animating multiple CALayers, but not in the same time space

Question

I have several CALayers that I'm trying to animate to a new zPosition with each layer slightly delayed from the others.

Each animation should take 0.25 seconds and start 0.05 seconds after the previous animation started. At the end of each animation the layer will be removed from the layer tree.

I've successfully been using the -animationDidStop:finished: delegate method to remove my layers as they finish but I've not been able to order the animations properly.

Is it possible to schedule animations in this way, and how?

Answer 1

I'd still like to hear others suggestions but I think I've solved this.

I'm now creating specific CAAnimation objects and specifying their beginTime property. I was doing this earlier and it wasn't working, what I finally realized is that for the beginTime property to be honored the animation has to be added to an CAAnimationGroup.

So, my code looks like this:

NSArray *layers = /* layers to be animated away */
CGFloat startOffset = 0.01;

for (NSInteger index = 0; index < layers.count; index++) {
    CALayer *layer = [layers objectAtIndex:index];

    CABasicAnimation *zoomOut = [CABasicAnimation animationWithKeyPath:@"zPosition"];
    zoomOut.toValue = [NSNumber numberWithDouble:400.0];
    zoomOut.beginTime = index * startOffset;

    CAAnimationGroup *group = [CAAnimationGroup animation];
    group.animations = [NSArray arrayWithObject:zoomOut];
    group.delegate = self;

    [layer addAnimation:group forKey:@"zoomAway"];
}

Answer 2

I have found that the BeginTime property does indeed work without putting the animations into a group if you base it's value as a delta from the value returned by QuartzCore's CACurrentMediaTime() function.

e.g. anim.beginTime = CACurrentMediaTime() + 0.05;

Answer 3

I wish I had the rep to comment, but the reason that setting anim.beginTime to CACurrentMediaTime() works is revealed by some other docs:

AVCoreAnimationBeginTimeAtZero Use this constant to set the CoreAnimation's animation beginTime property to be time 0. The constant is a small, non-zero, positive value which prevents CoreAnimation from replacing 0.0 with CACurrentMediaTime. Available in iOS 4.0 and later. Declared in AVAnimation.h.

So the normal setting of beginTime to 0 is shorthand for setting it to CACurrentMediaTime(). So you can use the to stagger the start of various groups.