zoomByDelta_(delta) {
    const map = this.getMap();
    const view = map.getView();
    if (!view) {
      // the map does not have a view, so we can't act
      // upon it
      return;
    }
    const currentZoom = view.getZoom();
    if (currentZoom !== undefined) {
      const newZoom = view.getConstrainedZoom(currentZoom + delta);
      if (this.duration_ > 0) {
        if (view.getAnimating()) {
          view.cancelAnimations();
        }
        view.animate({
          zoom: newZoom,
          duration: this.duration_,
          easing: easeOut,
        });
      } else {
        view.setZoom(newZoom);
      }
    }
  }
@param {number} delta Zoom delta.
@private