title: How to detect keyboard frame changes when dismiss the keyboard interactively?
body: <p>Consider this scenario, I have a textview with keyboard Dismiss interactively set in storyboard, so when user scroll down and able to dismiss keyboard interactively.
  I have constraints on the textview to bottom to make sure it is always fully displayed on the view.</p>
  
  <p>Current problem is, when user gradually scroll down to dismiss the keyboard, I can not detect the keyboard frame changes. I tried <code>UIKeyboardWillHideNotification</code> and <code>UIKeyboardWillChangeFrameNotification</code>, they were only called after the keyboard dismissed.</p>
  
  <p>So my question is, how can we detect keyboard frame changes simultaneously when dismiss the keyboard interactively?</p>