title: size(forChildContentContainer container: UIContentContainer,withParentContainerSize parentSize: CGSize) does not work
body: <p>Apple says:</p>
  <blockquote>
  <p>&quot;Container view controllers use this method to return the sizes for
  their child view controllers. UIKit calls the method as part of the
  default implementation of the viewWillTransition(to:with:) method for
  view controllers&quot;</p>
  </blockquote>
  <p>And when i first time launch program:</p>
  <pre><code>class ViewController: UIViewController {
                         
  override func viewDidLoad() {
      super.viewDidLoad()
      
      let child = ViewControllerChild()
      self.addChildViewController(child)
      self.view.addSubview(child.view)
                      
     
      child.view.translatesAutoresizingMaskIntoConstraints = true
      child.view.autoresizingMask = [.flexibleBottomMargin, .flexibleRightMargin]       
  }
  
  override func size(forChildContentContainer container: UIContentContainer, withParentContainerSize parentSize: CGSize) -&gt; CGSize {
      return CGSize(width: 100, height: 100)
  }
  
  override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
      super.viewWillTransition(to: size, with: coordinator)
  }
  </code></pre>
  <p>}
  I get full parent vc view's size child view controller's view not (100, 100). So what the real point to override it, if i can respond to trait changes by redefining child vc view's frame in (1st load in viewdidload) then in preferredContentSizeDidChange function?</p>
  <p><a href="https://i.stack.imgur.com/HZWOw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HZWOw.png" alt="enter image description here" /></a></p>