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/*如果你想实现一个扩展子Drawable的动画drawable，那么你可以通过setCallBack(android.graphics.drawable.Drawable.Callback)来把你实现的该接口注册到动画drawable 
*中。可以实现对动画的调度和执行 
*/   
public static interface Callback {  /** * 当drawable重画时触发，这个点上drawable将被置为不可用（起码drawable展示部分不可用） * @param 要求重画的drawable */  public void invalidateDrawable(Drawable who);  /** * drawable可以通过该方法来安排动画的下一帧。可以仅仅简单的调用postAtTime(Runnable, Object, long)来实现该方法。参数分别与方法的参数对 *应 * @param who The drawable being scheduled. * @param what The action to execute. * @param when The time (in milliseconds) to run */  public void scheduleDrawable(Drawable who, Runnable what, long when);  /** *可以用于取消先前通过scheduleDrawable(Drawable who, Runnable what, long when)调度的某一帧。可以通过调用removeCallbacks(Runnable,Object)来实现 * @param who The drawable being unscheduled. * @param what The action being unscheduled. */  public void unscheduleDrawable(Drawable who, Runnable what);  }