title: detect changes to UIWebView's scroll view's contentSize
body: <p>I'm trying to set a UIView at the bottom of the content of a UIScrollView, do to so I set the view's position to the scrollview's contentsize height. But my scrollview is a subview of a UIWebView so when images are loaded, the contentsize height changes and my view which should be at the bottom of the scrollview ends up in the middle...</p>
  
  <p>So I am looking for a way to be notified when the scrollview's contentsize changes. I've tried to subclass it and change the setter for contentsize to send a NSNotification:</p>
  
  <pre><code>@implementation UIScrollView (Height)
  
  -(void)setContentSize:(CGSize)contentSize
  {
      _contentSize=contentSize;
      [[NSNotificationCenter defaultCenter] postNotification:[NSNotification notificationWithName:@"scrollViewContentSizeChanged" object:nil]];
  }
  
  @end
  </code></pre>
  
  <p>But on compile I get and error saying:</p>
  
  <blockquote>
    <p>"_OBJC_IVAR_$_UIScrollView._contentSize", referenced from:
          -[UIScrollView(Heigth) setContentSize:] in MyClass.o
    ld: symbol(s) not found for architecture armv7</p>
  </blockquote>
  
  <p>Any idea how the setter should be subclassed ?</p>
  
  <p>Thanks !</p>