[android] returning null instead of the Source object in the MapboxMap#removeSource when it fails

[osana]

closes #12526

[tobrun]

Thank you for picking this up @osana. Seeing that we return the source object to the user instead of true/false feels a bit strange to me. Take for example following codeblock:

geoJsonSource = mapboxMap.removeSource(geoJsonSource);
if(geoJsonSource == null){
 // we couldn’t remove source
}

I think it makes more sense following the core setup to return true or false on the public api. If we would do this, we should also need to fix removeLayer as well.

thoughts @mapbox/maps-android ?

[LukasPaczos]

I agree with @tobrun, remove methods might be more readable if they return boolean values indicating whether removal has been successful. Users can always fetch the layer/source before removal with get methods which should cover current use cases.

[osana]

@tobrun @LukasPaczos Do you think it makes sense to change removeLayer return value
in this PR?

[LukasPaczos]

Sure, shouldn't be an issue.

[LukasPaczos]

Every other remove method besides this one drops the reference to the layer, so the one kept in Java is probably going to become invalid. In order to make our intended logic possible, that a user fetches the layer with getLayer (or just keeps the reference from the start) and can keep after it's removed from the map (successfully or not), we need to keep the reference valid.

[osana]

@tobrun @LukasPaczos Capturing offline conversation of what needs to happen in this PR

1. MapboxMap has three removeLayer methods. At the moment two of them invalidate existing jav areferences to the removed layer.
   

   mapbox-gl-native/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/maps/MapboxMap.java

   Line 354 in 1ceea72

   * Removes the layer. Any references to the layer become invalid and should not be used anymore

   
   

   mapbox-gl-native/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/maps/MapboxMap.java

   Line 376 in 1ceea72

   * Removes the layer. Any other references to the layer become invalid and should not be used anymore

We need the above two methods keep the reference alive ( transfer ownership from native to java peer) as it is done in

mapbox-gl-native/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/maps/MapboxMap.java

Line 365 in 1ceea72

* Removes the layer. The reference is re-usable after this and can be re-added

TODO: javadoc update & native_map_view.cpp change

2. Add test to verify the above is working, ie the reference to Layer in java is still valid after the Layer was removed

3. MapboxMap has two methods to removeSource. They result in the same native method call.
   But Javadoc does not reflect that

mapbox-gl-native/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/maps/MapboxMap.java

Line 437 in 1ceea72

* Removes the source. Any references to the source become invalid and should not be used anymore

mapbox-gl-native/platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/maps/MapboxMap.java

Line 448 in 1ceea72

* Removes the source, preserving the reference for re-use

in both cases the reference should be kept alive

4. Add test to verify 2 is working, ie the reference to Source is still valid in java after the Source was removed

[osana]

@tobrun could you ask someone with more knowledge of c++ how this should be done . Lines 846 -848 are not correct.

[LukasPaczos]

Would be great to add test covering a scenario when removal fails. Otherwise LGTM 👍

[LukasPaczos]

NIT: new line

[LukasPaczos]

🚀