title: How to only enable button when two text fields are not empty - Swift + storyboard
body: <p>Recently, I am writing my own project with swift.
  I use storyboard and create two text fields and one button.
  How do I enable the button only when two text fields are not empty?</p>
  <p>Here is what I had so far</p>
  <pre><code>@IBAction func enableNextBtn(_ sender: UITextField) {
      if !sender.text!.isEmpty {
         nextBtn.isEnabled = true
      } else {
         nextBtn.isEnabled = false
      }
  }
  </code></pre>