Delay of Adding Event Handlers

Question

This is follow up to this thread. I use a Dispatcher to delay setting Event Handlers:

Private Delegate Sub ManageHandles()

Private Sub LoadMe(sender As Object, e As RoutedEventArgs) Handles Me.Loaded
....
    Dim d As New ManageHandles(AddressOf HandleEvents)
    Dispatcher.BeginInvoke(DispatcherPriority.Background, d)Dim d As New ManageHandles(AddressOf HandleEvents)
    Dispatcher.BeginInvoke(DispatcherPriority.Background, d)
End Sub

Private Sub HandleEvents()
	AddHandler lbxPeople.SelectionChanged, AddressOf SelectPerson
	AddHandler cvsPeople.Filter, AddressOf PeopleFilter
End Sub

However, the filter event is still happening during the Load event. Specifically, the PeopleFilter event is fired immediately after the AddHandler cvsPeople.Filter is specified. But using the Dispatcher was supposed to prevent that from happening.

Answer 1

Hi @RogerSchlueter-7899 ,

Dispatcher.BeginInvoke delays when you hook the handlers, it does not stop WPF from immediately doing two normal initialization actions:

1. ListBox SelectionChanged fires because the control sets or synchronizes its selection (e.g. selects the first item) during data binding.
2. CollectionViewSource.Filter fires because as soon as a filter handler is attached WPF refreshes the view to ensure no unfiltered data is shown.

So the first calls are inevitable. The practical fix is to make those early invocations harmless until your initialization finishes.

I suggest using a readiness flag (plus optional DeferRefresh batching) so the first automatic refresh and selection change are ignored, then performing one real refresh once ready.

Private _ready As Boolean = False
Private Sub LoadMe(sender As Object, e As RoutedEventArgs) Handles Me.Loaded
    cvsPeople = CType(Resources("cvsPeople"), CollectionViewSource)
    cvsPeople.Source = ocPeople
    Using cvsPeople.DeferRefresh()
        AddHandler cvsPeople.Filter, AddressOf PeopleFilter
        AddHandler lbxPeople.SelectionChanged, AddressOf SelectPerson
    End Using          ' Filter + selection may fire; _ready still False
    _ready = True
    cvsPeople.View.Refresh()   ' First real filter pass
End Sub
 
Private Sub PeopleFilter(sender As Object, e As FilterEventArgs)
    If Not _ready Then
        e.Accepted = True
        Return
    End If
    Dim p = TryCast(e.Item, Person)
    e.Accepted = (p IsNot Nothing) AndAlso ShouldShowPerson(p)
End Sub
 
Private Sub SelectPerson(sender As Object, e As SelectionChangedEventArgs)
    If Not _ready Then Return
    ' Real selection logic here
End Sub

Hope this helps. Feel free to reach out if there's any problem