title: Why is contentView property of NSWindow of type id?
body: <p>Why is the <code>contentView</code> property of the <code>NSWindow</code> class of type <code>id</code> instead of <code>NSView</code></p>
  
  <p>This does not make sense to me, why should the contentView be anything other than a <code>NSView</code> subclass.</p>
  
  <p>So in my case I had to type it like this in order to access its frame:</p>
  
  <pre><code>NSView *contentView = self.window.contentView; // returns an `id`
  CGRect frame = contentView.frame
  </code></pre>
  
  <p>Instead of this, which the compiler does not like:</p>
  
  <pre><code>CGRect frame =  self.window.contentView.frame; // This does not compile
  </code></pre>