- (void)replaceViewController:(UIViewController *)existingViewController withViewController:(UIViewController *)newViewController inContainerView:(UIView *)containerView completion:(void (^)(void))completion

 {

     // Add initial view controller

     if (!existingViewController && newViewController) {

         [newViewController willMoveToParentViewController:self];

         [newViewController beginAppearanceTransition:YES animated:NO];

         [self addChildViewController:newViewController];

         newViewController.view.frame = containerView.bounds;

         [containerView addSubview:newViewController.view];

         [newViewController didMoveToParentViewController:self];

         [newViewController endAppearanceTransition];

         if (completion) completion();

     }

     // Remove existing view controller

     else if (existingViewController && !newViewController) {

         [existingViewController willMoveToParentViewController:nil];

         [existingViewController beginAppearanceTransition:NO animated:NO];

         [existingViewController.view removeFromSuperview];

         [existingViewController removeFromParentViewController];

         [existingViewController didMoveToParentViewController:nil];

         [existingViewController endAppearanceTransition];

         if (completion) completion();

     }

     // Replace existing view controller with new view controller

     else if ((existingViewController != newViewController) && newViewController) {

         [newViewController willMoveToParentViewController:self];

         [existingViewController willMoveToParentViewController:nil];

         [existingViewController beginAppearanceTransition:NO animated:NO];

         [existingViewController.view removeFromSuperview];

         [existingViewController removeFromParentViewController];

         [existingViewController didMoveToParentViewController:nil];

         [existingViewController endAppearanceTransition];

         [newViewController beginAppearanceTransition:YES animated:NO];

         newViewController.view.frame = containerView.bounds;

         [self addChildViewController:newViewController];

         [containerView addSubview:newViewController.view];

         [newViewController didMoveToParentViewController:self];

         [newViewController endAppearanceTransition];

         if (completion) completion();

     }

 }