Question

I try to do this: There is an view which has been rotated already to some value, lets say 15 degrees.

Imagine a view rotated by 15 degrees.

Next, I only know, that I want this view to rotate from it’s current rotation to 40 degrees.

Imagine how that view now rotates from 15 to 40 degrees.

For practice, I want to do that with CAKeyFrameAnimation. But the thing is, all I can do there is to hard-specify from where to where. There’s no convenient “pick up current state” thing like UIView animations have, right? How would I go about it?

Also, I want to specify always 3 keyframes for that rotation, no matter how much the needed amount of rotation is.

Imagine how that view rotates from 15 to 40 degrees, using 3 keyframes to do so

But another big problem: It might happen, that another animation has to kick in with a different target. lets say a view is currently rotation from 20 to 100 degrees, and just somewhere inbetween the animation a new animation is kicked and wants that view to go to 15 degrees. With UIView animations that’s simple. How to do that with CAKeyFrameAnimation?

Basically I want it to behave like UIView animations which pick up current state 😉

CALayer* theLayer = rotatedView.layer;

CAKeyframeAnimation* animation;
animation = [CAKeyframeAnimation animationWithKeyPath:@"transform.rotation.z"];

animation.duration = 2;
animation.repeatCount = 1;
animation.removedOnCompletion = NO;
animation.fillMode = kCAFillModeForwards;

animation.values = [NSArray arrayWithObjects:
                    [NSNumber numberWithFloat:(10.0 / 180.0) * M_PI],
                    [NSNumber numberWithFloat:(20.0 / 180.0) * M_PI],
                    [NSNumber numberWithFloat:(30.0 / 180.0) * M_PI], nil];

[theLayer addAnimation:animation forKey:@"transform.rotation.z"];

Answer 1

You can use theLayer.transform or theLayer.affineTransform to get the current transform matrix (before the animation), then use iphone sdk CGAffineTransform getting the angle of rotation of an object to get the angle to start with.