How to hide game's state when going to background - ios

Question

I am writing a logic puzzle game in SpriteKit that plays on the iPad and is against the clock, and am struggling to hide the puzzle neatly when the app goes into the background.
The issue is that user shouldn't be able to double-tap on the home button and see the full puzzle in the App Switcher, as this would allow them to work through it without the clock running.
This is the solution I have come up with:
In the Singleton GameManager there is a variable that is a SKTexture(), to hold a screenshot texture, and in my AppDelegate I have:
func applicationWillResignActive(_ application: UIApplication) {
NotificationCenter.default.post(name: NSNotification.Name(rawValue: "resigning"), object: self)
let tempBackground = UIImageView(image: UIImage(cgImage: GameManager.shared.puzzleImage.cgImage()))
tempBackground.frame = CGRect(x: 0, y: 0, width: 1024, height: 768)
tempBackground.tag = 1000
self.window?.addSubview(tempBackground)
self.window?.bringSubviewToFront(tempBackground)
}
This sends out a notification when it's about to resign the focus, which is picked up by my GameScene, which then creates a screenshot and stores it to the variable in my GameManager. This is then added as a subview in the AppDelegate.
class GameScene: SKScene {
override func didMove(to view: SKView) {
// Layout puzzle here
NotificationCenter.default.addObserver(self, selector: #selector(appToBackground), name: NSNotification.Name(rawValue: "resigning") , object: nil)
}
#objc private func appToBackground() {
// Save state of puzzle and hide it
GameManager.shared.puzzleImage = SKView().texture(from: self)!
}
}
This all works. But not brilliantly. There is a noticeable time-lag between double-tapping and the tempBackground being added - the App Switcher shows the puzzle in detail very briefly and then changes the image to the hidden puzzle.
When the app comes back into focus, the following is called in AppDelegate:
func applicationDidBecomeActive(_ application: UIApplication) {
if let tempBackground = self.window?.viewWithTag(1000) {
tempBackground.removeFromSuperview()
}
}
But when the app returns to focus, the tempBackground is shown, then there is a very brief glimpse of the puzzle in all its detail, before once again showing the hidden puzzle.
I may well have gone about this completely the wrong way, but after reading various archive questions and articles on the internet, this seemed to be way to go.
What I'd like to know is: is there any way that I can have the hidden puzzle shown in the App Switcher immediately and avoid the flash of puzzle detail when returning to the puzzle?
Thanks.

Answer 1

There is no issue in adding subview logic, but the issue is in where are you triggering it, as per apple docs, you should add subview in applicationDidEnterBackground and should remove you subview and prepare your app to display in applicationWillEnterForeground
As I have created Xcode project in Xcode 12, my project has scene delegate, here is the code I used and O/P is shown in gif below
func sceneWillResignActive(_ scene: UIScene) {
guard let view = Bundle.main.loadNibNamed("HiddenView", owner: nil, options: [:])?[0] as? HiddenView else { return }
view.tag = 1000
view.frame = UIApplication.shared.windows[0].frame
UIApplication.shared.windows[0].addSubview(view)
UIApplication.shared.windows[0].bringSubviewToFront(view)
}
func sceneWillEnterForeground(_ scene: UIScene) {
if let tempBackground = UIApplication.shared.windows[0].viewWithTag(1000) {
tempBackground.removeFromSuperview()
}
}
EDIT 1:
As OP has mentioned, that he is not using Scene Delegate I am updating the answer for AppDelegate
func applicationWillResignActive(_ application: UIApplication) {
guard let view = Bundle.main.loadNibNamed("HiddenView", owner: nil, options: [:])?[0] as? HiddenView else { return }
view.tag = 1000
view.frame = UIApplication.shared.windows[0].frame
UIApplication.shared.windows[0].addSubview(view)
UIApplication.shared.windows[0].bringSubviewToFront(view)
}
func applicationDidBecomeActive(_ application: UIApplication) {
if let tempBackground = UIApplication.shared.windows[0].viewWithTag(1000) {
tempBackground.removeFromSuperview()
}
}

Answer 2

I have a solution, although it feels more like a work-around than a proper solution.
The problem is that iOS takes a snapshot of the screen before sending the app into the background and uses this when the app returns to the foreground.
This snapshot can be replaced by using either a predefined view (see #Sandeep's solution) or creating a new snapshot of the screen once the puzzle has been hidden (my original attempted solution).
However, removing this dummy in the applicationDidBecomeActive means that the original snapshot taken by the iOS is shown briefly before updating the screen.
There is a function UIApplication.shared.ignoreSnapshotOnNextApplicationLaunch() which can be entered into the applicationWillResignActive() function, but it doesn't seem to be working all the time - just most of it. In fact, on the Apple Developer's forum there's a comment about how it's inconsistent and a bug was reported. But that was over a year ago and it's still inconsistent.
Instead, in this function add the dummy not to the self.window? directly, but to the view of the rootViewController, using
self.window?.rootViewController!.view.addSubview(dummyImage)
Then don't actually remove this when the app comes back into the foreground. Instead, use Notification to alert GameScene that the app is back in the foreground. This can then run a method which adds a UITapGestureRecognizer to the scene. This in turn can be linked to a method that fades out the dummy image, before removing both it and the UITapGestureRecognizer once it's completed.
Perhaps not the most elegant solution, but it seems to work.