title: Resize current view using NSLayoutConstraints
body: <p>I'm trying to set the layout of a view using <code>NSLayoutConstraint</code>s.</p>
  
  <p>I can set things up within the view correctly, but I also need to be able to resize the current view (the one that I'm setting the constraints for) if the constraints require it.  In other words, I want to set a fixed content size with a flexible window, rather than the reverse.</p>
  
  <p>So basically, I have:</p>
  
  <pre><code>NSDictionary *views = [NSDictionary dictionaryWithObjectsAndKeys:self.v1, @"v1",
                         self.v2, @"v2",
                         nil];
  // Set horizontal constraints
  [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"|-[v1]-|"
                                                               options:0
                                                               metrics:nil
                                                                 views:views]];
  [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"|-[v2]-|"
                                                               options:0
                                                               metrics:nil
                                                                 views:views]];
  // Set vertical constraints
  [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[v1]-[v2]-|"
                                                               options:0
                                                               metrics:nil
                                                                 views:views]];
  // Initialise height constraint for v1
  self.v1Height = [NSLayoutConstraint constraintWithItem:self.v1
                                                  attribute:NSLayoutAttributeHeight
                                                  relatedBy:NSLayoutRelationEqual
                                                     toItem:nil
                                                  attribute:NSLayoutAttributeNotAnAttribute
                                                 multiplier:1
                                                   constant:50];
  </code></pre>
  
  <p>If I later change <code>self.v1Height</code> (by removing it, recreating it, and then readding it), I would like the frame of <code>self</code> to expand (or contract) to accommodate the changed height.</p>
  
  <p>I'm not sure if it's relevant (as I'm more familiar with iOS than OS X), but this is the content view of a <code>NSPopover</code>.</p>
  
  <p>What constraint do I need to add to achieve this?</p>