

posted about 4 years ago

CC BY-SA 4.0

4y ago

Alexei‭

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