UISplitViewController's detailViewController is nil when overrideUserInterfaceStyle is set to .dark

Question

I have a UISplitViewController with two UINavigationControllers as primary and detail view controllers, created using Storyboard. The primary has a static UITableViewController and clicking on a row would set the detail view to load in the detail view navigation controller.

Everything is working as expected, except when I set overrideUserInterfaceStyle to "dark mode". When that is done, the UISplitViewController's viewControllers array has only 1 element in it - the primary view controller.

I tried to recreate the problem using a simplified example, and succeeded in reproducing the issue.

The reproducible test case is at https://github.com/sridharrajagopal/TestApp

protocol RootSettingsViewControllerSelectionDelegate : AnyObject {
    func viewControllerSelected(_ vc: String)
}

class RootSettingsViewController: UITableViewController, MFMailComposeViewControllerDelegate {
    
    weak var delegate: RootSettingsViewControllerSelectionDelegate?
    
    override func viewDidLoad() {
        super.viewDidLoad()

        if (UIDevice.current.userInterfaceIdiom == .pad) {
            let indexPath = IndexPath(row: 0, section: 0)
            tableView.selectRow(at: indexPath, animated: true, scrollPosition: UITableView.ScrollPosition.top)
            tableView.delegate?.tableView!(tableView, didSelectRowAt: indexPath)
        }
    }
    
    
    
    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        if (indexPath.section == 2 && indexPath.row == 2)  {
            // emailUs()
        } else if (indexPath.section == 0 && indexPath.row == 0) {
            delegate?.viewControllerSelected("A")
        } else if (indexPath.section == 0 && indexPath.row == 1) {
            delegate?.viewControllerSelected("B")
        } else if (indexPath.section == 0 && indexPath.row == 2) {
            delegate?.viewControllerSelected("C")
        } 
        if let detailViewController = delegate as? SettingsSecondaryNavigationViewController {
            splitViewController?.showDetailViewController(detailViewController, sender: nil)
        }
    }
    
}

class SettingsSecondaryNavigationViewController: UINavigationController {

    override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
        super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
}

extension SettingsSecondaryNavigationViewController: RootSettingsViewControllerSelectionDelegate {
    func viewControllerSelected(_ vc: String) {
        if (vc == "A") {
            let detailViewController = UIViewController()
            detailViewController.view.backgroundColor = .red
            self.viewControllers = [detailViewController]
        } else if (vc == "B") {
            let detailViewController = UIViewController()
            detailViewController.view.backgroundColor = .green
            self.viewControllers = [detailViewController]
            
        } else if (vc == "C") {
            let detailViewController = UIViewController()
            detailViewController.view.backgroundColor = .blue
            self.viewControllers = [detailViewController]
            
        } else if (vc == "D") {
            let detailViewController = UIViewController()
            detailViewController.view.backgroundColor = .purple
            self.viewControllers = [detailViewController]
            
        }
        
    }

}

class SettingsSplitViewController: UISplitViewController, UISplitViewControllerDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        delegate = self
        
        let detailViewController =
          (self.viewControllers.last as? SettingsSecondaryNavigationViewController)
        
        let primaryViewController =
          (self.viewControllers.first as? UINavigationController)?
            .topViewController as? RootSettingsViewController
        
        // This is where it bombs, as detailViewController is nil 
        primaryViewController!.delegate = detailViewController!

        preferredDisplayMode = UISplitViewController.DisplayMode.oneBesideSecondary
    }
    
    func splitViewController(
        _ splitViewController: UISplitViewController,
        collapseSecondary secondaryViewController: UIViewController,
        onto primaryViewController: UIViewController
    ) -> Bool {
        
        return true
    }
}

I am quite stumped at this time. If overrideUserInterfaceStyle is .light or .unspecified (match system settings), everything's working fine.

I see that when loading the app, the SettingsSecondaryNavigationViewController's viewDidLoad gets called when overrideUserInterfaceStyle is .light or .unspecified (even before loading the actual UISplitViewController, but doesn't get called when .dark mode.

My first question is if what I'm doing is ok, or has some inherent problems that's manifesting as the "dark mode" issue.

Any insights or pointers would be greatly appreciated!

Answer 1

I have filed a bug report with Apple.

I have worked around the issue by creating the primary and detail view controllers programmatically in the UISplitViewController’s viewDidLoad using the storyboard ID to load the viewControllers from the Interface Builder storyboard.

override func viewDidLoad() {
    super.viewDidLoad()
    
    loadViewControllers()
    
    delegate = self

    preferredDisplayMode = UISplitViewController.DisplayMode.oneBesideSecondary
}

private func loadViewControllers() {
    
    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    
    let primaryViewController = storyboard.instantiateViewController(withIdentifier: "RootSettingsViewController") as! RootSettingsViewController
    
    let detailViewController = storyboard.instantiateViewController(withIdentifier: "SettingsSecondaryNavigationViewController") as! SettingsSecondaryNavigationViewController
            
    primaryViewController.delegate = detailViewController
    
    let navController = UINavigationController(rootViewController: primaryViewController)
    
    self.viewControllers = [navController, detailViewController]
}