customizing timing of an animation
Timing is an important part of animations, and with Core Animation you specify precise timing information for your animations through the methods and properties of the CAMediaTiming protocol. Two Core Animation classes adopt this protocol. The CAAnimation class adopts it so that you can specify timing information in your animation objects. The CALayer also adopts it so that you can configure some timing-related features for your implicit animations, although the implicit transaction object that wraps those animations usually provides default timing information that takes precedence.
timing 是动画的重要部分之一，通过CAMediaTiming协议的方法和属性_可以为动画_指定精确的时间信息。 有两个核心动画类采用了CAMediaTiming协议。CAAnimation遵守了CAMediaTiming协议_所以_可以为动画对象指定精确的世界信息。 CALayer遵守了CAMediaTiming协议_所以 _可以为隐式动画配置时间相关的特征，尽管，包装动画的隐式事务提供了时间信息。
When thinking about timing and animations, it is important to understand how layer objects work with time. Each layer has its own local time that it uses to manage animation timing. Normally, the local time of two different layers is close enough that you could specify the same time values for each and the user might not notice anything. However, the local time of a layer can be modified by its parent layers or by its own timing parameters. For example, changing the layer’s speed property causes the duration of animations on that layer (and its sublayers) to change proportionally.
考虑时间和动画时，理解layer对象与时间如何工作_是非常重要的。每个动画有自己的本地时间_这个时间用于管理动画的时间。通常，两个不同的layer的本地时间 非常接近，所以，你指定同一个值，用户也不能觉察到。然而，本地时间可以被父layer或本身的时间参数修改。例如，修改layer 的speed属性导致layer 上的动画的持续时间比例变化。
To assist you in making sure time values are appropriate for a given layer, the CALayer class defines the convertTime:fromLayer: and convertTime:toLayer: methods. You can use these methods to convert a fixed time value to the local time of a layer or to convert time values from one layer to another. The methods take into account the media timing properties that might affect the local time of the layer and return a value that you can use with the other layer. Listing 5-3 shows an example that you should use regularly to get the current local time for a layer. The CACurrentMediaTime function is a convenience function that returns the computer’s current clock time, which the method takes and converts to the layer’s local time.
为了帮助你确认给定layer 的时间值，CALayer 类定义了convertTime:fromLayer: and convertTime:toLayer:两个方法。使用这两个方法，从固定时间转化到其他layer的时间。
的本地时间，或者将时间从一个layer转为另一个layer的时间。
Listing 5-3 Getting a layer’s current local time
CFTimeInterval localLayerTime = [myLayer convertTime:CACurrentMediaTime() fromLayer:nil];
Once you have a time value in the layer’s local time, you can use that value to update the timing-related properties of an animation object or layer. With these timing properties, you can achieve some interesting animation behaviors, including:
Use the beginTime property to set the start time of an animation. Normally, animations begin during the next update cycle. You can use the beginTime parameter to delay the animation start time by several seconds. The way to chain two animations together is to set the begin time of one animation to match the end time of the other animation.
If you delay the start of an animation, you might also want to set the fillMode property to kCAFillModeBackwards. This fill mode causes the layer to display the animation’s start value, even if the layer object in the layer tree contains a different value. Without this fill mode, you would see a jump to the final value before the animation starts executing. Other fill modes are available too.
The autoreverses property causes an animation to execute for the specified duration and then return to the starting value of the animation. You can combine this property with the repeatCount property to animate back and forth between the start and end values. Setting the repeat count to a whole number (such as 1.0) for an autoreversing animation causes the animation to stop on its starting value. Adding an extra half step (such as a repeat count of 1.5) causes the animation to stop on its end value.
Use the timeOffset property with group animations to start some animations at a later time than others.
Pausing and Resuming Animations
To pause an animation, you can take advantage of the fact that layers adopt the CAMediaTiming protocol and set the speed of the layer’s animations to 0.0. Setting the speed to zero pauses the animation until you change the value back to a nonzero value. Listing 5-4 shows a simple example of how to both pause and resume the animations later.
Listing 5-4 Pausing and resuming a layer’s animations
-(void)pauseLayer:(CALayer*)layer {
CFTimeInterval pausedTime = [layer convertTime:CACurrentMediaTime() fromLayer:nil];
layer.speed = 0.0;
layer.timeOffset = pausedTime;
}
-(void)resumeLayer:(CALayer*)layer {
CFTimeInterval pausedTime = [layer timeOffset];
layer.speed = 1.0;
layer.timeOffset = 0.0;
layer.beginTime = 0.0;
CFTimeInterval timeSincePause = [layer convertTime:CACurrentMediaTime() fromLayer:nil] - pausedTime;
layer.beginTime = timeSincePause;
}
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.
* The conversion from parent time to local time has two stages:
*
* 1. conversion to "active local time". This includes the point at
* which the object appears in the parent's timeline, and how fast it
* plays relative to the parent.
*
* 2. conversion from active to "basic local time". The timing model
* allows for objects to repeat their basic duration multiple times,
* and optionally to play backwards before repeating. */