Playing around with hosting SwiftUI Views in an NSWindow from inside a View 🙃 (also works for the NSStatusBar item!)

ContentView.swift

import SwiftUI

struct ContentView: View {

    @State var now = Date()
    @State var showWindow = false
    @State var text: String = ""

    let timer = Timer.publish(every: 1, on: .current, in: .common).autoconnect()

    var body: some View {
        ZStack {

            // Uncomment for a status item that does nothing other than show the title text.
            // StatusItemView(title: .constant("\(now.timeIntervalSince1970)"))

            // Attempt 2: Show a view in a popover from the status item
            StatusItemViewWithPopover(title: .constant("\(now.timeIntervalSince1970)")) {
                VStack(alignment: .leading, spacing: 20) {
                    Text("Here's a view inside SwiftUI")
                        .font(.title)
                    Text("\(self.now.timeIntervalSince1970)")
                }
                .padding(100)
                .frame(minWidth: 400, idealWidth: 400, minHeight: 400, idealHeight: 400)
            }
            
            WindowView(isVisible: $showWindow) {
                Group {
                    Text("\(self.now.timeIntervalSince1970)")
                        .frame(width: 100)
                    Button(
                        action: {
                            self.showWindow.toggle()
                        },
                        label: {
                            Text("Close")
                        }
                    )
                }
                .padding(40)
            }

            HStack {
                Text("\(self.now.timeIntervalSince1970)")
                    .onReceive(timer) { _ in
                        self.now = Date()
                    }

                Button(
                    action: {
                        self.showWindow.toggle()
                    },
                    label: {
                        Text("Tap")
                    }
                )
                .frame(maxWidth: .infinity, maxHeight: .infinity)
            }
        }
    }
}

struct WindowView<WindowContent>: NSViewControllerRepresentable where WindowContent: View {
    typealias NSViewControllerType = NSHostingController<AnyView>

    @Binding var isVisible: Bool
    var windowContent: () -> WindowContent

    init(isVisible: Binding<Bool>, @ViewBuilder windowContent: @escaping () -> WindowContent) {
        self._isVisible = isVisible
        self.windowContent = windowContent
    }

    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    func makeNSViewController(context: NSViewControllerRepresentableContext<WindowView>) -> NSHostingController<AnyView> {
        // Return an empty view as we don't want to render anything where this WindowView is hosted.
        return NSHostingController(rootView: AnyView(EmptyView()))
    }

    func updateNSViewController(_ nsViewController: WindowView.NSViewControllerType, context: NSViewControllerRepresentableContext<WindowView>) {

        context.coordinator.hostingViewController.rootView = AnyView(self.windowContent())

        // Ensure that the visiblity has changed.
        if isVisible != context.coordinator.window.isVisible {
            if isVisible {
                context.coordinator.window.makeKeyAndOrderFront(nil)
            } else {
                context.coordinator.window.orderOut(nil)
            }
        }
    }

    static func dismantleNSViewController(_ nsViewController: NSHostingController<AnyView>, coordinator: WindowView<WindowContent>.Coordinator) {
        print(#function)
    }

    class Coordinator: NSObject {
        var parent: WindowView

        let window = NSWindow(
            contentRect: NSRect(x: 0, y: 0, width: 480, height: 300),
            styleMask: [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView],
            backing: .buffered,
            defer: true
        )

        let hostingViewController = NSHostingController(rootView: AnyView(EmptyView()))

        init(_ windowManager: WindowView) {
            self.parent = windowManager
            window.contentViewController = hostingViewController
        }

    }
}

struct StatusItemView: NSViewControllerRepresentable {
    typealias NSViewControllerType = NSHostingController<AnyView>

    @Binding var title: String

    init(title: Binding<String>) {
        self._title = title
    }

    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    func makeNSViewController(context: NSViewControllerRepresentableContext<StatusItemView>) -> NSHostingController<AnyView> {
        // Return an empty view as we don't want to render anything where this WindowView is hosted.
        return NSHostingController(rootView: AnyView(EmptyView()))
    }

    func updateNSViewController(_ nsViewController: StatusItemView.NSViewControllerType, context: NSViewControllerRepresentableContext<StatusItemView>) {

        context.coordinator.statusItem.button?.title = title

        // Could also add support for all of these!

//        statusItem.button?.toolTip = "Focus"
//        statusItem.image = NSImage(named: "Account")
//        statusItem.alternateImage = NSImage(named: "StatusHighlighted")
//        statusItem.action = #selector(onPress(sender:))
//        statusItem.target = self
//        statusItem.length = NSStatusItem.variableLength
//        statusItem.length = 80 // For now, a fixed length fixes an issue

    }

    class Coordinator: NSObject {
        var parent: StatusItemView

        let statusItem = NSStatusBar.system.statusItem(withLength: NSStatusItem.variableLength)

        init(_ statusItemView: StatusItemView) {
            self.parent = statusItemView
        }

    }
}

StatusItemWithPopover.swift

struct StatusItemViewWithPopover<PopoverContent>: NSViewControllerRepresentable where PopoverContent: View {
    typealias NSViewControllerType = NSHostingController<AnyView>

    @Binding var title: String
    var popoverContent: () -> PopoverContent

    init(title: Binding<String>, @ViewBuilder popoverContent: @escaping () -> PopoverContent) {
        self._title = title
        self.popoverContent = popoverContent
    }

    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    func makeNSViewController(context: NSViewControllerRepresentableContext<StatusItemViewWithPopover>) -> NSHostingController<AnyView> {
        // Return an empty view as we don't want to render anything where this WindowView is hosted.
        return NSHostingController(rootView: AnyView(EmptyView()))
    }

    func updateNSViewController(_ nsViewController: StatusItemViewWithPopover.NSViewControllerType, context: NSViewControllerRepresentableContext<StatusItemViewWithPopover>) {
        context.coordinator.statusItem.button?.title = title
        context.coordinator.hostingViewController.rootView = AnyView(popoverContent())
        // Could also add support for all of these!

//        statusItem.button?.toolTip = "Focus"
//        statusItem.image = NSImage(named: "Account")
//        statusItem.alternateImage = NSImage(named: "StatusHighlighted")
//        statusItem.action = #selector(onPress(sender:))
//        statusItem.target = self
//        statusItem.length = NSStatusItem.variableLength
//        statusItem.length = 80 // For now, a fixed length fixes an issue

    }

    class Coordinator: NSObject {
        var parent: StatusItemViewWithPopover

        let statusItem = NSStatusBar.system.statusItem(withLength: NSStatusItem.variableLength)
        let popover: NSPopover
        var popoverMonitor: AnyObject?

        let hostingViewController = NSHostingController(rootView: AnyView(EmptyView()))

        init(_ statusItemView: StatusItemViewWithPopover) {
            self.parent = statusItemView

            popover = NSPopover()
            super.init()

            popover.animates = false
            popover.contentViewController = hostingViewController

            statusItem.button?.action = #selector(onPress(sender:))
            statusItem.button?.target = self
        }

        @objc func onPress(sender: AnyObject) {
            if popover.isShown == false {
                openPopover()
            }
            else {
                closePopover()
            }
        }

        func openPopover() {
            if let statusView = statusItem.button {
                popover.animates = false
                popover.show(relativeTo: NSZeroRect, of: statusView, preferredEdge: NSRectEdge.minY)
                popoverMonitor = NSEvent.addGlobalMonitorForEvents(matching: .leftMouseDown, handler: { (event: NSEvent!) -> Void in
                    self.closePopover()
                }) as AnyObject
            }
        }

        func closePopover() {
            popover.close()

            if let monitor: AnyObject = popoverMonitor {
                NSEvent.removeMonitor(monitor)
                popoverMonitor = nil
            }
        }

    }
}