Imports System.ComponentModel
Imports System.Collections.Specialized
Imports System.Windows
Imports System.Windows.Controls.Primitives

Public Class ScrollToTopOnItemsSourceChange

   Public Shared ReadOnly ScrollToTopOnItemsSourceChangeProperty As DependencyProperty =
      DependencyProperty.RegisterAttached("ScrollToTopOnItemsSourceChange",
                                          GetType(Boolean),
                                          GetType(ScrollToTopOnItemsSourceChange),
                                          New UIPropertyMetadata(False, AddressOf OnScrollToTopOnItemsSourceChangePropertyChanged))

   Public Shared Function GetScrollToTopOnItemsSourceChange(ByVal obj As DependencyObject) As Boolean
      Return CBool(obj.GetValue(ScrollToTopOnItemsSourceChangeProperty))
   End Function

   Public Shared Sub SetScrollToTopOnItemsSourceChange(ByVal obj As DependencyObject, ByVal value As Boolean)
      obj.SetValue(ScrollToTopOnItemsSourceChangeProperty, value)
   End Sub

   Private Shared Sub OnScrollToTopOnItemsSourceChangePropertyChanged(ByVal obj As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
      Dim itemsControl = TryCast(obj, ItemsControl)
      If itemsControl Is Nothing Then
         Throw New Exception("ScrollToTopOnItemsSourceChange Property must be attached to an ItemsControl based control.")
      End If

      Dim descriptor As DependencyPropertyDescriptor = DependencyPropertyDescriptor.FromProperty(itemsControl.ItemsSourceProperty, GetType(ItemsControl))
      If descriptor IsNot Nothing Then
         If CBool(e.NewValue) Then
            descriptor.AddValueChanged(itemsControl, AddressOf ItemsSourceChanged)
         Else
            descriptor.RemoveValueChanged(itemsControl, AddressOf ItemsSourceChanged)
         End If
      End If
   End Sub

   Private Shared Sub ItemsSourceChanged(ByVal sender As Object, ByVal e As EventArgs)
      Dim itemsControl = TryCast(sender, ItemsControl)
      DoScrollToTop(itemsControl)
      Dim collection = TryCast(itemsControl.ItemsSource, INotifyCollectionChanged)
      If collection IsNot Nothing Then
         AddHandler collection.CollectionChanged, Sub(o, args) DoScrollToTop(itemsControl)
      End If
   End Sub

   Private Shared Sub DoScrollToTop(itemsControl As ItemsControl)
      Dim eventHandler As EventHandler = Nothing
      eventHandler = New EventHandler(Sub()
                                         If (itemsControl.ItemContainerGenerator.Status = GeneratorStatus.ContainersGenerated) Then
                                            Dim scrollViewer As ScrollViewer = GetVisualChild(Of ScrollViewer)(itemsControl)
                                            scrollViewer.ScrollToTop()
                                            RemoveHandler itemsControl.ItemContainerGenerator.StatusChanged, eventHandler
                                         End If
                                      End Sub)
      AddHandler itemsControl.ItemContainerGenerator.StatusChanged, eventHandler
   End Sub

   Private Shared Function GetVisualChild(Of T As Visual)(ByVal parent As DependencyObject) As T
      Dim child As T = Nothing
      Dim numVisuals As Integer = VisualTreeHelper.GetChildrenCount(parent)
      For i = 0 To numVisuals - 1
         Dim v = CType(VisualTreeHelper.GetChild(parent, i), Visual)
         child = If(TryCast(v, T), GetVisualChild(Of T)(v))
         If child IsNot Nothing Then
            Exit For
         End If
      Next

      Return child
   End Function

End Class