CABasicAnimation 按home鍵后台之后，再切回來動畫就停止了

解決方法：

1. CABasicAnimation *thisAnimation = [CABasicAnimtaion animationWithKeyPath:@"transform.rotation.z"];

    thisAnimation.removedOnCompletion = NO; 

/* When true, the animation is removed from the render tree once its
 * active duration has passed. Defaults to YES. */

@property(getter=isRemovedOnCompletion) BOOL removedOnCompletion;

2. 對UIApplicationWillEnterForegroundNotification增加observer, 然后在其中重啟動畫...

   舊動畫直接removeAllAnimations, 然后重新創建和添加動畫.

3. 

   1）. 所在類注冊監聽 
       [notify addObserver:self    selector:@selector(doAnimation:)    name:UIApplicationDidBecomeActiveNotification object:nil];
   2）. 實現自定義的方法doAnimation:，再次方法中添加動畫
       - (void)doAnimation:(NSNotification *)notify{
           .....
           [obj addAnimate];
           ..... 
       }