ObservableList and CollectionChanged event

[Sergej Mertens]

Hi,

I have two objects "Invoice" and "Details". The Invoice class holds a field of ObservableCollection<Details> and therefore has a Property called "InvoiceDetails". In Invoice I create the ObservableCollection-Object and add an eventhandler to the CollectionChanged event.

I now load a previously saved instance from Invoice and add a new Detail to the Collection. All went fine, but the event does not fire.
I tried the same with a BindingList and it's the same behaviour: no event fires.

Example (in VB):

public class Invoice
   Dim _details as ObservableCollection(Of Details) = new ObservableCollection(Of Details)()

   public sub New()
      AddHandler _details.CollectionChanged, AddressOf details_CollectionChanged
   end sub
  
   public property InvoiceDetails() as ObservableCollection(Of Details)
      get
         return _details
      end get
   end property

   private sub details_CollectionChanged(sender as Object, e as EventArgs)
      MessageBox.Show("Detail added")
   end sub


end class

and in a form I call:
dim result as Invoice = (From i in scope.Extent(of Invoice)() Where i.Id = 1 Select i).First()
result.Add(new Detail())

My tasks to solve are:
- I have an object and need to know every change of all sub- und listed objects in this instance. They all implement INotifyPropertyChanged for tracking the changed object and property
- I have a collection to which an instance is added, removed or changed. I now have to recalculate totals and subtotals in the "master" object
- Tracking changes of Collections while this collection is databound to e.g. an UltraWinGrid

Thanks,
Daniel Schilling

Edit: Typo

[PetarP]

Hi Daniel Schilling,
the event is not firing because you have not subscribed on the right place. If you let your class implement the
IInstanceCallbacks interface than you will have access to the lifecycle events of the persistent objects. This interface contains 3 methods that need to be implemented:
PostLoad() - fires when the first persistent field of a class is accessed. You can subscribe your collection to the event handler here
PreRemove(Iobjectscope scope) - this emthod is fired when an object is being removed from the scope
PreStore() - this method is fired when an object is being saved to the database.
To give you an example. If we have 2 classes, Details and Invoices and we let the Invoice implement the IInstanceCallbacks than we will have something similar to that:

<Telerik.OpenAccess.Persistent(IdentityField:="invoiceId_Renamed")> _
    Public Class Invoice
        Implements IInstanceCallbacks
        Private invoiceId_Renamed As Integer
        <Telerik.OpenAccess.FieldAlias("invoiceId_Renamed")> _
        Public Property InvoiceId() As Integer
            Get
                Return invoiceId_Renamed
            End Get
            Set(ByVal value As Integer)
                invoiceId_Renamed = value
            End Set
        End Property
        Private invoiceDescription_Renamed As String
        <Telerik.OpenAccess.FieldAlias("invoiceDescription_Renamed")> _
        Public Property InvoiceDescription() As String
            Get
                Return invoiceDescription_Renamed
            End Get
            Set(ByVal value As String)
                invoiceDescription_Renamed = value
            End Set
        End Property
        Private details_Renamed As ObservableCollection(Of Detail)

        Public Property Details() As ObservableCollection(Of Detail)
            Get
                Return details_Renamed
            End Get
            Set(ByVal value As ObservableCollection(Of Detail))
                details_Renamed = value
            End Set
        End Property

        Public Sub PostLoad() Implements Telerik.OpenAccess.IInstanceCallbacks.PostLoad
            AddHandler details_Renamed.CollectionChanged, AddressOf details_CollectionChanged
        End Sub

        Public Sub PreRemove(ByVal objectScope As Telerik.OpenAccess.IObjectScope) Implements Telerik.OpenAccess.IInstanceCallbacks.PreRemove

        End Sub

        Public Sub PreStore() Implements Telerik.OpenAccess.IInstanceCallbacks.PreStore

        End Sub
        Private Sub details_CollectionChanged(ByVal sender As Object, ByVal e As System.Collections.Specialized.NotifyCollectionChangedEventArgs)
            Console.WriteLine("Changing")
        End Sub
    End Class

I believe that you will find this article useful.

Greetings,
PetarP
the Telerik team

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