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preferredcontentheight's Introduction

PreferredContentHeight

Purpose

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.

Structure & Problem

The main ViewController opens a PopupViewController that embeds a UINavigationController whose root view controller is the ContentViewController. Pfiou ๐Ÿ˜…

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.

Solution

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.

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