Having a UINavigationController in the master view of a UISplitViewController in iOS 8

Short answer, you can control this behaviour via the UISplitViewControllerDelegate methods:

splitViewController:collapseSecondaryViewController:ontoPrimaryViewController:
splitViewController:separateSecondaryViewControllerFromPrimaryViewController:

I suspect what you really want to do is deal with the situation where you have an iOS 8 UISplitViewController-based app where your primary and detailed views are both UINavigationControllers and there are some viewControllers (within these navigation controllers) that you want to appear only on the primary or detail side of the split view. The answer below deals with this. It also copes with the situation where you sometimes wish for a view to replace the views in the Detail navigation controller, rather than getting pushed there.

A small caveat: the code below does not deal with all possible cases and has some assumptions:

• We don't expect anything can change on the Detailed navigation controller stack when the split view is collapsed and those views are obscured by a detailed view above them.
• Our UIViewController subclasses all have a shouldDisplayInDetailedView and shouldReplaceDetailedView property
• We assume that we only push views onto the detailed navigation controller that have the shouldDisplayInDetailedView property set.
• View controllers are added to the Detail side via splitViewController:showDetailViewController: or a pushViewController:animated: on the navigationController property of a view within a detailed view (in either expanded or collapsed state).
• View controllers that should replace the view controllers in the Detail navigation controller are only added via splitViewController:showDetailViewController: and only from interaction with view in the Primary view controller, i.e., this can only happen if the Primary view controller is not obscured when in collapsed state.
• We have a BlankViewController to display in the Detail View when the split view controller gets expanded but we only have view controllers that should stay on the Primary side.

I don't recommend implementing only one side of the splitViewController:collapseSecondaryViewController:ontoPrimaryViewController: / splitViewController: separateSecondaryViewControllerFromPrimaryViewController: logic and depending on the default implementation for the other side. Apple do some strange things like putting the UINavigationViewController from the Detail side into the primary side as one of the viewControllers in the Primary navigation controller stack but then pushing other view controllers above it, which even if you understand completely still can't be replicated from your own code. Thus its best to handle both sides of the process yourself.

This is what I use:

#pragma mark -
#pragma mark Split View Controller delegate.

- (BOOL)splitViewController:(UISplitViewController *)splitViewController showViewController:(UIViewController *)vc sender:(id)sender
{
    //Standard behaviour.  This won't get called in our case when the split view is collapsed and the primary view controllers are obscured.
    return NO;
}

// Since we treat warnings as errors, silence warning about unknown selector below on UIViewController subclasses.
#pragma GCC diagnostic ignored "-Wundeclared-selector"


- (BOOL)splitViewController:(UISplitViewController *)splitViewController showDetailViewController:(UIViewController *)vc sender:(id)sender
{
    if (splitViewController.collapsed == NO)
    {
        // The navigation controller we'll be adding the view controller vc to.
        UINavigationController *navController = splitViewController.viewControllers[1];

        UIViewController *topDetailViewController = [navController.viewControllers lastObject];
        if ([topDetailViewController isKindOfClass:[BlankViewController class]] ||
           ([vc respondsToSelector:@selector(shouldReplaceDetailedView)] && [vc performSelector:@selector(shouldReplaceDetailedView)]))
        {
            // Replace the (expanded) detail view with this new view controller.
            [navController setViewControllers:@[vc] animated:NO];
        }
        else
        {
            // Otherwise, just push.
            [navController pushViewController:vc animated:YES];
        }
    }
    else
    {
        // Collapsed.  Just push onto the conbined primary and detailed navigation controller.
        UINavigationController *navController = splitViewController.viewControllers[0];
        [navController pushViewController:vc animated:YES];
    }

    // We've handled this ourselves.
    return YES;
}

- (BOOL)splitViewController:(UISplitViewController *)splitViewController collapseSecondaryViewController:(UIViewController *)secondaryViewController ontoPrimaryViewController:(UIViewController *)primaryViewController
{
    UINavigationController *primaryNavController = (UINavigationController *)primaryViewController;
    UINavigationController *secondaryNavController = (UINavigationController *)secondaryViewController;
    UIViewController *bottomSecondaryView = [secondaryNavController.viewControllers firstObject];
    if ([bottomSecondaryView isKindOfClass:[BlankViewController class]])
    {
        NSAssert([secondaryNavController.viewControllers count] == 1, @"BlankViewController is not only detail view controller");
        // If our secondary controller is blank, do the collapse ourself by doing nothing.
        return YES;
    }

    // We need to shift these view controllers ourselves.
    // This should be the primary views and then the detailed views on top.
    // Otherwise the UISplitViewController does wacky things like embedding a UINavigationController inside another UINavigation Controller, which causes problems for us later.
    NSMutableArray *newPrimaryViewControllers = [NSMutableArray arrayWithArray:primaryNavController.viewControllers];
    [newPrimaryViewControllers addObjectsFromArray:secondaryNavController.viewControllers];
    primaryNavController.viewControllers = newPrimaryViewControllers;

    return YES;
}

- (UIViewController *)splitViewController:(UISplitViewController *)splitViewController separateSecondaryViewControllerFromPrimaryViewController:(UIViewController *)primaryViewController
{
    UINavigationController *primaryNavController = (UINavigationController *)primaryViewController;

    // Split up the combined primary and detail navigation controller in their component primary and detail view controller lists, but with same ordering.
    NSMutableArray *newPrimaryViewControllers = [NSMutableArray array];
    NSMutableArray *newDetailViewControllers = [NSMutableArray array];
    for (UIViewController *controller in primaryNavController.viewControllers)
    {
        if ([controller respondsToSelector:@selector(shouldDisplayInDetailedView)] && [controller performSelector:@selector(shouldDisplayInDetailedView)])
        {
            [newDetailViewControllers addObject:controller];
        }
        else
        {
            [newPrimaryViewControllers addObject:controller];
        }
    }

    if (newDetailViewControllers.count == 0)
    {
        // If there's no detailed views on the top of the navigation stack, return a blank view  (in navigation controller) for detailed side.
        UINavigationController *blankDetailNavController = [[UINavigationController alloc] initWithRootViewController:[[BlankViewController alloc] init]];
        return blankDetailNavController;
    }

    // Set the new primary views.
    primaryNavController.viewControllers = newPrimaryViewControllers;

    // Return the new detail navigation controller and views.
    UINavigationController *detailNavController = [[UINavigationController alloc] init];
    detailNavController.viewControllers = newDetailViewControllers;
    return detailNavController;
}

Swift 4 Version with minor changes to make it work with my code:

func splitViewController(_ splitViewController: UISplitViewController, showDetail vc: UIViewController, sender: Any?) -> Bool {
    if !isCollapsed {
        // in expanded mode set new VC as top view controller of the detail nav controller
        if let detailNavigationController = viewControllers[1] as? UINavigationController {
           detailNavigationController.setViewControllers([vc], animated: false)
        }
    } else {
        // in collapsed mode push the new view controller on the master nav controller
        if let masterNavigationController = viewControllers[0] as? UINavigationController {
            masterNavigationController.pushViewController(vc, animated: true)
        }
    }
    return true
}

func splitViewController(_ splitViewController: UISplitViewController, collapseSecondary secondaryViewController: UIViewController, onto primaryViewController: UIViewController) -> Bool {

    let masterNavigationController = primaryViewController as? UINavigationController
    let detailNavigationController = secondaryViewController as? UINavigationController
    let episodeDetailViewController = detailNavigationController?.viewControllers.first as? EpisodeDetailTableViewController
    if episodeDetailViewController?.episode == nil {
        // detail view is blank. We do not need to push this onto the master
        return true
    }

    guard var newMasterViewControllers = masterNavigationController?.viewControllers else { return false }
    newMasterViewControllers.append(contentsOf: detailNavigationController?.viewControllers ?? [])
    masterNavigationController?.setViewControllers(newMasterViewControllers, animated: false)
    return true
}


func splitViewController(_ splitViewController: UISplitViewController, separateSecondaryFrom primaryViewController: UIViewController) -> UIViewController? {
    let masterNavigationViewController = primaryViewController as? UINavigationController

    var newMasterViewControllers = [UIViewController]()
    var newDetailViewControllers = [UIViewController]()

    for vc in masterNavigationViewController?.viewControllers ?? [] {
        if vc is PodcastsTableViewController || vc is EpisodesTableViewController {
            newMasterViewControllers.append(vc)
        } else {
            newDetailViewControllers.append(vc)
        }
    }

    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    let detailNavigationController = storyboard.instantiateViewController(withIdentifier: "splitViewDetailViewController") as! UINavigationController

    if newDetailViewControllers.count == 0 {
        let emptyEpisodeDetailViewController = storyboard.instantiateViewController(withIdentifier: "episodeDetail")
        newDetailViewControllers.append(emptyEpisodeDetailViewController)
    }

    masterNavigationViewController?.setViewControllers(newMasterViewControllers, animated: false)
    detailNavigationController.setViewControllers(newDetailViewControllers, animated: false)
    return detailNavigationController
}