理解在图层上暂停和恢复动画 [英] Comprehend pause and resume animation on a layer

问题描述

我正在《 Core Animation编程指南》中学习"Animation"，并且无法理解暂停并在图层上恢复动画.

I am studying Animation in Core Animation Programming Guide and I get stuck on comprehending pause and resume animation on a layer.

该文档告诉我如何在没有明确说明的情况下暂停和恢复动画.我认为关键是要了解什么是CAlayer的timeOffset和beginTime方法.

The document tells me how to pause and resume animation without clear explanation. I think the key is to understand what is timeOffset and beginTime method of CAlayer.

这些代码是暂停并恢复动画.在resumeLayer方法中，layer.beginTime = timeSincePause;这行确实让我感到困惑.

These code is pause and resume animation. In resumeLayer method, layer.beginTime = timeSincePause; this line really make me confused.

-(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;
}

任何帮助将不胜感激.

推荐答案

在CAMediaTiming的头文件中，我们可以看到以下代码:

In the header file of CAMediaTiming, we can see these codes:

/* The begin time of the object, in relation to its parent object, if
 * applicable. Defaults to 0. */

@property CFTimeInterval beginTime;

/* The basic duration of the object. Defaults to 0. */

@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. */

@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. */

@property CFTimeInterval timeOffset;

重要的公式是:

t =(tp-开始)*速度+偏移量

此公式定义了全局时间(或父时间tp)如何映射到图层的本地时间.这个公式可以解释所有列出的代码:

This formula defines how the global time (or parent time, tp) maps to the local time of the layer. And this formula can explain everything of the listed codes:

1. 在时间A，动画被暂停.设置速度= 0，并且timeOffset = pauseTime之后，该图层的本地时间等于pauseTime.而且本地时间不会再增加了，因为速度= 0;
2. 在时间B，恢复动画.设置速度= 1.0，timeOffset = 0，beginTime = 0之后，图层的本地时间等于全局时间(或tp)，即(timePause + timeSinacePause).但是我们需要动画从时间点#A开始，因此我们设置beginTime = timeSincePaused，然后图层的本地时间等于timePause.当然，这会导致动画从暂停点继续播放.

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