title: Detect if user copied text from textfield
body: <p>I want to use the word that user copies from my app textfield.</p>
  
  <p>I found this code:</p>
  
  <pre><code>NotificationCenter.default.addObserver(self, selector: #selector(clipboardChanged), name: UIPasteboard.changedNotification, object: nil)
  </code></pre>
  
  <p>But 'clipboardChanged' function not calling when copying text from textfield.</p>
  
  <p>I use Swift 4.2</p>