title: Update / run code while typing in text field [Swift / iOS]
body: <p>I'm making a basic app for iOS, and I was hoping somebody would be able to help me.</p>
  
  <p>Basically the app takes numbers from text fields and enters them into a formula which is output to a label. This calculation is performed when I tap a button.</p>
  
  <p>Is there a way to automatically calculate this after every keypress/number entered, so I don't have to press the "Calculate" button every time?</p>
  
  <p>Thanks</p>