How to synchronize OpenGL drawing with UIKit updates

Question

In our app we have UIScrollView above CAEAGLLayer. UIScrollView contains some UIViews (red rectangles). In CAEAGLLayer we draw white rectangles. Centers of white rectangles

Answer 1

In lack of a proper answer I'd like to share my thoughts:

There are two ways of drawing involved: Core Animation (UIKit) and OpenGL. In the end, all drawing is done by OpenGL but the Core Animation part is rendered in backboardd (or Springboard.app, before iOS 6) which serves as a kind of window server.

To make this work your app's process serializes the layer hierarchy and changes to its properties and passes the data over to backboardd which in turn renders and composes the layers and makes the result visible.

When mixing OpenGL with UIKit, the CAEAGLLayer's rendering (which is done in your app) has to be composited with the rest of the Core Animation layers. I'm guessing that the render buffer used by the CAEAGLLayer is somehow shared with backboardd to have a fast way of transferring your application's rendering. This mechanism does not necessarily has to be synchronized with the updates from Core Animation.

To solve your issue it would be necessary to find a way of synchronizing the OpenGL rendering with the Core Animation layer serialization and transfer to backboardd. Sorry for not being able to present a solution but I hope these thoughts and guesses help you to understand the reason for the problem.