title: Why weak IBOutlet NSLayoutConstraint turns to nil when I make it inactive?
body: <p>I have an <code>IBOutlet NSLayoutConstraint</code> in my app. Very simple:</p>
  
  <pre><code>@property (nonatomic, weak) IBOutlet NSLayoutConstraint* leftConstraint;
  </code></pre>
  
  <p>At some point I want to deactivate this constraint:</p>
  
  <pre><code>self.leftConstraint.active = NO;
  </code></pre>
  
  <p>It happens in a method called from <code>cellForRowAtIndexPath:</code>. And the constraint becomes nil right after the line above. However if I declare this property as <code>strong</code> then it's ok, it doesn't turn to nil. Can anybody explain why it happens?</p>