title: Binding NSOutlineView's selection index paths
body: <p>I have a custom <code>&lt;NSOutlineViewDataSource&gt;</code>, for which I would its <code>NSOutlineView</code> to update a <code>selectionIndexPaths</code> property. I can't get <code>bind:toObject:withKeyPath:options:</code> to actually bind properly. Basically, I need my data source to know which item is selected.</p>
  
  <p>When I bind to <a href="http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/CocoaBindingsRef/BindingsText/NSTableView.html#//apple_ref/doc/uid/NSTableView-DontLinkElementID_623" rel="nofollow"><code>selectionIndexes</code></a> (declared by NSTableView) or <a href="http://developer.apple.com/library/mac/documentation/Cocoa/Reference/CocoaBindingsRef/BindingsText/NSOutlineView.html#//apple_ref/doc/uid/NSOutlineView-DontLinkElementID_328" rel="nofollow"><code>selectionIndexPaths</code></a>  (declared by NSOutlineView), which the documentation indicates I should be able to (unless for some reason that's only available in IB), I get a runtime exception that the outline view isn't "key value coding-compliant" for either of those keys.</p>
  
  <p>When I bind to <a href="http://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSTableView_Class/Reference/Reference.html#//apple_ref/doc/uid/20000118-SW79" rel="nofollow"><code>selectedRowIndexes</code></a> (a fallback, since I would need to then take that information and reverse-engineer it to figure out which parent/child pair is selected), it updates the data source once, with the <code>NSIndexSet</code> in the data source frozen to its initial value. I don't set it manually, either. Further, I can't even bind <code>selectedRowIndexes</code> in the other direction, so the data source can update the outline view's selection. I get the same "key value coding-compliant" exception on the outline view.</p>
  
  <p>I have an <code>@property</code> defined in my data source class, with a backing ivar, and I'm binding it from a controller class with references to the data source and the outline view, after data has been loaded, in <code>-awakeFromNib</code>.</p>