app_for_reddit/Reddit App/Views/Components/WindowFullScreenObserver.swift

import AppKit
import SwiftUI

struct WindowFullScreenObserver: NSViewRepresentable {
    @Binding var isFullScreen: Bool

    func makeCoordinator() -> Coordinator {
        Coordinator(isFullScreen: $isFullScreen)
    }

    func makeNSView(context: Context) -> WindowObservationView {
        let view = WindowObservationView()
        view.onWindowChange = { [weak coordinator = context.coordinator] window in
            coordinator?.observe(window: window)
        }
        return view
    }

    func updateNSView(_ nsView: WindowObservationView, context: Context) {
        context.coordinator.observe(window: nsView.window)
    }

    final class Coordinator {
        @Binding private var isFullScreen: Bool
        private var observers: [NSObjectProtocol] = []
        private weak var observedWindow: NSWindow?

        init(isFullScreen: Binding<Bool>) {
            _isFullScreen = isFullScreen
        }

        func observe(window: NSWindow?) {
            guard window !== observedWindow else {
                if let window {
                    updateFullScreenState(for: window)
                }
                return
            }

            removeObservers()
            observedWindow = window

            guard let window else {
                isFullScreen = false
                return
            }

            updateFullScreenState(for: window)

            let center = NotificationCenter.default
            observers = [
                center.addObserver(
                    forName: NSWindow.didEnterFullScreenNotification,
                    object: window,
                    queue: .main
                ) { [weak self] _ in
                    self?.updateFullScreenState(for: window)
                },
                center.addObserver(
                    forName: NSWindow.didExitFullScreenNotification,
                    object: window,
                    queue: .main
                ) { [weak self] _ in
                    self?.updateFullScreenState(for: window)
                },
            ]
        }

        private func updateFullScreenState(for window: NSWindow) {
            isFullScreen = window.styleMask.contains(.fullScreen)
        }

        private func removeObservers() {
            observers.forEach { NotificationCenter.default.removeObserver($0) }
            observers.removeAll()
        }

        deinit {
            removeObservers()
        }
    }
}

final class WindowObservationView: NSView {
    var onWindowChange: ((NSWindow?) -> Void)?

    override func viewDidMoveToWindow() {
        super.viewDidMoveToWindow()
        onWindowChange?(window)
    }

    override func hitTest(_ point: NSPoint) -> NSView? {
        nil
    }
}

private struct FullScreenLayoutModifier: ViewModifier {
    @State private var isFullScreen = false

    func body(content: Content) -> some View {
        VStack(spacing: 0) {
            if isFullScreen {
                AppTheme.sidebarBackground
                    .frame(height: AppWindow.fullScreenTopInset)
                    .frame(maxWidth: .infinity)
                    .allowsHitTesting(false)
            }

            content
                .frame(maxWidth: .infinity, maxHeight: .infinity)
        }
        .frame(maxWidth: .infinity, maxHeight: .infinity)
        .background {
            WindowFullScreenObserver(isFullScreen: $isFullScreen)
        }
    }
}

extension View {
    func fullScreenLayoutWhenNeeded() -> some View {
        modifier(FullScreenLayoutModifier())
    }
}