How to access own window within SwiftUI view?

SwiftUI Lift-Cycle (SwiftUI 2+)

Here is a solution (tested with Xcode 13.4), to be brief only for iOS

1. We need application delegate to create scene configuration with our scene delegate class

@main
struct PlayOn_iOSApp: App {
    @UIApplicationDelegateAdaptor(AppDelegate.self) var appDelegate

    // ...
}

class AppDelegate: NSObject, UIApplicationDelegate {


    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        let configuration = UISceneConfiguration(name: nil, sessionRole: connectingSceneSession.role)
        if connectingSceneSession.role == .windowApplication {
            configuration.delegateClass = SceneDelegate.self
        }
        return configuration
    }
}

2. Declare our SceneDelegate and confirm it to both (!!!+) UIWindowSceneDelegate and ObservableObject

class SceneDelegate: NSObject, ObservableObject, UIWindowSceneDelegate {
    var window: UIWindow?   // << contract of `UIWindowSceneDelegate`

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let windowScene = scene as? UIWindowScene else { return }
        self.window = windowScene.keyWindow   // << store !!!
    }
}

3. Now we can use our delegate anywhere (!!!) in view hierarchy as EnvironmentObject, because (bonus of confirming to ObservableObject) SwiftUI automatically injects it into ContentView

    @EnvironmentObject var sceneDelegate: SceneDelegate
    
    var body: some View {
         // ...       
            .onAppear {
                if let myWindow = sceneDelegate.window {
                    print(">> window: \(myWindow.description)")
                }
            }
    }

Complete code in project is here

UIKit Life-Cycle

Here is the result of my experiments that looks appropriate for me, so one might find it helpful as well. Tested with Xcode 11.2 / iOS 13.2 / macOS 15.0

The idea is to use native SwiftUI Environment concept, because once injected environment value becomes available for entire view hierarchy automatically. So

1. Define Environment key. Note, it needs to remember to avoid reference cycling on kept window

struct HostingWindowKey: EnvironmentKey {

#if canImport(UIKit)
    typealias WrappedValue = UIWindow
#elseif canImport(AppKit)
    typealias WrappedValue = NSWindow
#else
    #error("Unsupported platform")
#endif

    typealias Value = () -> WrappedValue? // needed for weak link
    static let defaultValue: Self.Value = { nil }
}

extension EnvironmentValues {
    var hostingWindow: HostingWindowKey.Value {
        get {
            return self[HostingWindowKey.self]
        }
        set {
            self[HostingWindowKey.self] = newValue
        }
    }
}

2. Inject hosting window in root ContentView in place of window creation (either in AppDelegate or in SceneDelegate, just once

// window created here

let contentView = ContentView()
                     .environment(\.hostingWindow, { [weak window] in
                          return window })

#if canImport(UIKit)
        window.rootViewController = UIHostingController(rootView: contentView)
#elseif canImport(AppKit)
        window.contentView = NSHostingView(rootView: contentView)
#else
    #error("Unsupported platform")
#endif

3. use only where needed, just by declaring environment variable

struct ContentView: View {
    @Environment(\.hostingWindow) var hostingWindow
    
    var body: some View {
        VStack {
            Button("Action") {
                // self.hostingWindow()?.close() // macOS
                // self.hostingWindow()?.makeFirstResponder(nil) // macOS
                // self.hostingWindow()?.resignFirstResponder() // iOS
                // self.hostingWindow()?.rootViewController?.present(UIKitController(), animating: true)
            }
        }
    }
}

Add the window as a property in an environment object. This can be an existing object that you use for other app-wide data.

final class AppData: ObservableObject {
    let window: UIWindow? // Will be nil in SwiftUI previewers

    init(window: UIWindow? = nil) {
        self.window = window
    }
}

Set the property when you create the environment object. Add the object to the view at the base of your view hierarchy, such as the root view.

let window = UIWindow(windowScene: windowScene) // Or however you initially get the window
let rootView = RootView().environmentObject(AppData(window: window))

Finally, use the window in your view.

struct MyView: View {
    @EnvironmentObject private var appData: AppData
    // Use appData.window in your view's body.
}