title: UIButtons won't click in simulator swift
body: <p>Sorry if this is a dumb question. I just can't find why the UIbuttons are not working. They were working for awhile. Not sure what happened. </p>
  
  <pre><code>import UIKit  
  
  class ViewController: UIViewController 
  {
  
  @IBOutlet weak var display: UILabel!
  
  var userIsInTheMiddleOfTypingNumber = false
  
  @IBAction func appendDigit(sender: UIButton) {
      let digit = sender.currentTitle!
      if userIsInTheMiddleOfTypingNumber {
          display.text = display.text! + digit
      }
      else {
          display.text = digit
          userIsInTheMiddleOfTypingNumber = true
      }
  }
  
  var operandStack = Array&lt;Double&gt;()
  
  @IBAction func enter() {
      userIsInTheMiddleOfTypingNumber = false
      operandStack.append(displayValue)
      println("operandStack = \(operandStack)")
  }
  
  var displayValue: Double {
      get {
          return NSNumberFormatter().numberFromString(display.text!)!.doubleValue
      }
      set {
          display.text = "\(newValue)"
          userIsInTheMiddleOfTypingNumber = false
      }
    }
  }
  </code></pre>