Found the solution. Remove NSToolbarDelegate inheritance from the SceneDelegate and add a SceneDelegate extension that inherits from NSToolbarDelegate. The extension is enclosed in #if targetEnvironment(macCatalyst)/#endif.

import UIKit

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

var window: UIWindow?
var tabBarController: UITabBarController!

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
    guard let windowScene = (scene as? UIWindowScene) else { return }

    let window = UIWindow(windowScene: windowScene)

    tabBarController = UITabBarController()
    tabBarController.viewControllers = [FirstViewController(), SecondViewController()]
    window.rootViewController = tabBarController
    self.window = window
    window.makeKeyAndVisible()

if targetEnvironment(macCatalyst)

    let toolbar = NSToolbar(identifier: "mainToolbar")
    toolbar.centeredItemIdentifier = NSToolbarItem.Identifier(rawValue: "mainTabsToolbarItem")
    toolbar.delegate = self

    windowScene.titlebar?.titleVisibility = .hidden
    windowScene.titlebar?.toolbar = toolbar

    tabBarController.tabBar.isHidden = true

endif

}

func sceneDidDisconnect(_ scene: UIScene) {
    // Called as the scene is being released by the system.
    // This occurs shortly after the scene enters the background, or when its session is discarded.
    // Release any resources associated with this scene that can be re-created the next time the scene connects.
    // The scene may re-connect later, as its session was not neccessarily discarded (see `application:didDiscardSceneSessions` instead).
}

func sceneDidBecomeActive(_ scene: UIScene) {
    // Called when the scene has moved from an inactive state to an active state.
    // Use this method to restart any tasks that were paused (or not yet started) when the scene was inactive.
}

func sceneWillResignActive(_ scene: UIScene) {
    // Called when the scene will move from an active state to an inactive state.
    // This may occur due to temporary interruptions (ex. an incoming phone call).
}

func sceneWillEnterForeground(_ scene: UIScene) {
    // Called as the scene transitions from the background to the foreground.
    // Use this method to undo the changes made on entering the background.
}

func sceneDidEnterBackground(_ scene: UIScene) {
    // Called as the scene transitions from the foreground to the background.
    // Use this method to save data, release shared resources, and store enough scene-specific state information
    // to restore the scene back to its current state.
}

}

// MARK: - NSToolbarDelegate

#if targetEnvironment(macCatalyst) extension SceneDelegate: NSToolbarDelegate {

func toolbar(_ toolbar: NSToolbar, itemForItemIdentifier itemIdentifier: NSToolbarItem.Identifier, willBeInsertedIntoToolbar flag: Bool) -> NSToolbarItem?
{
    if (itemIdentifier.rawValue == "mainTabsToolbarItem")
    {
        let group = NSToolbarItemGroup.init(itemIdentifier: itemIdentifier,
                                            titles: ["First", "Second"],
                                            selectionMode: .selectOne,
                                            labels: ["First view", "Second view"],
                                            target: self, action: #selector(toolbarGroupSelectionChanged))
        group.setSelected(true, at: 0)
        return group
    }

    return nil
}

func toolbarDefaultItemIdentifiers(_ toolbar: NSToolbar) -> [NSToolbarItem.Identifier] {
    return [NSToolbarItem.Identifier(rawValue: "mainTabsToolbarItem")]
}

func toolbarAllowedItemIdentifiers(_ toolbar: NSToolbar) -> [NSToolbarItem.Identifier] {
    return self.toolbarDefaultItemIdentifiers(toolbar)
}

@objc func toolbarGroupSelectionChanged(_ sender: NSToolbarItemGroup) {
    print("toolbarGroupSelectionChanged( \(sender.selectedIndex) )")
    tabBarController.selectedIndex = sender.selectedIndex
}

} #endif

kylehowells / ikyle.me-code-examples

macCatalyst-titlebar-tabbar-swift SceneDelegate Issue #2

if targetEnvironment(macCatalyst)

endif