ngAfterViewInit() {
  const searchLessons$ = fromEvent<any>(this.input.nativeElement, 'keyup')
    .pipe(
      map(event => event.target.value),
      debounceTime(400),
      distinctUntilChanged(),
      // switchMap cancels prior calls.
      switchMap(search => this.loadLessons(search))
    );

  const initialLessons$ = this.loadLessons();

  this.lessons$ = concat(initialLessons$, searchLessons$);
}