-(void)viewDidAppear:(BOOL)animated

{

    [self.selectedViewController endAppearanceTransition];

}

-(void)viewWillAppear:(BOOL)animated

{

    [self.selectedViewController beginAppearanceTransition: YES animated: animated];

}

-(void) viewWillDisappear:(BOOL)animated

{

    [self.selectedViewController beginAppearanceTransition: NO animated: animated];

}

-(void) viewDidDisappear:(BOOL)animated

{

    [self.selectedViewController endAppearanceTransition];

}

这个在自定义tabBarcontroller的类里面写

报错：Unbalanced calls to begin/end appearance transitions for XXXX

报这样的警告，是因为：过于快速切换子视图控制器，导致，前一个切换动作还没有完成，就进行了下一个切换！

自定义tabBarcontroller必须实现以下几个方法