序言
CAAnimation是一个抽象类，遵循了CAMediaTiming协议和CAAction协议！我们不要直接使用CAAnimation类，而是使用其子类：
CATransition：提供渐变效果，如推拉push效果,消退fade效果,揭开reveal效果CAAnimationGroup：允许多个动画同时播放CABasicAnimation： 提供了对单一动画的实现CAKeyframeAnimation： 关键桢动画,可以定义动画路线CAPropertyAnimation：属性动画，通常不直接使用，而是使用CABasicAnimation子类
创建对象
我们看到有一个工厂方法来创建CAAnimation对象，因此，我们通常都使用这个方法来创建动画：
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+ (instancetype)animation;
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当然不同类型的子类使用的方法不一样，对于继承于CAPropertyAnimation的子类，都可以通过属性路径来创建：
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/* Creates a new animation object with its `keyPath' property set to
* 'path'. */
+ (instancetype)animationWithKeyPath:(nullable NSString *)path;
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遵守了CAMediaTiming协议
这个协议是是用于配置动画的相关属性的，英文部分是官方的注释，中文部分为笔者的理解，下面一一讲解：
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/* The begin time of the object, in relation to its parent object, if
* applicable. Defaults to 0. */
// 获取或设置动画的开始时间，默认为0
@property CFTimeInterval beginTime;
/* The basic duration of the object. Defaults to 0. */
// 获取或设置动画的时长，也就是整个动画的总时长
@property CFTimeInterval duration;
/* The rate of the layer. Used to scale parent time to local time, e.g.
* if rate is 2, local time progresses twice as fast as parent time.
* Defaults to 1. */
// 获取或设置动画的播放速度，默认为1，若设置为2，则以两倍的速度播放。
// 如果设置小于1，则相当于慢放。
@property float speed;
/* Additional offset in active local time. i.e. to convert from parent
* time tp to active local time t: t = (tp - begin) * speed + offset.
* One use of this is to "pause" a layer by setting `speed' to zero and
* `offset' to a suitable value. Defaults to 0. */
// 获取或设置当前播放的进度，默认为0。有这么一种使用场景：设置speed为0，
// 然后设置这个timeOffset为合适的值，就可以暂停动画了
@property CFTimeInterval timeOffset;
/* The repeat count of the object. May be fractional. Defaults to 0. */
// 获取或设置动画播放次数，默认为0表示只播放一次。
// 设置为HUGE_VALF表示无限制播放次数
@property float repeatCount;
/* The repeat duration of the object. Defaults to 0. */
// 获取或设置重复播放的动画时长，不要与repeatCount混合使用
@property CFTimeInterval repeatDuration;
/* When true, the object plays backwards after playing forwards. Defaults
* to NO. */
// 获取或设置是否回放。
// 如果设置为YES，在动画播放完成时，就会以动画的效果回到起点
// 如果设置为NO，播放完成时，就会停留在终点
@property BOOL autoreverses;
/* Defines how the timed object behaves outside its active duration.
* Local time may be clamped to either end of the active duration, or
* the element may be removed from the presentation. The legal values
* are `backwards', `forwards', `both' and `removed'. Defaults to
* `removed'. */
// 获取或设置动画完成时的动作
// forwards表示动画完成时，也回到起点而不是留在终点
// backwards表示动画完成时，就停留在终点
// removed表示完成时就移除，默认就是removed
@property(copy) NSString *fillMode;
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遵守了CAAction协议
这个协议只有一个方法，我们可以调用此方法来触发指定的事件，这样接收者就可以接收到代理。
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/* Called to trigger the event named 'path' on the receiver. The object
* (e.g. the layer) on which the event happened is 'anObject'. The
* arguments dictionary may be nil, if non-nil it carries parameters
* associated with the event. */
- (void)runActionForKey:(NSString *)event
object:(id)anObject
arguments:(nullable NSDictionary *)dict;
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CAAnimationDelegate代理
CAAnimation为这么个属性：
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/* The delegate of the animation. This object is retained for the
* lifetime of the animation object. Defaults to nil. See below for the
* supported delegate methods. */
@property(nullable, strong) id delegate;
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我们只要指定了代理，就可以实现这两个代理方法：
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/* Called when the animation begins its active duration. */
// 动画开始时的回调
- (void)animationDidStart:(CAAnimation *)anim;
/* Called when the animation either completes its active duration or
* is removed from the object it is attached to (i.e. the layer). 'flag'
* is true if the animation reached the end of its active duration
* without being removed. */
// 动画停止的回调，可以通过flag判断动画是否是完成还是暂停
- (void)animationDidStop:(CAAnimation *)anim finished:(BOOL)flag;
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removedOnCompletion属性
当我们动画完成时，如果希望动画就自动移除的话，我们可以设置此属性为YES，默认值为YES。如果我们想要循环或者执行多次动画，就将此属性设置为NO
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/* When true, the animation is removed from the render tree once its
* active duration has passed. Defaults to YES. */
// 当duration值已经达到时，是否将动画自动从渲染树上移除
@property(getter=isRemovedOnCompletion) BOOL removedOnCompletion;
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timingFunction属性
这个属性是用于指定动画移动的步调是什么样式，比如线性。
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/* A timing function defining the pacing of the animation. Defaults to
* nil indicating linear pacing. */
@property(nullable, strong) CAMediaTimingFunction *timingFunction;
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关于CAMediaTimingFunction类，主要是这向个方法。当创建时，我们+functionWithName:工厂方法来创建系统已经提供的样式。
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/* A convenience method for creating common timing functions. The
* currently supported names are `linear', `easeIn', `easeOut' and
* `easeInEaseOut' and `default' (the curve used by implicit animations
* created by Core Animation). */
+ (instancetype)functionWithName:(NSString *)name;
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其中这个name有这几个变量对应的：
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// 线性动画
CA_EXTERN NSString * const kCAMediaTimingFunctionLinear
__OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
// 快速进入动画
CA_EXTERN NSString * const kCAMediaTimingFunctionEaseIn
__OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
// 快速出来动画
CA_EXTERN NSString * const kCAMediaTimingFunctionEaseOut
__OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
// 快速进入出来动画
CA_EXTERN NSString * const kCAMediaTimingFunctionEaseInEaseOut
__OSX_AVAILABLE_STARTING (__MAC_10_5, __IPHONE_2_0);
// 默认动画是curve动画，也就是曲线动画
CA_EXTERN NSString * const kCAMediaTimingFunctionDefault
__OSX_AVAILABLE_STARTING (__MAC_10_6, __IPHONE_3_0);
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如果我们想要让其移动动画是按贝塞尔曲线的路径行动，那么可以用这两个方法来创建：
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/* Creates a timing function modelled on a cubic Bezier curve. The end
* points of the curve are at (0,0) and (1,1), the two points 'c1' and
* 'c2' defined by the class instance are the control points. Thus the
* points defining the Bezier curve are: '[(0,0), c1, c2, (1,1)]' */
+ (instancetype)functionWithControlPoints:(float)c1x :(float)c1y :(float)c2x :(float)c2y;
- (instancetype)initWithControlPoints:(float)c1x :(float)c1y :(float)c2x :(float)c2y;
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