iPhone dev Stupidity 134: Core Animation doc reading

* CALayer must be hosted with UIView. UIView provides the event-handling for underlying layers, while the layers provide display of the content.

  - for iPhone UIView.layer

  - for Mac OS, you need to enable it: theView.wantsLayer = YES

* Sublayers do not clip to parent by default (UIView does). To enable, theLayer.masksToBounds = YES

* To position content image in a CALayer:

  - kCAGravityResize

  - kCAGravityResizeAspect

  - kCAGravityResizeAspectFill

  will do a much better job than write your own resize & fill code

* You can stop an animation by: removeAnimationForKey:

* AffineTransform类描述了一种二维仿射变换的功能，它是一种二维坐标到二维坐标之间的线性变换，保持二维图形的“平直性”（译注： straightness，即变换后直线还是直线不会打弯，圆弧还是圆弧）和“平行性”（译注：parallelness，其实是指保二维图形间的相对位置关系不变，平行线还是平行线，相交直线的交角不变。）。仿射变换可以通过一系列的原子变换的复合来实现，包括：平移（Translation）、缩放（Scale）、翻转（Flip）、旋转（Rotation）和错切（Shear）。

* When an instance of CAAnimation receives the runActionForKey:object:arguments: message it responds by adding itself to the layer’s animations, causing the animation to run:

- (void)runActionForKey:(NSString *)key 

                 object:(id)anObject 

              arguments:(NSDictionary *)dict 

{ 

     [(CALayer *)anObject addAnimation:self forKey:key]; 

} 

*