title: UILabel appear in debug view hierarchy but not in the application
body: <p>I have a UIView that I added in ViewController through the Interface Builder.</p>
  
  <p>In my code, I create a label in this UIView every time that a button is pressed:</p>
  
  <pre><code>func addToDisplay(stringToDisplay: String) {
      let label = UILabel(frame: CGRectMake(0, 0, 10, 10))
      label.text = stringToDisplay
      label.textColor = UIColor.redColor()
      label.textAlignment = .Right
      label.sizeToFit()
      label.adjustsFontSizeToFitWidth = true
      label.minimumScaleFactor = 0.5
      label.setTranslatesAutoresizingMaskIntoConstraints(false)
  
      var lastLabel: UILabel?
  
      if displayView.subviews.count &gt; 0 {
          lastLabel = displayView.subviews[displayView.subviews.count - 1] as? UILabel
      }
  
      displayView.addSubview(label)
  
      var viewsDictionary = ["displayView": displayView, "label": label]
  
      // add constraints
      if let lastLabel = lastLabel {
          viewsDictionary["lastLabel"] = lastLabel
          displayView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:[label(==20)]-0-[lastLabel]", options: nil, metrics: nil, views: viewsDictionary))
      } else {
          displayView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:[label(==20)]-0-|", options: nil, metrics: nil, views: viewsDictionary))
      }
      displayView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-0-[label]-0-|", options: nil, metrics: nil, views: viewsDictionary)) 
  }
  </code></pre>
  
  <p>When I press the button, nothing appear on my device (nor on the simulator) but when I check the view debug hierarchy, it's there. I don't understand why. Any idea?</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>Here are two screenshots:</p>
  
  <p><img src="https://i.stack.imgur.com/i9xwC.png" alt="enter image description here"></p>
  
  <p><img src="https://i.stack.imgur.com/7nWXT.png" alt="enter image description here"></p>
  
  <p><strong>EDIT2:</strong></p>
  
  <p>Solved by changing alpha to something different than 0...</p>