- (void)transitionToViewController:(UIViewController *)toViewController
{
   UIViewController *oldTopViewController = _topViewController;

   [self addChildViewController:toViewController];
   [toViewController didMoveToParentViewController:self];

   toViewController.view.frame = self.topViewController.view.frame;

   if ([self isViewLoaded]) {
       [toViewController beginAppearanceTransition:YES animated:YES];
       [self.view addSubview:toViewController.view];
       [toViewController endAppearanceTransition];
   }

   [UIView transitionFromView:oldTopViewController.view
toView:toViewController.view duration:0.5
options:UIViewAnimationOptionTransitionCrossDissolve completion:^ (BOOL
finished) {
       [oldTopViewController.view removeFromSuperview];
       [oldTopViewController willMoveToParentViewController:nil];
       [oldTopViewController beginAppearanceTransition:NO animated:YES];
       [oldTopViewController removeFromParentViewController];
       [oldTopViewController endAppearanceTransition];
       _topViewController = toViewController;
   }];
}