title: How to observe NSScroller changes?
body: <p>I have an <code>NSScrollView</code> subclass and I would like to update another <code>NSView</code> based on the current scroll position. I tried KVC-observing the <code>value</code> of <code>[self horizontalScroller]</code> but that never gets called.</p>
  
  <pre><code>// In awakeFromNib
  [[self horizontalScroller] addObserver:self
                              forKeyPath:@"value"
                                 options:NSKeyValueObservingOptionNew
                                 context:NULL];
  
  // Later in the file
  - (void)observeValueForKeyPath:(NSString *)keyPath 
                    ofObject:(id)object 
                      change:(NSDictionary *)change 
                     context:(void *)context {
      if (object == [self horizontalScroller] &amp;&amp; [keyPath isEqualToString:@"value"]) {
          // This never gets called
      }
  }
  </code></pre>
  
  <p>Do you see an error in my reasoning or know a better method of how to observe the scrolling of an <code>NSScrollview</code>?</p>