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

After digging around a little I'd like to extend my previous answer:

Your OpenGL rendering is immediately done from within the scrollViewDidScroll: method while the UIKit drawing is performed later, during normal CATransaction updates from the runloop.

To synchronize UIKit updates with the OpenGL rendering just enclose both in one explicit transaction and flush it to force UIKit to commit the changes to backbaordd immediately:

- (void)scrollViewDidScroll:(UIScrollView *)aScrollView {
    [CATransaction begin];
    [overlayView updateVisibleRect:CGRectMake(...)];
    [openGlView updateVisibleRect:CGRectMake(...)];
    [CATransaction flush]; // trigger a UIKit and Core Animation graphics update
    [CATransaction commit];
}