SwiftUI macOS observe app focus

focus.swift

struct Example: View {
  @State private var isWindowActive: Bool = false

  var body: some View {
    Text("Is Window Active? \(isWindowActive ? "yes" : "no")")
      .modifier(WindowStateModifier(isWindowActive: $isWindowActive))
  }
}

struct WindowStateModifier: ViewModifier {
  var isWindowActive: Binding<Bool>

  func body(content: Content) -> some View {
    content
      .onAppear {
        self.isWindowActive.wrappedValue = true
      }
#if os(macOS)
      .onReceive(NotificationCenter.default.publisher(for: NSApplication.didBecomeActiveNotification), perform: { _ in // swiftlint:disable:this
        self.isWindowActive.wrappedValue = true
      })
      .onReceive(NotificationCenter.default.publisher(for: NSApplication.willResignActiveNotification), perform: { _ in // swiftlint:disable:this
        self.isWindowActive.wrappedValue = false
      })
#endif
  }
}