Make state an observable property of ol.source.Source

[pagameba]

This changes ol.source.Source to extend ol.Object and adds state as an observable property so that it is easier to detect when a source's state has changed.

[pagameba]

While I can confirm this works (change:state is triggered), I couldn't find a convenient place to add a test. Ideally it would be in the vectorsource.test.js file, but the change:state event is only triggered by specific format sources (eg ol.source.GeoJSON) and there are no existing tests for these that I can see. I can add a new test file (test/spec/ol/source/geojson.test.js or staticvectorsource.test.js) if its deemed desirable.

[elemoine]

I don't think there are new tests to add for this change.

[tschaub]

It looks to me like with this change the change event will not be dispatched when the state is changed (and the revision will not be incremented). There are some fragile and unguessable things that may break. Tests would prove otherwise.

[elemoine]

You're very right Tim. We need to be very careful here.

[ahocevar]

@pagameba Your application could also just listen for the change event and check the state in the listener. Then this pull request would be obsolete.

[pagameba]

Did you read my cautionary tale? change is fired from the source when features change. state becomes ready once and is still ready when features change. If you change features in response to change and testing for ready then you end up with recursion. This can be avoided by deregistering the event listener when state is ready for the first time, but it is very easy to get tripped up on this.

I'm happy to add tests, I just need some guidance on where to put them.

Also, probably dispatchChangeEvent() and seems like an omission.

[pagameba]

Discussed with @elemoine, @tschaub, and @ahocevar (in details I didn't always follow), there is a larger issue in the library concerning the use of ol.Object and propertychange vs ol.Observable and the more generic change event. It seems inadvisable to proceed with this PR until the larger architectural issue has been discussed and an approach selected.