title: combineLatest is fired only for first event
body: <p>I have 3 events which I want to combine, and when one of them is fired,  to call the service. I used <code>combineLatest</code>, but it works only if the first event is fired by the <code>filterChanged</code>.</p>
  <p><code>filterChanged</code> is an local event, while the rest are events comed from the child component. How can I fix this, to emit one of the events, even from the page initialization?</p>
  <pre><code>filterChanged = new EventEmitter&lt;boolean&gt;();
  paginatorChanged = new EventEmitter&lt;MatPaginator&gt;();
  sortChanged = new EventEmitter&lt;{}&gt;();
  
  ngAfterViewInit(): void {
     combineLatest(
        this.paginatorChanged,
        this.sortChanged,
        this.filterChanged
     ).pipe(
        startWith([null, null, null]),
        switchMap(value =&gt; {
        const paginator = value[0];
        const sort = value[1];
        const filters = value[2];
      
        return this.service.getFiltered(paginator, sort, filters);
     })).subscribe(data =&gt; this.data = data);
  }    
  
  applyFilter(): void {
    this.filterChanged.emit(true);
  }
  
  onPaginatorChanged(paginator): void {
    this.paginatorChanged.emit(paginator);
  }
  
  onSortChanged(sort): void {
    this.sortChanged.emit(sort);
  }
  </code></pre>
  <p>thanks</p>