javascript - View Animation of OpenLayers

When I learned View Animation of OpenLayers I encountered a part of the code that I did not understand.

The source address is http://openlayers.org/en/latest/examples/animation.html.

  function flyTo(location, done) {
    var duration = 2000;
    var zoom = view.getZoom();
    var parts = 2;
    var called = false;

    function callback(complete) {
      --parts;
      if (called) {
        return;
      }
      if (parts === 0 || !complete) {
        called = true;
        done(complete);
      }
    }

    view.animate({
      center: location,
      duration: duration
    }, callback);
    view.animate({
      zoom: zoom - 1,
      duration: duration / 2
    }, {
      zoom: zoom,
      duration: duration / 2

    }, callback);
  }

What does "parts" and "called" mean? I changed "parts" value to 1 or 3 and did not see any change. It could also execute this method when I deleted the "callback".