iOS - Detecting change of contents of CALayer

Question

I have a CALayer which has a CAKeyFrameAnimation.

Is there a way to detect the change of contents of CALayer?

It's like, whenever the contents (image) of CALayer is changed due to CAKeyFrameAnimation, I want to play a short sound with AVAudioPlayer.

UPDATE

I solved it by doing like this.

- (void) viewDidLoad
{
    // init CADisplayLink to catch the changing moment
    CADisplayLink *displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(checkContents:)];
    [displayLink setFrameInterval:6] // checking by every 0.1 sec (6 frames)
    [displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];
}

- (void) checkContents:(CADisplayLink *) sender;
{
    // _newImage and _oldImage are class variables. (UIImage)
    // animatingView is a `UIImageView` where its layer's contents is being changed by `CAKeyFrameAnimation`

    CALayer *presentLayer = [animatingView.layer presentationLayer];
    _newImage = presentLayer.contents;
    if ( ![_newimage isEqual:_oldImage] )
    {
        NSLog(@"Image changed! From %@ to %@", _oldImage, _newImage);
    }
    _oldImage = _newImage;
}

of course, do not forget to invalidate CADisplayLink when it is no more needed.

Answer 1

Seems like you need a CADisplayLink to do the job, like described here: Detecting collision, during a CAKeyFrameAnimation

Basically:

A CADisplayLink object is a timer object that allows your application to synchronize its drawing to the refresh rate of the display.

Your application creates a new display link, providing a target object and a selector to be called when the screen is updated. Next, your application adds the display link to a run loop.

Source: Apple's documentation on CADisplayLink