如何在将项目绑定到ItemsSource时过滤集合？

I created a treeview which modelizes directories and files on my hard drive. Each treeviewItem has a checkbox, binded to a isSelected property. What I'd like to achieve is display for each parent node the count of selected files on the total files count (10 / 12 10 files on twelve total selected).I created a treeview which modelizes directorie