title: WPF Treeview visible Items when Scrolling
body: <p>I'm using a Multiselect Treeview in MVVM WPF App. A ReactiveCollection is binded as the treeview source. Implemented paging mechanism internally for the source and populated the collection with dummy items. </p>
  
  <p>Now I need to how many items (rows) in the treeview are actually visible in the GUI at any point of time and when the user scrolls. Basically, I want to know the first and last item index in the visible view at any given time and I will implement paging based on this item(row) index and scrolling. </p>
  
  <p>And this logic to detect item index must be implemented as a behaviour for itemscontrol, so that I can reuse it for any itemscontrol.</p>