View Animation of OpenLayers

Question

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".

Answer 1

callback is an optional so you don't need to pass it.
Then 'parts' and 'called' is not about OpenLayers.

• Epulari: What I want to know is their role in this place.