SwiftUI perform UI change in background

Question

I am currently implementing a secure window which blocks the View if the app is in the background or shown in the app switcher. However, if the scenePhase is set to background, the View doesn't change in the app switcher. If the View is changed when the scenePhase goes to inactive, the changes are made visible in the app switcher, however, we get unwanted behavior if e.g. the user uses the reachability swipe, the secure window pops in and out. I tried to implement it with a SceneDelegate, but nothing changes in the behavior.

Is there any way to apply UI changes when the scenePhase hits background? I saw this thread here asking the same thing, but apparently it was a bug back then.

Sample Code:

import SwiftUI

@main
struct SwiftUIView: App {
    @State private var showSecureWindow: Bool = false
    @Environment(\.scenePhase) var scenePhase
    
    var body: some Scene {
        WindowGroup {
            if showSecureWindow {
                EmptyView()
            } else {
                Text("Hello")
            }
        }
        .onChange(of: scenePhase) { newScenePhase in
            switch newScenePhase {
            case .active:
                showSecureWindow = false
            case .inactive:
                //showSecureWindow = true // <- This works, but unwanted behavior
                break
            case .background:
                showSecureWindow = true // <- This doesn't work
            @unknown default:
                break
            }
        }
    }
}

Answer 1

So the way the app switcher works is, when your app enters the background, the system takes a screenshot. That screenshot is used to show the preview in the app switcher.

This occurs immediately after the app has entered the background.

The problem here, is that the .onChange block fires off on the next runloop after the environment value has changed. So the app switcher screenshot has already been taken.

You want to perform the change immediately. So you need to base your view logic directly on the scenePhase value.

Like so:

import SwiftUI

@main
struct SwiftUIView: App {
    @Environment(\.scenePhase) var scenePhase
    
    var body: some Scene {
        WindowGroup {
            if scenePhase == .background {
                EmptyView()
            } else {
                Text("Hello")
            }
        }
    }
}