traitCollectionDidChange not called on window

Question

I have a situation where I need to inject a full screen view on the window. For whatever reason, inserting the full screen view on `window.rootViewController.view' is not an option. The full screen view responds to trait collection changes dynamically for portrait and landscape layouts.

The problem is that when this full screen view is injected into the root window, it doesn't seem to respond to any trait collection change events (when device is rotated). If I inject this same full screen view inside of any UIViewController's view, it works fine - trait collection events fire as expected. It's only when I try and inject on the window that these trait change events don't fire.

Is there anything I need to do to get trait collection changes to work on the window level? Or is it known that this only works within UIViewControllers and its views?

// This works fine 
UIWindow *window = [[UIApplication sharedApplication].delegate window];
[window.rootViewController.view addSubview:self.fullScreenControlsView];

// View is visible but traitCollectionDidChange does not fire 
UIWindow *window = [[UIApplication sharedApplication].delegate window];
[window addSubview:self.fullScreenControlsView];