Why does UIScrollView pause my CADisplayLink?

Question

I have a view backed by a CAEAGLLayer, which is inside a UIScrollView. When I begin scrolling, the CADisplayLink that calls the -draw method of openGL view stops getting called.

I verified that my runloop start / stop methods don't get called when scrolling. The -draw method simply doesn't get called as soon as scrolling begins, and resumes getting called as soon as scrolling ends.

Does UIKit stop a CADisplayLink from firing as soon as scrolling starts?

The display link is added to the run loop like this:

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

Maybe there is a conflict with this run loop mode and UIScrollView? Are there other run loop modes or alternative solutions to keep a CADisplayLink firing even when a UIScrollView is scrolling?

I thought there can be more than just one CADisplayLink in any application. Is that wrong?

Answer 1

You're not in NSDefaultRunLoopMode while scrolling a UIScrollView; you're in UITrackingRunLoopMode. So any timer scheduled only for the former won't fire in the latter. You can add your CADisplayLink to multiple run loop modes by calling addToRunLoop:forMode: repeatedly, or call it once with NSRunLoopCommonModes, which covers both modes.

They talked about this in detail, and other issues with integrating scroll views with GL, at WWDC 2012 in Session 223: "Enhancing User Experience with Scroll Views"; I recommend watching the video, as there's lots of other stuff in there that's likely relevant to your situation.

---

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
  }