title: How to detect backspace/delete was pressed on UITextField
body: <p>There has been lots of questions on how to detect when a user presses <code>DELETE</code> button on a <code>UITextField</code>. However, playing around with it I realised it is only possible to trigger an <code>event/action</code> to run custom code when there is actually content in the <code>UITextField</code>. If the <code>UITextField</code> is empty and the user hits <code>DELETE</code>, no action will be fired on <code>iOS</code>.</p>
  
  <p>I tried the following actions: <code>Editing Changed</code>, <code>Editing Did Begin</code>, <code>Editing Did End</code>, <code>Value Changes</code>. Also tried the delegate approach: 
   <code>textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -&gt; Bool</code></p>
  
  <p>The task I am looking to achieve is related to having a "type your pin code" screen where a user can type in a 6-digits numeric code gotten from SMS message. Since each <code>UITextField</code> represents a digit, once the user hits <code>DELETE</code>, the app should clear the <code>UITextField</code> just before the current one.</p>
  
  <p>Any ideas whether this is possible on iOS?</p>