Hi. I'm trying to implement a flip animation to be used in board game like iPhone-application. The animation is supposed to look like a game piece that rotates and changes to the color of its back (kind of like an Reversi piece). I've managed to create an animation that flips the piece around its orthogonal axis, but when I try to flip it around a diagonal axis by changing the rotation around the z-axis the actual image also gets rotated (not surprisingly). Instead I would like to rotate the image "as is" around a diagonal axis.

I have tried to change layer.sublayerTransform but with no success.

Here is my current implementation. It works by doing a trick to resolve the issue of getting a mirrored image at the end of the animation. The solution is to not actually rotate the layer 180 degrees, instead it rotates it 90 degrees, changes image and then rotates it back.

+ (void)flipLayer:(CALayer *)layer
          toImage:(CGImageRef)image
        withAngle:(double)angle {
  const float duration = 0.5f;

  CAKeyframeAnimation *diag = [CAKeyframeAnimation
                               animationWithKeyPath:@"transform.rotation.z"];
  diag.duration = duration;
  diag.values = [NSArray arrayWithObjects:
                 [NSNumber numberWithDouble:angle],
                 [NSNumber numberWithDouble:0.0f],
                 nil];
  diag.keyTimes = [NSArray arrayWithObjects:
                   [NSNumber numberWithDouble:0.0f],
                   [NSNumber numberWithDouble:1.0f],
                   nil];
  diag.calculationMode = kCAAnimationDiscrete;

  CAKeyframeAnimation *flip = [CAKeyframeAnimation
                               animationWithKeyPath:@"transform.rotation.y"];
  flip.duration = duration;
  flip.values = [NSArray arrayWithObjects:
                 [NSNumber numberWithDouble:0.0f],
                 [NSNumber numberWithDouble:M_PI / 2],
                 [NSNumber numberWithDouble:0.0f],
                 nil];
  flip.keyTimes = [NSArray arrayWithObjects:
                   [NSNumber numberWithDouble:0.0f],
                   [NSNumber numberWithDouble:0.5f],
                   [NSNumber numberWithDouble:1.0f],
                   nil];
  flip.calculationMode = kCAAnimationLinear;

  CAKeyframeAnimation *replace = [CAKeyframeAnimation
                                  animationWithKeyPath:@"contents"];
  replace.duration = duration / 2;
  replace.beginTime = duration / 2;
  replace.values = [NSArray arrayWithObjects:(id)image, nil];
  replace.keyTimes = [NSArray arrayWithObjects:
                      [NSNumber numberWithDouble:0.0f], nil];
  replace.calculationMode = kCAAnimationDiscrete;

  CAAnimationGroup *group = [CAAnimationGroup animation];
  group.removedOnCompletion = NO;
  group.duration = duration;
  group.timingFunction = [CAMediaTimingFunction
                          functionWithName:kCAMediaTimingFunctionLinear];
  group.animations = [NSArray arrayWithObjects:diag, flip, replace, nil];
  group.fillMode = kCAFillModeForwards;

  [layer addAnimation:group forKey:nil];
}