title: ForeceFully set view height to zero
body: <p>I have a view which has many <code>subview</code>. Height of view is depend upon height of <code>subViews</code>.But in a case I want view to be of height 0 regardless of its <code>subView</code> height. I to achieve that. 
  for example there is a view called view and it has 2 <code>subView(image and label)</code> <code>view1</code> and <code>view2</code>.
  constraint are like this. </p>
  
  <pre><code> view1.top = view.top + 1
  view1.height = 30
  view2.top = view1.bottom
  view.bottom = view2.bottom.
  </code></pre>