title: Unresponsive UIButton in subview added to UIStackView
body: <p>In the detailViewController of a <code>UISplitView</code> I have subviews added to a <code>UIStackView</code> inside a <code>UIScrollView</code>.</p>
  
  <p>Just using system buttons without <code>subviews</code> or <code>images</code>, results in responsive buttons, but <code>subviews</code> seem to interfere.</p>
  
  <p>Enabling touch is specifically coded.  I have attempted to keep each view inside the containing view so there will be no overlap to invalidate receiving touch events, but not sure if this is done properly.
  Each <code>subview</code> contains a <code>label</code> and a custom button with an image. The subview is then added to the <code>stackview</code>, and the <code>stackview</code> to the <code>scrollview</code>.  </p>
  
  <p>Thanks for any help.</p>
  
  <pre><code>override func viewDidLoad() {
      super.viewDidLoad()
  
      scrollView = UIScrollView()
      scrollView.translatesAutoresizingMaskIntoConstraints = false
      view.addSubview(scrollView)
  
      // Constrain the scroll view within the detailView
      view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|[scrollView]|", options: .AlignAllCenterX, metrics: nil, views: ["scrollView": scrollView]))
      view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|[scrollView]|", options: .AlignAllCenterX, metrics: nil, views: ["scrollView": scrollView]))
  
  
      stackView = UIStackView()
  
      stackView.frame = CGRectMake(0,0,view.frame.width, 1000)
      stackView.translatesAutoresizingMaskIntoConstraints = false
      stackView.axis = .Vertical
      scrollView.contentSize = CGSizeMake(400, 1000)
      scrollView.addSubview(stackView)
  
      // Constrain the stackView within the scrollView
  scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|[stackView]|", options: NSLayoutFormatOptions.AlignAllCenterX, metrics: nil, views: ["stackView": stackView]))
  scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|[stackView]", options: NSLayoutFormatOptions.AlignAllCenterX, metrics: nil, views: ["stackView": stackView]))
  
  
      let selectedGroup: Group = GroupArray[5]
  
      let descriptorsArray = selectedGroup.descriptorsArray
  
      for descriptor in descriptorsArray {
          // Create a subview for each descriptor
  
          let subView = UIView()
          subView.frame = CGRectMake(0 , 0, self.stackView.frame.width-10, 54)
          subView.backgroundColor = UIColor.yellowColor()
  
          subView.heightAnchor.constraintEqualToConstant(54.0).active = true
          // Create a label for Descriptor subview
  
          let label = UILabel(frame: CGRectMake(20, 0, 200, 50))
          label.text = descriptor.name
          label.font = UIFont.boldSystemFontOfSize(22.0)
          label.textAlignment = .Left
          label.textColor = UIColor.brownColor()
          label.backgroundColor = UIColor.greenColor()
          label.heightAnchor.constraintEqualToConstant(50.0).active = true
          subView.addSubview(label)
  
          // Create a button for Checkbox
          let btn = UIButton()
          btn.frame = CGRectMake(220, 0, 50, 50)
          btn.backgroundColor = UIColor.blueColor()
          btn.setImage(UIImage(named:"checked.png"), forState: UIControlState.Normal)
  
          btn.heightAnchor.constraintLessThanOrEqualToConstant(50.0)
          btn.widthAnchor.constraintLessThanOrEqualToConstant(50.0)
  
          btn.addTarget(self, action: "btnPressed:", forControlEvents: UIControlEvents.TouchUpInside)
  
          subView.addSubview(btn)
          btn.userInteractionEnabled = true
          subView.userInteractionEnabled = true
          stackView.userInteractionEnabled = true
          scrollView.userInteractionEnabled = true
          stackView.addArrangedSubview(subView)
  
  
      }
  }
  
  func btnPressed(sender: UIButton!) {
  
      print("btn Pressed")
  
  }
  </code></pre>