[ios] Calculate size of a MGLMapViewOpenGLImpl renderable from MGLMapView bounds

Fixes: #14807

platform/ios/src/MGLMapView+Impl.h

@@ -48,6 +48,9 @@ public:
     // the rendering context and reduce memory while in the background.
     virtual UIImage* snapshot() = 0;
 
+    // Called when UIView's layout has changed.
+    virtual void layoutChanged() {};
+
     // Called by the view delegate when it's time to render.
     void render();
 

platform/ios/src/MGLMapView+OpenGL.h

@@ -55,5 +55,6 @@ public:
     UIView* getView() override;
     void deleteView() override;
     UIImage* snapshot() override;
+    void layoutChanged() override;
     // End implementation of MGLMapViewImpl
 };

platform/ios/src/MGLMapView+OpenGL.mm

@@ -28,6 +28,14 @@
 
 @end
 
+namespace {
+CGFloat contentScaleFactor() {
+    return [UIScreen instancesRespondToSelector:@selector(nativeScale)]
+        ? [[UIScreen mainScreen] nativeScale]
+        : [[UIScreen mainScreen] scale];
+}
+} // namespace
+
 class MGLMapViewOpenGLRenderableResource final : public mbgl::gl::RenderableResource {
 public:
     MGLMapViewOpenGLRenderableResource(MGLMapViewOpenGLImpl& backend_)
@@ -124,9 +132,7 @@ void MGLMapViewOpenGLImpl::createView() {
     resource.glView = [[GLKView alloc] initWithFrame:mapView.bounds context:resource.context];
     resource.glView.delegate = resource.delegate;
     resource.glView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
-    resource.glView.contentScaleFactor = [UIScreen instancesRespondToSelector:@selector(nativeScale)]
-                                    ? [[UIScreen mainScreen] nativeScale]
-                                    : [[UIScreen mainScreen] scale];
+    resource.glView.contentScaleFactor = contentScaleFactor();
     resource.glView.contentMode = UIViewContentModeCenter;
     resource.glView.drawableStencilFormat = GLKViewDrawableStencilFormat8;
     resource.glView.drawableDepthFormat = GLKViewDrawableDepthFormat16;
@@ -137,7 +143,6 @@ void MGLMapViewOpenGLImpl::createView() {
     eaglLayer.presentsWithTransaction = NO;
 
     [mapView insertSubview:resource.glView atIndex:0];
-    size = resource.framebufferSize();
 }
 
 UIView* MGLMapViewOpenGLImpl::getView() {
@@ -247,6 +252,12 @@ UIImage* MGLMapViewOpenGLImpl::snapshot() {
     return resource.glView.snapshot;
 }
 
+void MGLMapViewOpenGLImpl::layoutChanged() {
+    const auto scaleFactor = contentScaleFactor();
+    size = { static_cast<uint32_t>(mapView.bounds.size.width * scaleFactor),
+             static_cast<uint32_t>(mapView.bounds.size.height * scaleFactor) };
+}
+
 EAGLContext* MGLMapViewOpenGLImpl::getEAGLContext() {
     auto& resource = getResource<MGLMapViewOpenGLRenderableResource>();
     return resource.context;

platform/ios/src/MGLMapView.mm

@@ -946,6 +946,10 @@ public:
 
     [self adjustContentInset];
 
+    if (_mbglView) {
+        _mbglView->layoutChanged();
+    }
+
     if (_mbglMap) {
         self.mbglMap.setSize([self size]);
     }