title: Adding NSProgressIndicators to an NSOutlineView subclass
body: <p>I'm using a heavily modified version of the fantastic <a href="https://github.com/Perspx/PXSourceList/" rel="nofollow">PXSourceList</a> control to emulate other popular OS X source lists. Unfortunately I've run into some issues while attempting to add NSProgressIndicator's to rows within said source list. </p>
  
  <p>PXSourceList acts as a subclass of the standard NSOutlineView control and comes packaged with an example SourceListItem class (a skeletal NSObject subclass). Rendering of the PXSourceList and its SourceListItem's is handled in PXSourceList's <code>drawRow:clipRect:</code> method, which is the source of my problem.</p>
  
  <p>I'm trying to find the best way of allowing each SourceListItem to optionally display an NSProgressIndicator. Instinct tells me I should be creating a custom cell, but I'm unsure as to how I should optimally implement a decent solution.</p>
  
  <p>Any and all help would be appreciated.</p>