淡入淡出更换 rootViewController

- (void)restoreRootViewController:(UIViewController *)rootViewController

{
 typedef void (^Animation)( void );
 UIWindow* window = self .window ;

 rootViewController .modalTransitionStyle = UIModalTransitionStyleCrossDissolve ;
 Animation animation = ^{
  BOOL oldState = [ UIView areAnimationsEnabled ];
  [ UIView setAnimationsEnabled : NO ];
  window .rootViewController = rootViewController;
  [ UIView setAnimationsEnabled :oldState];
 };

 [ UIView transitionWithView :window
      duration : 0 .5f
       options :UIViewAnimationOptionTransitionCrossDissolve
    animations :animation
    completion :nil ];
}