Layer size/bounds during a core animation - macos

Question

I am working on a Mac app for Mac OS X 10.6+ and need to redraw the contents of a CAOpenGLLayer while an animation is occurring. I think I've read up on all the necessary parts, but it is just not working for me. I set up the animation as follows:
[CATransaction setAnimationDuration:SLIDE_DURATION];
CABasicAnimation *drawAnim = [CABasicAnimation animationWithKeyPath:#"drawIt"];
[drawAnim setFromValue:[NSNumber numberWithFloat:0]];
[drawAnim setToValue:[NSNumber numberWithFloat:1]];
[drawAnim setDuration:SLIDE_DURATION];
[chartView.chartViewLayer addAnimation:drawAnim forKey:#"drawIt"];
chartFrame.origin.x += controlFrame.size.width;
chartFrame.size.width -= controlFrame.size.width;
chartView.chartViewLayer.frame = CGRectMake(chartFrame.origin.x,
chartFrame.origin.y,
chartFrame.size.width,
chartFrame.size.height);
The drawIt property is a custom property whose only purpose is to be called to draw the layer during the successive animation frames. It order to get this to work, you have to add this to the chartViewLayer's class:
+ (BOOL)needsDisplayForKey:(NSString *)key
{
if ([key isEqualToString:#"drawIt"])
{
return YES;
}
else
{
return [super needsDisplayForKey:key];
}
}
So this seems to all be working fine. However, I need to get the current (animated) size of the layer before drawing it. I've found various conflicting information on how to get this out of the presentation layer. Here is what I've tried when the layer's:
drawInCGLContext:(CGLContextObj)glContext
pixelFormat:(CGLPixelFormatObj)pixelFormat
forLayerTime:(CFTimeInterval)timeInterval
displayTime:(const CVTimeStamp *)timeStamp
method is called during the animation. I've tried getting the size using KVC, and by querying either the frame or the bounds.
CALayer *presentationLayer = [chartViewLayer presentationLayer];
//bounds.size.width = [[[chartViewLayer presentationLayer]
// valueForKeyPath:#"frame.size.width"] intValue];
//bounds.size.height = [[[chartViewLayer presentationLayer]
// valueForKeyPath:#"frame.size.height"] intValue];
//bounds.size = presentationLayer.bounds.size;
bounds.size = presentationLayer.frame.size;
NSLog(#"Size during animation: %f, %f", bounds.size.width, bounds.size.height);
In all cases, the returned value is the end result of the animation rather than the transitional values. My understanding is that using the presentationLayer should give the transitional values.
So is this just broken or am I missing some critical step? Thanks for any help.