Update ViewModel.cs

Author: friedcucumber

CS/HierarchicalDataTemplate/ViewModel.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 
 namespace HierarchicalDataTemplate {
 
@@ -10,22 +11,22 @@ public ViewModel() {
         }
     }
     public class Data {
-        public List<Category> Categories { get; set; }
+        public ObservableCollection<Category> Categories { get; set; }
         public Data() {
-            Categories = new List<Category>();
-            List<Item> subitems = new List<Item>();
+            Categories = new ObservableCollection<Category>();
+            ObservableCollection<Item> subitems = new ObservableCollection<Item>();
             subitems.Add(new Item() { ItemName = "Chair", Description = "A red chair." });
             subitems.Add(new Item() { ItemName = "Table", Description = "An old table." });
             Categories.Add(new Category() { CategoryName = "Furniture", Items = subitems });
-            List<Item> books = new List<Item>();
+            ObservableCollection<Item> books = new ObservableCollection<Item>();
             books.Add(new Item() { ItemName = "Dictionary", Description = "My old French-English Dictionary" });
             Categories.Add(new Category() { CategoryName = "Books", Items = books });
         }
     }
     public class Category {
         public string CategoryName { get; set; }
         public string Description { get; set; }
-        public List<Item> Items { get; set; }
+        public ObservableCollection<Item> Items { get; set; }
     }
     public class Item {
         public string ItemName { get; set; }