[core] Implement "smart setStyle"

[jfirebaugh]

Implements automatic "smart setStyle" behavior for existing style-setting APIs: Map::setStyleURL and Map::setStyleJSON. Both of these APIs will now smoothly transition between the prior and new style (providing equivalent layers/sources/etc. use the same IDs).

Besides the obvious behavioral change, several changes are worth calling out for consideration of their downstream impact:

• Paint property mutations now transition by default over a 300ms duration. This can be overridden via Map::setTransitionOptions (already existed), or within the style via the "transition" property (support added in this PR). A 300ms transition was always the intended default behavior according to the style specification, and is what's implemented in gl-js, but for whatever reason was not the default for gl-native. This change is included because it makes smart setStyle look cool.
• For Map::setStyleURL, the prior style remains in effect until the asynchronous network request for the new style has completed. Previously, the map would have an empty style in the interim. This caused flashing and flickering which, if not fixed, would be especially noticeable with smart setStyle.
• The internal Style object held by Map has a lifetime coincident with the Map lifetime itself. This was the most straightforward implementation of the prior point, but I think it's also a reasonable change on the grounds that it eliminates corner cases from the API. Specifically:
  • A Map now actually has an (empty) style from the get go. Previously, runtime styling APIs were explicitly no-ops or errors prior to a style being loaded.
  • Map::setStyleURL/setStyleJSON update the existing internal style object in place, rather than creating a new one. There isn't currently a Map::getStyle method, so this is unlikely to be an externally visible change. If we add Map::setStyle(std::unique_ptr<Style>), it will replace the existing style object, rather than updating it in place.

Fixes #7893.

TODO:

• Enable set-style-* integration tests and make sure they pass

[brunoabinader]

Beautiful @jfirebaugh!

[tmpsantos]

RIP style classes

[jfirebaugh]

Updated the description and this is ready for review.

[1ec5]

Paint property mutations now transition by default over a 300ms duration.

#9270 tracks documenting this default on iOS and macOS.

[1ec5]

within the style via the "transition" property (support added in this PR)

Does the root property affect the transition to or from the style it’s on? If the former, then is it the case that, by the time the developer has the opportunity to set this property, the transition will’ve already commenced? If the latter, this documentation would need to be updated.

[1ec5]

A Map now actually has an (empty) style from the get go. Previously, runtime styling APIs were explicitly no-ops or errors prior to a style being loaded.

-[MGLMapView styleURL] currently compensates by returning the “default style URL” (currently Streets v10). I wonder if we should account for this change by making the property nullable instead of null-resettable or (if we’re concerned about backwards-compatibility) return about:blank as the URL. The iOS and macOS SDKs automatically load Streets unless otherwise configured, so this edge case would only occur while loading the initial style or if Streets fails to load (due to a missing access token).

/cc @boundsj

[jfirebaugh]

Does the root property affect the transition to or from the style it’s on?

The value from the new style takes precedence, so the former.

If the former, then is it the case that, by the time the developer has the opportunity to set this property, the transition will’ve already commenced?

No, the user has opportunities to override the style default. When using setStyleJSON, you can do:

map.setStyleJSON(newJSON);
map.setTransitionOptions({ Duration::zero() });

When using setStyleURL, you can call setTransitionOptions in an onDidFinishLoadingStyle handler.

[1ec5]

Sounds good. In that case, the MGLStyle.transition documentation should mention that it affects inter-style transitions as well.

A more natural way to expose transition options to the SDKs would involve something like -[MGLMapView setStyleURL:duration:delay:], since a transition is by definition distinct from the start and end states. With these changes, it would be feasible to implement such a method at the SDK level, but we might need to hold off until #6386 (comment) is completely unblocked.

[lbud]

🎉

[anandthakker]

Beautiful 🎉

[nitrag]

So to clarify, mapViewDidFinishLoadingMap won't fire after a style change? Only onDidFinishLoadingStyle?

Referencing discussion in: #6180

My current code:

func mapViewDidFinishLoadingMap(_ mapView: MGLMapView) {
        //Fired each time map is changed (changing styles)
        //
        // Set up Map Data/Styles only once!
        //
        guard let style = mapView.style else {
            print("Can't load styles yet! You shouldn't be seeing this!!")
            return
        }

        //add sources
        //add layers
        ....
}

[qrh672114236]

setStyleUrl

{y.mango_measure}[Setup]: loading style failed: Attempt to invoke virtual method 'java.lang.String okhttp3.HttpUrl.host()' on a null object reference