[core] Several methods assume style object is created

[tmpsantos]

We have several methods that are currently crashing because map.setStyleURL is async. For instance addLayer, addClass, isLoaded and several others. We should have a signal (MapChangeDidFinishLoadingStyle?) emitted right after MapContext::loadStyleJSON.

On Android for instance MapChangeDidFinishLoadingMap is used to indicate that the style was loaded as a whole but we need something to indicate that the style JSON arrived (and the style object was created). Otherwise we might ended up rendering a partial state and later add a custom layer or class.

This new signal would be better implemented after we get rid of the context thread (#2909) because we can safely call view.notifyMapChange from MapContext::loadStyleJSON after that.

Finally, we should maybe throw an runtime exception when calling the methods that depend on a style when the style json is not yet loaded.

/cc @brunoabinader @jfirebaugh

[brunoabinader]

Nice catch @tmpsantos - I think it would be useful if we can also add a onStyleLoaded observer that could e.g. trigger a callback on the client.

[1ec5]

@friedbunny, for iOS/macOS, how does this relate to #2775? Do we need to add a new delegate method, or is an existing one suitable?

[friedbunny]

for iOS/macOS, how does this relate to #2775? Do we need to add a new delegate method, or is an existing one suitable?

There isn’t currently a suitable delegate method. MapKit doesn’t care to inform you of style loading events, so there’s no analogue to draw upon.

[jfirebaugh]

I agree with the utility of a notification for this event -- it's style.load in gl-js. I'm less keen on expanding the MapChange enumeration, for reasons detailed in #6383, though I wouldn't block a PR for this on the full refactor suggested there.