Simple NSPageController example throws an unknown subview warning and stops working - swift

Question

I'm trying to get a very basic NSPageController to work (in book mode, not history mode). It will successfully transition once, and then stop working.
I suspect I'm creating the NSImageViews I'm loading into it wrong, but I can't figure out how.
The storyboard has a the SamplePageController which holds in initial hard-coded NSImageView.
I suspect I'm missing something really obvious here, since all of the tutorial's I've found for NSPageController are in Objective C not swift, and tend to focus on the history view mode.
The code is:
import Cocoa
class SamplePageController: NSPageController, NSPageControllerDelegate {
private var images = [NSImage]()
#IBOutlet weak var Image: NSImageView!
//Gets an object from arranged objects
func pageController(pageController: NSPageController, identifierForObject object: AnyObject) -> String {
let image = object as! NSImage
let image_name = image.name()!
let temp = arrangedObjects.indexOf({$0.name == image_name})
return "\(temp!)"
}
func pageController(pageController: NSPageController, viewControllerForIdentifier identifier: String) -> NSViewController {
let controller = NSViewController()
let imageView = NSImageView(frame: Image.frame)
let intid = Int(identifier)
let intid_u = intid!
imageView.image = images[intid_u]
imageView.sizeToFit()
controller.view = imageView
return controller
// Does this eventually lose the frame since we're returning the new view and then not storing it and the original ImageView is long gone by then?
// Alternatively, are we not sizing the imageView appropriately?
}
override func viewDidLoad() {
super.viewDidLoad()
images.append(NSImage(named:"text")!)
images.append(NSImage(named:"text-2")!)
arrangedObjects = images
delegate = self
}
}

Answer 1

In this case your pageController.view is set to your window.contentView and that triggers the warning. What you need to do is add a subview in the window.contentView and have your pageController.view point to that instead.
The reason for the warning is that since NSPageController creates snapshots (views) of your content history, it will add them at the same level as your pageController.view to transition between them: that means it will try to add them to pageController.view.superview.
And if your pageController.view is set to window.contentView, you are adding subviews to the window.contentView.superview, which is not supported:
New since WWDC seed: NSWindow has never supported clients adding subviews to anything other than the contentView.
Some applications would add subviews to the contentView.superview (also known as the border view of the window). NSWindow will now log when it detects this scenario: "NSWindow warning: adding an unknown subview:".
Applications doing this will need to fix this problem, as it prevents new features on 10.10 from working properly. See titlebarAccessoryViewControllers for official API.