title: iOS - Updating auto layout constraint constant stops working after pushing another view controller then returning
body: <p>I have a UIViewController designed in IB (Storyboard) that has a UIView containing a few controls. The UIView has an auto layout constraint to fix its height.  In my UIViewController subclass I have an <code>IBOutlet NSLayoutConstraint</code> (ivar) wired to the height constraint so that I can adjust it in code.  Depending on user events, I expand/shrink the height of the UIView by modifying the constraint's <code>constant</code> property between 80 and 44, which also slides up/down a UITableView which is constrained to the bottom of the UIView. I have used this technique before and all works well.</p>
  
  <p>I am now using 4 instances of this UIViewController within a parent container UIViewController with a UITabBar.  When the UITabBar tab selection is changed, I swap in the appropriate instance using <code>insertSubView</code>.  The problem is that the UIView fails to expand/shrink in the first child view controller tab page. In the other 3 tab page instances it works fine.  Also in the 3 tab pages where it works, if I push another view controller onto the stack and then return back, the resizing of the UIView no longer works.</p>
  
  <p>In the case where the expand/shrink fails, I have used <code>NSLog</code> to monitor the value of the constraint's <code>constant</code> property and it is being changed in every instance, but the height of the UIView is not being affected.  As if the constraint is no longer set to work on the UIView??</p>
  
  <p>I'm using Xcode 5.1 and my app is targetting iOS7.</p>