Our applications state service uses a Firefly Semantics Slice Object Store ( OStore) to store an authenticated User instance.

export const OSK: OStoreStart = {
  USER: { value: null },
};
export interface OSKI extends KeyObsValueReset {
  USER: ObsValueReset;
}
const OS: OStore<OSKI> = new OStore<OSKI>(OSK);

The application will also use a reference an Observable<boolean> that notifies whether the user is authenticated.

const isAuthenticated$: Observable<boolean>

We wish to derive this Observable reference from the USER state in our object store.

Approach

We will pipe the value emitted by the USER obs Observable through map where we map it to a boolean value.

const isAuthenticated$: Observable<boolean> = OS.S.USER.obs.pipe(
  map((r) => {
    return !!r;
  })
);

Demo

When we update the value of USER to a non null value, true is logged. When USER is set to null false is logged:

Derived Reactive Observable State with Slice | Task

Scenario

Approach

Demo