Can't get a CALayer to update its drawLayer: DURING a bounds animation

Question

I'm trying to animate a custom UIView's bounds while also keeping its layer the same size as its parent view. To do that, I'm trying to animate the layers bounds alongside its parent view. I need the layer to call drawLayer:withContext AS its animating so my custom drawing will change size correctly along with the bounds.

drawLayer is called correctly and draws correctly before I start the animation. But I can't get the layer to call its drawLayer method on EACH step of the bounds animation. Instead, it just calls it ONCE, jumping immediately to the "end bounds" at the final frame of the animation.

// self.bg is a property pointing to my custom UIView
self.bg.layer.needsDisplayOnBoundsChange = YES;
self.bg.layer.mask.needsDisplayOnBoundsChange = YES;

[UIView animateWithDuration:2 delay:0 options:UIViewAnimationOptionCurveEaseOut|UIViewAnimationOptionAutoreverse|UIViewAnimationOptionRepeat animations:^{

    [CATransaction begin];
    self.bg.layer.bounds = bounds;
    self.bg.layer.mask.bounds = bounds;
    [CATransaction commit];

    self.bg.bounds = bounds;
} completion:nil];

Why doesn't the bounds report a change AS its animating (not just the final frame)? What am I doing wrong?

Answer 1

This might or might not help...

Many people are unaware that Core Animation has a supercool feature that allows you to define your own layer properties in such a way that they can be animated. An example I use is to give a CALayer subclass a thickness property. When I animate it with Core Animation...

CABasicAnimation* ba = [CABasicAnimation animationWithKeyPath:@"thickness"];
ba.toValue = @10.0f;
ba.autoreverses = YES;
[lay addAnimation:ba forKey:nil];

...the effect is that drawInContext: or drawLayer:... is called repeatedly throughout the animation, allowing me to change repeatedly the way the layer is drawn in accordance with its current thickness property value at each moment (an intermediate value in the course of the animation).

It seems to me that that might be the sort of thing you're after. If so, you can find a working downloadable example here:

https://github.com/mattneub/Programming-iOS-Book-Examples/tree/master/ch17p498customAnimatableProperty

Discussion (from my book) here:

http://www.apeth.com/iOSBook/ch17.html#_making_a_property_animatable