Platform bindings for within expression

[chloekraw]

Tracking the development of platform bindings for the new within expression (mapbox/mapbox-gl-native#16157; mapbox/mapbox-gl-native#16194)

If it's possible, because this isn't too complex, it would be great to get this merged this week before the next alpha release.

cc @julianrex @zmiao

[1ec5]

I think the most natural NSExpression implementation of this expression operator would be SELF IN %@ (alternatively %@ CONTAINS SELF), where %@ is an MGLFeature. Since we’d be overloading the IN and CONTAINS operators, this implementation will only recognize operands it can lexically detect as MGLFeature instances.

As always, if the developer needs something syntactically closer to JSON, they can use MGL_FUNCTION() or +[NSExpression expressionWithJSONObject:].

[1ec5]

This feature depends on gl-native v1.2.0.

[zmiao]

@1ec5 here is the Android binding for within expression, please have a look https://github.com/mapbox/mapbox-gl-native-android/pull/198/files, please be aware currently toGeoJSON is not implemented in MGLConversion.h, it also caused problems with android_conversion.hpp, extra adaptation is done in mapbox/mapbox-gl-native#16243