ObservableUtils.ts

import { combineLatest, concat, interval, Observable } from 'rxjs';
import { debounceTime, map, skip, switchMap, take, tap } from 'rxjs/operators';

export interface ShowMessage {
  showMessage: boolean;
}

export class ObservableUtils {
  /**
   * Generate an observable that will emit twice, instantly and after delay. Second time the show message will be set to false.
   */
  public static showAndHide<T>(observable: Observable<T>, control: ShowMessage, delay: number): Observable<T> {
    return combineLatest([observable, observable.pipe(debounceTime(delay))]).pipe(
      map(([value, delayedValue]) => {
        control.showMessage = true;
        if (value === delayedValue || !value) {
          control.showMessage = false;
        }
        return value;
      })
    );
  }

  /**
   * Generate a new interval observable that will be reset when observable param emits
   * @param resetObservable observable that will reset interval every time it emits
   * @param intervalMillis interval time in milliseconds
   * @returns An interval observable
   */
  public static intervalWithReset(resetObservable: Observable<any>, intervalMillis: number): Observable<number> {
    return resetObservable.pipe(
      switchMap(() => {
        // SwitchMap generates new interval each time resetObservable emits and dropps previous inner observable
        return interval(intervalMillis);
      })
    );
  }

  /**
   * Generates an observable that will emit first top values and next emisions will be delayed with debounceTime
   * @param observable observable who will emit the values
   * @param dueTime delay time. Emisions between delays will be dropped
   * @param top number of items to be emited without delay
   * @returns An observable with some pipes
   */
  public static debounceAfterTop<T>(observable: Observable<T>, dueTime: number, top: number = 1): Observable<T> {
    return concat(observable.pipe(take(top)), observable.pipe(skip(top), debounceTime(dueTime)));
  }
}