iphone - How can I enforce an specific direction (i.e. clockwise) of rotation in Core Animation?

Question

I am rotating a view with this:

CGAffineTransform rotatedTransform = CGAffineTransformRotate(CGAffineTransformIdentity, rotationValue);

I have an object which I want to spin around for about 320 degrees. Now Core Animation is clever and just rotates it as much as needed, doing that by rotating it with -40 degrees. So the object rotates the other way around with a shorter amount of movement.

I want to constrain it to rotate clockwise. Would I have to do that by changing animations in little steps, or is there an more elegant way?

Answer 1

The following snippet rotates a view called someView by using key-framed animation. The animation consists of 3 frames spread over 1 second, with the view rotated to 0o, 180o and 360o in the first, second and last frames respectively. Code follows:

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

animation.duration = 1.0;
animation.cumulative = YES;
animation.repeatCount = 1;
animation.removedOnCompletion = NO;
animation.fillMode = kCAFillModeForwards;

animation.values = [NSArray arrayWithObjects:
    [NSNumber numberWithFloat:0.0 * M_PI],
    [NSNumber numberWithFloat:0.5 * M_PI],
    [NSNumber numberWithFloat:1.0 * M_PI], nil];

animation.keyTimes = [NSArray arrayWithObjects:
    [NSNumber numberWithFloat:0.0],
    [NSNumber numberWithFloat:0.5],
    [NSNumber numberWithFloat:1.0], nil];

animation.timingFunctions = [NSArray arrayWithObjects:
    [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionLinear],
    [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionLinear], nil];

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

If you're after counterclockwise animation, you should use negative values. For a slightly more basic animation, you can use CABasicAnimation:

CALayer* layer = someView.layer;
CABasicAnimation* animation;
animation = [CABasicAnimation animationWithKeyPath:@"transform.rotation.z"];

animation.fromValue = [NSNumber numberWithFloat:0.0 * M_PI];
animation.toValue = [NSNumber numberWithFloat:1.0 * M_PI];

animation.duration = 1.0;
animation.cumulative = YES;
animation.repeatCount = 1;
animation.removedOnCompletion = NO;
animation.fillMode = kCAFillModeForwards;

[layer addAnimation:rotationAnimation forKey:@"transform.rotation.z"];