Getting transform.rotation.z value from an animated view

Question

I'm spinning continuously an UIImageView using CABasicAnimation and I want to be able to get the rotation angle.

-(void)viewDidAppear:(BOOL)animated{

    [self spinAnimationOnView:self.myImageView
                        duration:2
                          repeat:HUGE_VAL];
}

- (void)spinAnimationOnView:(UIView*)view duration:(CGFloat)duration repeat:(float)repeat{
    CABasicAnimation* rotationAnimation;
    rotationAnimation = [CABasicAnimation animationWithKeyPath:@"transform.rotation.z"];
    rotationAnimation.toValue = [NSNumber numberWithFloat: M_PI * 2.0];
    rotationAnimation.duration = duration;
    rotationAnimation.repeatCount = repeat;

    [view.layer addAnimation:rotationAnimation forKey:@"rotationAnimation"];
}

The animation behaves as expected. But then I try to get the rotation angle like this:

CGFloat angle = [(NSNumber *)[self.myImageView valueForKeyPath:@"layer.transform.rotation.z"] floatValue];

NSLog(@"ROTATION = %f", angle);

And the output is always 0. Any thoughts?

Answer 1

While an animation is in progress, you need to use the CALayer's presentationLayer to get the state of the layer. See the CALayer docs.

CGFloat angle = [[self.myImageView valueForKeyPath:@"layer.presentationLayer.transform.rotation.z"] floatValue];

NSLog(@"ROTATION = %f", angle);