Question

I’m using animateWithDuration to have one of my UIVIew rotating endlessly. Now, I need to get the current value of my UIView’s rotation at a specific time.
I’ve tried to use the following function :

-(float)getCurrentRotation{
CGAffineTransform transform = ((UIImageView*)[self.subviews objectAtIndex:0]).transform;
return (atan2(transform.b, transform.a));
}

but it always returns the same value (M_PI/2) as it’s the value I’ve specified in my initial call to animateWithDuration :

[UIView animateWithDuration:4 delay:0 options:( UIViewAnimationOptionRepeat|UIViewAnimationOptionCurveLinear) animations:^{
    CGAffineTransform transform = CGAffineTransformMakeRotation(M_PI/2);
    ((UIImageView*)[self.subviews objectAtIndex:0]).transform = transform;
}
                 completion:^(BOOL finished){
                 }];

Is there a way to have the current value of the rotation ?

Thanks.

Answer 1

When using CoreAnimation based animation, your view properties (bounds, transform…) never actually change. What changes are the properties of the layer displayed on screen while animating (see this documentation page), which is accessible using view.layer.presentationLayer.

Thus, the following line will get you the actual transform at the time you ask for it:

CGAffineTransform transform = CATransform3DGetAffineTransform([(CALayer*)[[[self.subviews objectAtIndex:0] layer] presentationLayer] transform]);

You must link against and include the QuartzCore framework, which defines CALayer, for this to work.

#import <QuartzCore/QuartzCore.h>