如何围绕对角线旋转 CALayer？

我正在尝试实现一个翻转动画，用于像 iPhone 应用程序这样的棋盘游戏。动画应该看起来像一个旋转并改变其背面颜色的游戏片段（有点像黑白棋棋子 http://en.wikipedia.org/wiki/Reversi）。我已经成功创建了一个围绕其正交轴翻转该片段的动画，但是当我尝试通过更改围绕 z 轴的旋转来围绕对角轴翻转它时，实际图像也会旋转（毫不奇怪）。相反，我想“按原样”绕对角轴旋转图像。

我尝试过改变layer.sublayerTransform但没有成功。

这是我当前的实现。它通过一个技巧来解决在动画结束时获得镜像图像的问题。解决方案是实际上不将图层旋转 180 度，而是将其旋转 90 度，更改图像，然后将其旋转回来。

最终版本：根据 Lorenzos 建议创建离散键控动画并计算每一帧的变换矩阵。此版本尝试根据图层大小估计所需的“引导”帧数量，然后使用线性键控动画。此版本以任意角度旋转，因此要绕对角线旋转，请使用 45 度角。

用法示例：

[someclass flipLayer:layer image:image angle:M_PI/4]

执行：

- (void)animationDidStop:(CAAnimationGroup *)animation
                finished:(BOOL)finished {
  CALayer *layer = [animation valueForKey:@"layer"];

  if([[animation valueForKey:@"name"] isEqual:@"fadeAnimation"]) {
    /* code for another animation */
  } else if([[animation valueForKey:@"name"] isEqual:@"flipAnimation"]) {
    layer.contents = [animation valueForKey:@"image"];
  }

  [layer removeAllAnimations];
}

- (void)flipLayer:(CALayer *)layer
            image:(CGImageRef)image
            angle:(float)angle {
  const float duration = 0.5f;

  CAKeyframeAnimation *rotate = [CAKeyframeAnimation
                                 animationWithKeyPath:@"transform"];
  NSMutableArray *values = [[[NSMutableArray alloc] init] autorelease];
  NSMutableArray *times = [[[NSMutableArray alloc] init] autorelease];
  /* bigger layers need more "guiding" values */
  int frames = MAX(layer.bounds.size.width, layer.bounds.size.height) / 2;
  int i;
  for (i = 0; i < frames; i++) {
    /* create a scale value going from 1.0 to 0.1 to 1.0 */
    float scale = MAX(fabs((float)(frames-i*2)/(frames - 1)), 0.1);

    CGAffineTransform t1, t2, t3;
    t1 = CGAffineTransformMakeRotation(angle);
    t2 = CGAffineTransformScale(t1, scale, 1.0f);
    t3 = CGAffineTransformRotate(t2, -angle);
    CATransform3D trans = CATransform3DMakeAffineTransform(t3);

    [values addObject:[NSValue valueWithCATransform3D:trans]];
    [times addObject:[NSNumber numberWithFloat:(float)i/(frames - 1)]];
  }
  rotate.values = values;
  rotate.keyTimes = times;
  rotate.duration = duration;
  rotate.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.duration = duration;
  group.timingFunction = [CAMediaTimingFunction
                          functionWithName:kCAMediaTimingFunctionLinear];
  group.animations = [NSArray arrayWithObjects:rotate, replace, nil];
  group.delegate = self;
  group.removedOnCompletion = NO;
  group.fillMode = kCAFillModeForwards;
  [group setValue:@"flipAnimation" forKey:@"name"];
  [group setValue:layer forKey:@"layer"];
  [group setValue:(id)image forKey:@"image"];

  [layer addAnimation:group forKey:nil];
}

原始代码：

+ (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];
}

你可以这样伪造：创建一个仿射变换，沿着对角线折叠图层：

A-----B           B
|     |          /
|     |   ->   A&D
|     |        /
C-----D       C

更改图像，并将 CALayer 转换回另一个动画。 这将产生图层绕其对角线旋转的错觉。

如果我没记错的话，矩阵应该是：

0.5 0.5 0
0.5 0.5 0
0   0   1

更新： 好吧，CAT 并不真的喜欢使用简并变换，但你可以这样近似：

CGAffineTransform t1 = CGAffineTransformMakeRotation(M_PI/4.0f);
CGAffineTransform t2 = CGAffineTransformScale(t1, 0.001f, 1.0f);
CGAffineTransform t3 = CGAffineTransformRotate(t2,-M_PI/4.0f);  

在我对模拟器的测试中，仍然存在问题，因为旋转发生的速度比平移快，因此对于纯黑色方块，效果有点奇怪。我想如果你有一个居中的精灵，周围有透明区域，那么效果将接近预期。然后您可以调整t3矩阵，看看是否能得到更有吸引力的结果。

经过更多研究，似乎应该通过关键帧为其自己的过渡设置动画，以获得对过渡本身的最大控制。假设你要在一秒钟内显示这个动画，你应该让十个矩阵每隔十分之一秒显示一次，而不使用 kCAAnimationDiscrete 进行插值；这些矩阵可以通过以下代码生成：

CGAffineTransform t1 = CGAffineTransformMakeRotation(M_PI/4.0f);
CGAffineTransform t2 = CGAffineTransformScale(t1, animationStepValue, 1.0f);
CGAffineTransform t3 = CGAffineTransformRotate(t2,-M_PI/4.0f);  

其中每个关键帧的animationStep值取自此进程：

{1 0.7 0.5 0.3 0.1 0.3 0.5 0.7 1}

也就是说：您将生成十个不同的变换矩阵（实际上是 9 个），将它们作为关键帧推送到每隔十分之一秒显示一次，然后使用“不插值”参数。您可以调整动画数量以平衡平滑度和性能*

*很抱歉可能出现错误，最后一部分是在没有拼写检查的情况下编写的。