title: StackView isHidden attribute not updating as expected
body: <p>I'm trying to update a <code>UIStackView</code> so that a field displays, should the value of a <code>UITextField</code> equal <code>"Other"</code>. Here is my code:</p>
  
  <pre><code>@IBOutlet var stackView: UIStackView!
  func updateView() {
      print("UPDATING")
      UIView.animate(withDuration: 0.25, animations: { () -&gt; Void in
           if(self.myTextField.text! == "Other") {
                print("SHOWING")
                self.stackView.arrangedSubviews[3].isHidden = false
           } else {
                print("HIDING")
                self.stackView.arrangedSubviews[3].isHidden = true
           }
           print("Is hidden: \(self.stackView.arrangedSubviews[3].isHidden )")
      })
  </code></pre>
  
  <p>An example output looks like this:</p>
  
  <pre><code>&gt; UPDATING
  &gt; HIDING
  &gt; Is hidden: true
  &gt; UPDATING
  &gt; SHOWING
  &gt; Is hidden: true
  </code></pre>
  
  <p>As you can see, the <code>isHidden</code> attribute is reported as <code>true</code>, regardless of what the code above has set it to. I can't really figure out why that might be, but perhaps someone on here can? Is there something obvious to check? Is there any reason why <code>isHidden</code> can't be updated? (note there are no errors appearing in the output).</p>