How to create a boolean observable that emits hot values with switchMap?

Question

How to create a boolean observable that emits hot values with switchMap?

Looking to develop a method named

isEmpty:Observable<boolean>

that generates a hot Observable<boolean> by employing a switchMap. Here's what I have so far:

  /**
   * Notifies observers when the store is empty.
   */
  protected notifyOnEmpty = new ReplaySubject<E[]>(1);

  /**
   * Check if the store is empty.
   * 
   * @return A hot {@link Observable<boolean>} indicating the status of the store (empty or not).
   * 
   * @example
     <pre>
    source.isEmpty();
    </pre>
  */
  isEmpty<E>():Observable<boolean> {
    const isCurrentlyEmpty = values(this.entries).length == 0;
    return this.notifyOnEmpty.pipe(startWith(isCurrentlyEmpty), 
                                   switchMap((entries:E[])=>entries.length == 0));
  }

The idea is that the store can use

notifyOnEmpty.next(Object.values(this.entries))

to inform subscribers about whether the store is empty.

However, I encounter an error with the switchMap statement:

[ts] Argument of type '(entries: E[]) => boolean' is not assignable to parameter of type '(value: E[], index: number) => ObservableInput'. Type 'boolean' is not assignable to type 'ObservableInput'. (parameter) entries: E[]

Any suggestions?

javascript angular typescript rxjs switchmap

Answer 1

Answer №1

The switchMap operator is utilized to choose a new observable for each value. A standard map function suffices to map each Array into a boolean:

import { map, startWith } from 'rxjs/operators';

// ...

isEmpty<E>():Observable<boolean> {
  return this.notifyOnEmpty.pipe(
    startWith(values(this.entries)), 
    map((entries:E[]) => entries.length == 0)
  );
}

Answer 2

The switchMap operator is utilized to choose a new observable for each value. A standard map function suffices to map each Array into a boolean:

import { map, startWith } from 'rxjs/operators';

// ...

isEmpty<E>():Observable<boolean> {
  return this.notifyOnEmpty.pipe(
    startWith(values(this.entries)), 
    map((entries:E[]) => entries.length == 0)
  );
}

How to create a boolean observable that emits hot values with switchMap?

Answer №1

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