Force CALayer's rotation direction?

Question

This one has been driving me nuts...

Considering "plane" is a CALayer, I rotate it in the X axis:

plane.transform = CATransform3DMakeRotation(180 * M_PI / 180.0f, 1.0f, 0.0f, 0.0f);

And this makes it rotate clockwise, which is perfect.

Now, at some point in time, I want it to return to 0 degrees, so I use:

plane.transform = CATransform3DMakeRotation(0 * M_PI / 180.0f, 1.0f, 0.0f, 0.0f);

The thing is... it does it anti-clockwise, which is not what I want :(

I'm guessing it uses the shortest rotation path, but even if I tell it to go to 360 degrees instead, when it's done and I tell it to restart the animation, it starts from 360 to go to 180, and it goes backwards instead of the right direction.

Is there a way to workaround that?

What I'd do in Actionscript would be:

if (plane.rotationX == 360) plane.rotationX = 0;

And it'd resume nicely, but if I do that using CATransform3DMakeRotation both transformations collide, because of the animation (I think) and it gets messed up.

Any help would be much appreciated!

Thanks

Answer 1

This question came up when I was searching to solve a similar issue. I wanted to rotate an image 180 degrees and then wanted to keeping going around for a full somersault. iOS wanted to take the shortest route back. The answer here didn't help me too much. I ended up looking up some stuff on keyPath animations and that solved my issue. Below is an example of the approach.

For the particular issue raised in the question, you'd need two different sets of keyFrameValues, one for the initial rotation and another for the second. But by putting in an intermediate value you can get the desired effect, i.e. take the longer path or force the choice of equally long paths, if that's what you want. I've left some variant code in comments, as they may help folk not familiar with CAKeyFrameAnimation follow the code / understand the range of options.

- (void) somersault;
{

    CALayer* layer = [self layer];
    NSMutableArray* keyFrameValues = [NSMutableArray array];
    [keyFrameValues addObject:[NSNumber numberWithFloat:0.0]];
    //[values addObject:[NSNumber numberWithFloat:M_PI*0.5]];
    [keyFrameValues addObject:[NSNumber numberWithFloat:M_PI]];
    [keyFrameValues addObject:[NSNumber numberWithFloat:M_PI*1.5]];
    [keyFrameValues addObject:[NSNumber numberWithFloat:M_PI*2.0]];

    CAKeyframeAnimation* animation = [CAKeyframeAnimation animationWithKeyPath:@"transform"];
    [animation setValues:keyFrameValues];
    [animation setValueFunction:[CAValueFunction functionWithName: kCAValueFunctionRotateX]];// kCAValueFunctionRotateZ]];

    [animation setDuration:0.9];
    //[animation setTimingFunction: [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut]];

    [layer addAnimation:animation forKey:nil];

    return;
}