weak event system

[yangwen]

I'm new to the arcGis JS API.

I'm trying to implement some fairly trivial and fundamental logic and came to the realization that there is a rather weak eventing system in the API.  I want to confirm my findings here.

In trying to implement a callback to trigger upon a zoom action, research led me to eventually find a sample code where extent changes are detected via a watcher on a MapView property.

From this example:  ArcGIS API for JavaScript Sandbox 

watchUtils.whenTrue(view, "stationary", function() {

// Get the new extent of the view only when view is stationary.
 if (view.extent) {
   
}

})

Really? An event as significant as zoom, drag are all inferred by watching the stationary flag?  

This article seems to confirm as such.  Disable all zooming on the view | ArcGIS API for JavaScript 4.5 

Or are there other APIs that I have missed? Thanks.

[YannCabon]

Hi Yang,

You don't really need to watch both properties. You can just watch for `stationary` and keep some info around:

function whenZoomChange(view, callback) {
  let oldScale;

  view.watch('stationary', (isStationary) => {
    if (!isStationary) {
      oldScale = view.scale;
    }
    else {
      if (oldScale !== view.scale) {
        callback();
      }
    }
  });
});
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On the academic discussion, we made the choice to favor watching properties over events as it gives more control over events.

- Events don't give us access to the current state of the emitter. If your code gets a mapview object and it's already zooming, without a property expressing that you cannot know it, while `stationary` gives out this information.

- Having events AND properties just blows up the API with redundant information.

[EvonFranklin]

Understood, that would give you guys more control over the API system then.