Q: What's the correct way to pass an object that will never change to an NSWindowController? I have an NSWindowController that shows a table and uses another controller as the data source and delegate for an NSTableView. This second controller displays information from an object, which is passed in by the NSWindowController. That controller in turns has the object set as a property by the AppDelegate. It looks like this:
class SomeWindowController: NSWindowController {
    var relevantThing: Thing!
    var someTableController: SomeTableController!
    @IBOutlet weak var someTable: NSTableView!

    override func windowDidLoad() {
        someTableController = SomeTableController(thing: relevantThing)
        someTable.dataSource = someTableController
        someTable.delegate = someTableController
    }
}

In the AppDelegate I then do something like
func applicationDidFinishLaunching(_ aNotification: Notification) {
    relevantThing = Thing()
    someWindowController = SomeWindowController()
    someWindowController.relevantThing = relevantThing
    someWindowController.showWindow(nil)
}

Is this a reasonable approach? I feel like the implicitly unwrapped optionals used in SomeWindowController might be bad form. Also, relevantThing is not allowed to change in my case, so I feel a let would be more correct. Maybe the relevantThing should be made constant and passed in through the initializers? Or would that break the init?(coder: NSCoder) initializer?
I'd greatly appreciate any suggestions, as I'm trying to get a feel for the right way to do things in Swift.

A: A few things:
Is there any reason your are creating your window controller in code and not loading it from a storyboard/xib?
Generally, a better practice is to put all your 'controller' that relates to a view in a NSViewController and use NSWindowController only for stuff that relates to the window itself (e.g. toolbar, window management, etc). 
Similarly to iOS, NSViewController is now integrated into the window/view lifecycle and responder chain. For many windows you don't even need to subclass NSWindowController.
XCode's app project template creates a storyboard with the window, main view and their controllers. This is a good starting point.
NSWindowController has a contentViewController property that is set to the NSViewController of the main content view (when loaded from storyboard). You generally don't need a separate view controller property for your view controller.
I think that usually, you want to minimize modifying your controllers from outside code and make them as independent as possible. This makes them more testable and reusable.
If your Thing instance is global for the entire application (as it appears from your code), you may want to consider adding it as a singleton instance to the Thing class and retrieving it from the NSViewController (e.g in viewDidLoad())
If you put your controllers/views in storyboard, you can connect the table's datasource/delegate there. And if this your main window, it can load and show it automatically when the app starts. But in any case, put your NSViewController/View wiring in the view controller.
If you want to separate logic between your main NSViewController into a more specialized view controller that handles a specific part of your view, you can use NSContainerView in Interface Builder to add additional view controllers to handle specific views.
