I created this project in order to understand how the mecanism preferredContentSize and func preferredContentSizeDidChange(forChildContentContainer:) is working to bubble up the size of a embedded view controller.

The main ViewController opens a PopupViewController that embeds a UINavigationController whose root view controller is the ContentViewController. Pfiou 😅

When the ContentViewController's preferredContentSize changes, UINavigationController.preferredContentSizeDidChange() is called.

I expected that the navigation controller would then set its preferredContentSize to bubble up the size by triggering PopupViewController.preferredContentSizeDidChange().

That is exactly which occurs when

• ContentViewController preferred width increase
• ContentViewController preferred width decrease
• ContentViewController preferred height increase
• ContentViewController preferred height decrease

Indeed, I don't know why, UINavigationController do not updates its preferredContentSize if the height of its active view controller decrease.

I made a simple navigation controller subclasse which correctly bubble up the preferredContentSize of its active view controller:

class CustomNavigationController: UINavigationController {
    override public func preferredContentSizeDidChange(forChildContentContainer container: UIContentContainer) {
        print("NAV UPDATING CONTENT SIZE")
        if self.navigationBar.isHidden {
            self.preferredContentSize = container.preferredContentSize
        }
        else {
            var size = container.preferredContentSize
            size.height += self.navigationBar.bounds.height
            self.preferredContentSize = size
        }
    }
}

Try it yourself.