title: Can CADisplayLink be run while in application is in background?
body: <p>I am using a CADisplayLink as a timer in my application. I am using a CADisplayLink because my app relies HEAVILY on the accuracy/precision of the CADisplayLink. NSTimer is not a suitable replacement in my case. </p>
  
  <p>The issue is that occasionally, I need the CADisplayLink to fire and call its selector, while the application is in the background. Is this even possible? I understand that the CADisplayLink is linked to the refresh rate of the application UI. Does this mean that, when my app is in the background, there is no UI, and thus it is impossible to use a CADisplayLink in this case? Or, is it possible to somehow use the CADisplayLink by changing the run loop that I add it to or by changing the <code>forMode</code> argument? or perhaps using some GCD trickery/work-around?</p>
  
  <p>Note: My app is using an AudioPlayer and thus I have background capabilities enabled. I am able to run other code while in the background. Just to test, I switched my CADisplayLink with an NSTimer, and the NSTimer fired and called its selector as I wanted it to. I just can't get the CADisplayLink to fire and I can't find any definitive documentation regarding the possibility of using it in the background</p>
  
  <p>Here is my current code pertaining to my display link:</p>
  
  <pre><code>func beginTimer(){
  
    dispatch_async(dispatch_get_main_queue()){
        self.displayLink = CADisplayLink(target: self, selector: "update")
        self.displayLink.addToRunLoop(NSRunLoop.currentRunLoop(), forMode: NSDefaultRunLoopMode)
    }
  }
  
  func update(){
    delegate?.executeCallback
   }
  </code></pre>