title: How do you extract the height from NSLayoutConstraint
body: <p>I wish to add extra height on my TabBar. I have used CarbonKit scrollable tabbar.
  When I ty to get the toolbar height  I manage to get a statement saying something is height of 40 (I assume it is the tabbar height). I wish to extract the 40 out and add extra 30 to it to the height. How do I do that?</p>
  
  <pre><code>    let carbonTabSwipeNavigation = CarbonTabSwipeNavigation(items: images, delegate: self)
  
      carbonTabSwipeNavigation.insert(intoRootViewController: self)
  
      let tabBarHeight = carbonTabSwipeNavigation.toolbarHeight
  
      print("the Tab Bar Height = " , tabBarHeight)
  </code></pre>
  
  <p>the Tab Bar Height = NSLayoutConstraint:0x2835d8e60 UIToolbar:0x10dd5d160.height == 40   (active)</p>
  
  <p>How to I get the 40 out of the statement? I try casting it to integer but fail</p>
  
  <pre><code>    carbonTabSwipeNavigation.setTabBarHeight(tabBarHeight+tabBarExtraHeight)
  </code></pre>
  
  <p>The above code is will give me error because I can't add them together. </p>