About the outlineView:objectValueForTableColumn:byItem: data source method, there is a note in document says that:
While this method is marked as @optional in the protocol, you must implement this method if you are not providing the data for the outline view using Cocoa bindings.
However, the SideBar demo of Apple samples doesn't implement this method, and can't find any cocoa bindings either, does anyone know why?
There is another method provided for this purpose as well, take a look at - (NSView *)outlineView:(NSOutlineView *)outlineView viewForTableColumn:(NSTableColumn *)tableColumn item:(id)item implemented in the demo app.