How to exclude certain AppKit views from restorable NSWindow?

Question

NSWindows can be made restorable so that their configuration is preserved between application launches.

https://developer.apple.com/documentation/appkit/nswindow/1526255-restorable

Windows should be preserved between launch cycles to maintain interface continuity for the user. During subsequent launch cycles, the system tries to recreate the window and restore its configuration to the preserved state. Configuration data is updated as needed and saved automatically by the system.

In a new macOS project, the NSWindow on a Storyboard is restorable by default:

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My problem comes when embedding an NSTabViewController in the NSWindow.

The NSTabView is inheriting the window's restorable state automatically, with no added code.

This makes the selected tab persist between app launches. I don't want that. I want it to always default to index 0. If the selected tab is restored, attempting to select a tab programmatically in viewDidLoad has unexpected results.

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How can I force certain AppKit UI elements to be excluded from NSWindow state restoration?

I want the Tab View to be un-restorable.

But I would like to keep other restorable benefits, such as restoring the previously-set window size.

How can single views be excluded from NSWindow state restoration?

Answer 1

The key to state restoration is the NSResponder method restoreStateWithCoder:

This method is part of the window restoration system and is called at launch time to restore the visual state of your responder object. The default implementation does nothing but specific subclasses (such as NSView and NSWindow) override it and save important state information. Therefore, if you override this method, you should always call super at some point in your implementation.

https://developer.apple.com/documentation/appkit/nsresponder/1526253-restorestate

So, to not restore a certain control, make this method a no-op.

It says that you "should always call super", but that restores the window state. So if you don't want window restoration, don't call super.

In the case of a Tab View, it evidently must be done in the NSTabView (subclass) itself. In other views, overriding this method on the View Controller may work.

class SomeTabView: NSTabView {
    
    override func restoreState(with coder: NSCoder) {
        // Do NOT restore state
    }
    
}

Answer 2

AFAIK, you cannot exclude a certain part of the UI from being restorable. It is an either ON or OFF thing for all elements. That's why I rarely use Apple's own restorability APIs, as more often than not, they are unreliable. I always do the restoration myself to get that fine control that you need. For simpler windows, however, I let the system do the restoration.

After this preamble, and to really answer your question, I rarely use viewDidLoad() to set up any windows, because as you found out that has some nasty consequences (e.g., the window might not exist yet!). I always do that in viewWillAppear(). For that to happen, you need to set up the following:

1. You need to have an ivar (let's call it tabViewController) to your NSTabViewController instance in your parent NSViewController (let's call it NSViewMainController)

2. Override prepare(for segue: NSStoryboardSegue, sender: Any?) in NSViewMainController and set up the NSTabViewController and its NSViewController children like this:

   override func prepare(for segue: NSStoryboardSegue, sender: Any?) {
      // set up the tabViewController ivar
      self.tabViewController = segue.destinationController as? NSTabViewController
   
      // set up the child NSViewControllers if you need to access them via their parent (otherwise this step is not needed)
      if let childControllers = tabViewController?.children {
         for controller in childControllers {
            if let controller = controller as? NSViewController1 {
               childController1 = controller
            }
            else if let controller = controller as? NSViewController2 {
               childController2 = controller
            }
            else if let controller = controller as? NSViewController3 {
               childController3 = controller
            }
         }
      }
   }
   

3. Override viewWillAppear() of NSViewMainController and then set up the desired tabView:

   guard let controller = tabViewController else { return }
   controller.selectedTabViewItemIndex = 0
   

Major caveat: Beware of viewWillAppear(), though... Unlike viewDidLoad(), this override can be called multiple times, and thus you need to take that into account in your code and react appropriately.