Switching Storyboard-Based Contextual Menus for NSOutlineView Based on Selection

Question

I like to build contextual menus in the storyboard, as opposed to doing it programmatically.

Typically, in an NSOutlineView, I want to display a particular context menu based on the current selection.

Rather than building a global menu and hiding/showing certain menu items, is there a way to switch menus dynamically by simply assigning the relevant @IBOutlet property?

class ViewController: NSViewController {
    @IBOutlet var menuOne: NSMenu!
    @IBOutlet var menuTwo: NSMenu!
    @IBOutlet var menuThree: NSMenu!
    // ...
}

extension ViewController: NSMenuDelegate {

    func menuNeedsUpdate(_ menu: NSMenu) {
        if // ... {
            menu = menuOne // Error: Cannot assign to value: 'menu' is a 'let' constant
        } else if // ... {
            menu = menuTwo // Error: Cannot assign to value: 'menu' is a 'let' constant
        } else if // ... {
            menu = menuThree // Error: Cannot assign to value: 'menu' is a 'let' constant
        }
    }

}

Apparently, another option is to subclass NSOutlineView and override the menu(for:) method, but I’m unsure how to access my @IBOutlet properties in that case:

class MyOutlineView: NSOutlineView {

    override func menu(for event: NSEvent) -> NSMenu? {
        // ...
        // How am I supposed to access my @IBOutlet properties?

        super.menu(for: event)
    }

}