Animation in OpenGL ES view freezes when UIScrollView is dragged on iPhone

Question

I have an animated transparent OpenGL ES subview (a modification of Apple's template EAGLView class) which draws a rotating sphere. Just like Apple's example, CADisplayLink is used on devices where available.

On the same screen, there is a UIScrollView containing UIButtons that can be selected. When the user scrolls the UIScrollView, the animation of my EAGLView freezes. This behavior is reproduced on iOS Simulator 4.2 and on iPhone OS 3.1.3 on an iPhone 2G device.

Any ideas on what to do to prevent pause of the EAGLView, apart from coding my own scroll view?

Answer 1

Whether CADisplayLink fires during scrolls depends on the mode with which you add it to the run loop. Probably you have this, somewhere:

[displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];

UIApplication adds a run loop mode, UITrackingRunLoopMode, for 'tracking in controls', which includes when a scrollview is scrolling. So at that point the runloop is switched out of the default mode and hence your display link (and also any timers, NSURLConnections, etc, added in the default mode) will fail to fire until default mode is restored.

Quick fix: change your code to:

[displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];

UITrackingRunLoopMode is considered one of the common modes.

Spending too much time interrupting UIKit can lead to some very poor control responsiveness, so you need to be careful. It'd be to diverge from the topic massively, but although OpenGL is modal and therefore not particularly threading friendly, you can use an EAGLSharegroup to do rendering on a separate thread and then push it onto the main thread.

---

An example in (2016) Swift3...

let d = CADisplayLink(target: self, selector: #selector(ThisClassName.updateAlpha))
d.add(to: RunLoop.current, forMode: RunLoopMode.commonModes)


//and then, for example...
func updateAlpha() {
  let a = leader.layer.presentation()?.value(forKey: "opacity") as! CGFloat
  follower.alpha = a
  }