Can Xcode Document-based app be set to always open in a new tab

Question

I have an macOS xcode swiftui document-based app. I can see that if I, "view->show tab bar" in the app, I get multiple tabs for free. However, if I do a "file open ...", it creates a new window. Now I eventually found that holding down option when selecting the file will open it in a new tab of an existing window. Does anyone know if it is possible to configure a document-based app to always open in a new tab rather than a new window each time?

Answer 1

Set window.tabbingMode to preferred in makeWindowControllers().

override func makeWindowControllers() {
    // Create the SwiftUI view that provides the window contents.
    let contentView = ContentView()

    // Create the window and set the content view.
    let window = NSWindow(
        contentRect: NSRect(x: 0, y: 0, width: 480, height: 300),
        styleMask: [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView],
        backing: .buffered, defer: false)
    window.isReleasedWhenClosed = false
    window.center()
    window.contentView = NSHostingView(rootView: contentView)
    
    window.tabbingMode = .preferred // open new document in tab
    
    let windowController = NSWindowController(window: window)
    self.addWindowController(windowController)
}

Answer 2

OK, for anyone who ends up here. Thanks to the hint from El Tomato, I have achieved (mostly) what I wanted. Key steps are:

1. Get the window in the application that has a tabGroup property that is not nil
2. Having got that, then use the addTabbedWindow function to add your window to the tabGroup
3. If the application has no window with a populated tabGroup property, then you have no tabs, so once you have a new window, invoke the toggleTabBar function to show the tabBar. The subsequent calls to makeViewControllers will find the window that contains the tabs.

Code from the makeWindowControllers for me was...

let tabsWindow = NSApplication.shared.windows.filter({$0.tabGroup != nil}).first
let window = NSWindow(
  contentRect: NSRect(x: 0, y: 0, width: 480, height: 300),
  styleMask: [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView],
  backing: .buffered, defer: false)
window.isReleasedWhenClosed = false
window.center()
window.contentView = NSHostingView(rootView: contentView)
if tabsWindow != nil {tabsWindow!.addTabbedWindow(window, ordered: .below)}
else { window.toggleTabBar(self) }