What event to use

Mar 28, 2016 at 10:19 PM
Hi. I use this control to display:

Category 1
 Subcategory1
 Subcategory2
 Subcategory3 
Category 2
 Subcategory1
 Subcategory2
 Subcategory3
etc.
I originally set the value IsExpanded="False". What event to use when I open/close a category and display the subcategories. I want to change the image from a closed to an open category.