iOS black screen when bringing app to foreground after some time in background

Question

I'm trying to remove all storyboards from our iOS app as they are a huge mess when working in a team with Git.

I'm now setting the initial ViewController in AppDelegate's application(_:didFinishLaunchingWithOptions:) method:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    window = UIWindow(frame: UIScreen.main.bounds)
    let launchViewController = LaunchView()
    window!.rootViewController = launchViewController
    window!.makeKeyAndVisible()

    [...]

    return true
}

LaunchView is a simple view controller responsible of routing the user to login or main screen depending if he/she is logged in.

Before this, LaunchView was set as initial in Main.storyboard.

I already removed these lines from Info.plist file:

<key>UIMainStoryboardFile</key>
<string>Main</string>

Everything is working fine, except when we leave the app in background for a couple hours without force-quitting it (I'm not sure how much time is needed to reproduce this) and then bring the app back to foreground, an all-black screen is shown, as if the root controller disappeared. And we have to kill the app and reopen it again in order to use it.

This is driving me crazy because it is really hard to reproduce, since if you leave the app in background only for a few minutes it works fine. I'm suspecting it has something to do with the app suspended state, but I'm not really sure.

Do you know where the problem might be?

Thank you!

EDIT

Maybe this has something to do with the problem: On the LaunchView, I'm sending the user to the main screen (if he is logged in) with the following code:

guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else { return }

let rootVC = UIStoryboard.main.rootViewController
if let snapshot = appDelegate.window?.snapshotView(afterScreenUpdates: true) {

    rootVC.view.addSubview(snapshot);
    appDelegate.window?.rootViewController = rootVC

    UIView.transition(with: snapshot, duration: 0.4, options: .transitionCrossDissolve, animations: {
        snapshot.layer.opacity = 0;
    }, completion: { (status) in
        snapshot.removeFromSuperview()
    })
}
else {
    appDelegate.window?.rootViewController = rootVC
}

Is it possible that iOS is removing rootViewController at some point?

Jose

This issue in simulator or device? Is it a universal application? – Ordinate

@Ordinate This occurs on a device, I didn't try it on the simulator. The app is for iPhone only – Jose

do you have LaunchScreen ? – Ratite

@Ratite yes, it is storyboard named Launch.storyboard. This is selected as Launch Screen File (UILaunchStoryboardName key in Info.plist). – Jose

Any other storyboard in the app? I mean in info.plist – Ordinate

please share your code for appDidEnterBackground – Meteorology

@Ordinate Yes, a lot of them (12 I think). Also, I still have Main.storyboard as I have the main view controller in there and didn't refactor it yet. – Jose

have you tried this? definesPresentationContext = true in viewDidLoad() – Meteorology

@JulianSilvestri In applicationDidEnterBackground I'm posting an `NSNotification' to default notification center, and using it in a couple observers to stop some processes running in background threads. Nothing UI-related. – Jose

You have to add <key>UIMainStoryboardFile</key> <string>Main</string> this in info plist – Ordinate

@JulianSilvestri no, I did not try definesPresentationContext = true, but I will and let you know, thanks! – Jose

@Ordinate I don't think so, because I'm not using Main.storyboard as the Main Interface, I'm only using it as a regular storyboard. – Jose

Share the code in AppDelegate – Ordinate

Without seeing what happens when app goes to background or returns from background, it feels like guessing in the dark. Could you share the AppDelegate code? If too much or too messy, focus on app going to background and app coming to foreground methods. – Sidran

@Ordinate @AlexIoja-Yang sorry, but the AppDelegate is huge. The app state related methods I only do something in are applicationDidBecomeActive and applicationDidEnterBackground. Both methods are sending an NSNotification to the Default center to inform interested observers, and these observers don't execute any UI-related code. I don't think this part has nothing to do with the problem, because it is doing exactly the same than before (when the problem didn't exist) – Jose

please share the complete code over this link "justpaste.it" and share the link here. – Brooke

@Brooke sorry but this won't be possible. I don't own neither this code nor its rights. I can't share the whole code. – Jose

@JulianSilvestri setting definesPresentationContext = true on viewDidLoad() didn't work :( – Jose

Answer 1

Turns out the problem was related to the transition effect involving a snapshot when swaping the window's rootViewController.

I removed the transition effect and simply changed the rootViewController. Now it's working fine!

In code, I changed this:

guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else { return }

let rootVC = UIStoryboard.main.rootViewController
if let snapshot = appDelegate.window?.snapshotView(afterScreenUpdates: true) {

    rootVC.view.addSubview(snapshot);
    appDelegate.window?.rootViewController = rootVC

    UIView.transition(with: snapshot, duration: 0.4, options: .transitionCrossDissolve, animations: {
        snapshot.layer.opacity = 0;
    }, completion: { (status) in
        snapshot.removeFromSuperview()
    })
}
else {
    appDelegate.window?.rootViewController = rootVC
}

To this:

guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else { return }
let rootVC = UIStoryboard.main.rootViewController
appDelegate.window?.rootViewController = rootVC

Thank you all for your help! I hope this answer is useful to somebody else.

Jose

You should not be changing the root view controller at all. – Ventriloquy

@Ventriloquy I'm changing it to avoid stacking all the other view controllers to my LaunchViewController. What should I do instead? – Jose

Have one eternal root view controller that is just a parent view controller and change its child. – Ventriloquy

@Ventriloquy Why? Is there any documentation or guidelines that says is a bad practice to change the rootViewController? – Jose

Well one obvious thing is that you could then have your transition that you just had to eliminate. Thus you would solve the problem the right way. – Ventriloquy

@Ventriloquy Ok, but I don't think that deserves a downvote. I accept that as a suggestion, although I think it makes no sense keeping that VC (which won't be used or seen anymore) in the stack. – Jose

On the contrary an unseen master parent view controller is a correct and standard technique straight from Apple Inc. – Ventriloquy

See #41145023 – Ventriloquy