[iOS] Transparent GLKView

Tie layer opacity to view opacity.

Fixes #3038.

CHANGELOG.md

@@ -46,6 +46,7 @@ Known issues:
 - The user dot’s callout view is now centered above the user dot. It was previously offset slightly to the left. ([#3261](https://github.com/mapbox/mapbox-gl-native/pull/3261))
 - Fixed an issue with small map views not properly fitting annotations within bounds. (#[3407](https://github.com/mapbox/mapbox-gl-native/pull/3407))
 - The map will now snap to north. ([#3403](https://github.com/mapbox/mapbox-gl-native/pull/3403))
+- The map view’s background can now be transparent or translucent, as long as the style’s background layer is transparent or translucent and `MGLMapView.opaque` is set to `NO`. ([#3096](https://github.com/mapbox/mapbox-gl-native/pull/3096))
 
 ## iOS 3.0.1
 

platform/ios/src/MGLMapView.mm

@@ -157,6 +157,8 @@ public:
     MBGLView *_mbglView;
     std::shared_ptr<mbgl::SQLiteCache> _mbglFileCache;
     mbgl::DefaultFileSource *_mbglFileSource;
+    
+    BOOL _opaque;
 
     NS_MUTABLE_ARRAY_OF(NSURL *) *_bundledStyleURLs;
     
@@ -257,6 +259,7 @@ std::chrono::steady_clock::duration MGLDurationInSeconds(float duration)
 - (void)commonInit
 {
     _isTargetingInterfaceBuilder = NSProcessInfo.processInfo.mgl_isInterfaceBuilderDesignablesAgent;
+    _opaque = YES;
 
     BOOL background = [UIApplication sharedApplication].applicationState == UIApplicationStateBackground;
     if (!background)
@@ -450,6 +453,7 @@ std::chrono::steady_clock::duration MGLDurationInSeconds(float duration)
     _glView.drawableStencilFormat = GLKViewDrawableStencilFormat8;
     _glView.drawableDepthFormat = GLKViewDrawableDepthFormat16;
     _glView.contentScaleFactor = [UIScreen instancesRespondToSelector:@selector(nativeScale)] ? [[UIScreen mainScreen] nativeScale] : [[UIScreen mainScreen] scale];
+    _glView.layer.opaque = _opaque;
     _glView.delegate = self;
     [_glView bindDrawable];
     [self insertSubview:_glView atIndex:0];
@@ -714,6 +718,16 @@ std::chrono::steady_clock::duration MGLDurationInSeconds(float duration)
     [super updateConstraints];
 }
 
+- (BOOL)isOpaque
+{
+    return _opaque;
+}
+
+- (void)setOpaque:(BOOL)opaque
+{
+    _glView.layer.opaque = _opaque = opaque;
+}
+
 // This is the delegate of the GLKView object's display call.
 - (void)glkView:(__unused GLKView *)view drawInRect:(__unused CGRect)rect
 {