SwiftUI macos NSWindow instance

Question

Using Xcode 12.3 and Swift 5.3 with the SwiftUI App lifecycle to build a macOS application, what is the best way to access and change the appearance and behaviour of the NSWindow?

Edit: What I'm really after is the NSWindow instance.

I've added an AppDelegate, but as I understand it the NSWindow is likely to be nil, so unavailable for modification, and simply creating one here similar to the AppKit App Delegate lifecycle method results in two windows appearing at launch.

One solution would be preventing the default window from appearing, and leaving it all to the applicationDidFinishLaunching method, but not sure this is possible or sensible.

The WindowStyle protocol looks to be a possible solution, but not sure how best to leverage that with a CustomWindowStyle at this stage, and whether that provides access to the NSWindow instance for fine-grained control.

class AppDelegate: NSObject, NSApplicationDelegate {        
    func applicationDidFinishLaunching(_ aNotification: Notification) {
      // In AppKit simply create the NSWindow and modify style.
      // In SwiftUI creating an NSWindow and styling results in 2 windows, 
      // one styled and the other default.
    }
}

@main
struct testApp: App {
    
    @NSApplicationDelegateAdaptor(AppDelegate.self) var appDelegate : AppDelegate

    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

Answer 1

Although I am not entirely sure this is exactly the right approach, based on the answer to this question: https://stackoverflow.com/a/63439982/792406 I have been able to access the NSWindow instance and modify its appearance.

For quick reference, here's a working example based on the original code provided by Asperi using xcode 12.3, swift 5.3, and the SwiftUI App Life cycle.

@main
struct testApp: App {    
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

class Store {
    var window: NSWindow
    
    init(window: NSWindow) {
        self.window = window
        self.window.isOpaque = false
        self.window.backgroundColor = NSColor.clear
    }
}

struct ContentView: View {
    @State private var window: NSWindow?
    var body: some View {
        VStack {
            Text("Loading...")
            if nil != window {
                MainView(store: Store(window: window!))
            }
        }.background(WindowAccessor(window: $window))
    }
}

struct MainView: View {

    let store: Store
    
    var body: some View {
        VStack {
            Text("MainView with Window: \(store.window)")
        }.frame(width: 400, height: 400)
    }
}

struct WindowAccessor: NSViewRepresentable {
    @Binding var window: NSWindow?
    
    func makeNSView(context: Context) -> NSView {
        let view = NSView()
        DispatchQueue.main.async {
            self.window = view.window
        }
        return view
    }
    
    func updateNSView(_ nsView: NSView, context: Context) {}
}