Container view controller

ViewControllerContainer.m

#pragma mark - container view controller 

- (CGRect)frameForContentViewController:(UIViewController*)contentViewController {
    return self.view.bounds;
}

- (void)addContentViewController: (UIViewController*)contentViewController; {
    [self addChildViewController:contentViewController];
    contentViewController.view.frame = [self frameForContentViewController:contentViewController];
    [self.view addSubview:contentViewController.view];
    [contentViewController didMoveToParentViewController:self];
}

- (void)removeContentViewController:(UIViewController*)contentViewController {
    [contentViewController willMoveToParentViewController:nil];
    [contentViewController.view removeFromSuperview];
    [contentViewController removeFromParentViewController];
}

- (void)transitionFromViewController:(UIViewController*)oldViewController
                    toViewController:(UIViewController*)newViewController
{
    [oldViewController willMoveToParentViewController:nil];
    [self addChildViewController:newViewController];
    
    newViewController.view.frame = [self frameForContentViewController:newViewController];
    newViewController.view.alpha = 0.0;
    
    [self transitionFromViewController:oldViewController
                      toViewController:newViewController
                              duration:0.25
                               options:0
                            animations:^{
                                oldViewController.view.alpha = 0.0;
                                newViewController.view.alpha = 1.0;
                            }
                            completion:^(BOOL finished) {
                                [oldViewController removeFromParentViewController];
                                [newViewController didMoveToParentViewController:self];
                            }];
}