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Wolfgang Pöchgraber
1878
Data binding update on UltraComboEditor
posted

Hi!

I'm using an UltraComboEditor that uses Data Binding to get an Id out of an underlying business object which is used to select appropriate item from the items list. If another item is selected in the combo the Id is changed in the business object via the data binding which is fine.

I handle the ValueChanged event of the UltraComboEditor since I have to update another property 'Name' on same business object. This business object implements the INotifyPropertyChanged interface i.e. when 'Name' is set event OnPropertyChanged("Name") is fired causing data binding to fetch all visible properties again. This causes also the UltraComboEditor to display the item associated with the Id from the business object which is the original one thus the selection can never be changed.

I found a solution which works but my feeling is that it is more a workaround. In the ValueChanged event handler of the UltraComboEditor I explicitely set the 'Id' before setting 'Name' I. What I don't like on this solution is that 'Id' is updated again through the data binding.

Comments are welcome,
  Wolfgang

Parents
  • 336
    posted

    I have the exact same problem. I think the main problem here is that combo doesn't update the value of the property on the object it is bound to until after the ValueChanged event has fired. So when code runs in the ValueChanged the value of the bound object property and the combobox value are different.

    This means that if you change any other property on the bound object if fires the PropertyChanged event which causes all the values to be copied to the bound controls again thus overwriting the change to the combo box value you just made and triggering the ValueChanged event again.

    Here's some contrived VB code that will hopefully makes sense. Just so you know, I'm typing this by hand and will leave some bits out - so its unlikely to compile  :)

    ----------------------------------------------------------------------------------------------

    Here's the business class (it's missing the propertychanged event):

    Public Class MyClass Implements INotifyPropertyChanged

     Public Property Property1() As MyEnum
            Get
                Return m_property1
            End Get
            Set(ByVal value As MyEnum)
                If m_property1<> value Then
                    m_property1= value
                    OnPropertyChanged("Property1")
                End If
            End Set
        End Property

    Public Property Property2() As Integer
            Get
                Return m_property2
            End Get
            Set(ByVal value As Integer)
                If m_property2<> value Then
                    m_property2= value
                    OnPropertyChanged("Property2")
                End If
            End Set
        End Property

    End Class

    ----------------------------------------------------------------------------------------------

    Here's the enum:

    Public Enum MyEnum
        Zero = 0
        One = 1
        Two = 2
    End Enum

    ----------------------------------------------------------------------------------------------

    And here's the binding code (m_myObject is an instance of MyClass held by the form):

    cboMyCombo.DataSource = [Enum].GetValues(GetType(MyEnum))
    cboMyCombo.DataBindings.Add("Value", m_myObject, "Property1", False, DataSourceUpdateMode.OnPropertyChanged)

    ----------------------------------------------------------------------------------------------

    Here's the ValueChanged event:

    Private Sub cboMyCombo_ValueChanged(ByVal sender As Object, ByVal e As EventArgs) Handles cboFrequency.ValueChanged

            ' At this point m_myObject.Property2 does not equal cboMyCombo.Value

     

            ' Next Line causes PropertyChanged event to fire ...

            m_myObject.Property2 = 5

     

        End Sub

    ----------------------------------------------------------------------------------------------

    The scenario is this:

    On the form I change the combo value from Zero to One. This causes the ValueChanged event to fire. In that event handler I set Property2 to 5. Assuming it was some other value before, this causes the object to trigger the PropertyChanged event.

    This in turn causes the bindings to refresh.At this point in time cboMyCombo.Value is One, but m_myObject.Property1 is still Zero, so the value of the combo box gets set back to Zero by the binding refresh and even worse the ValueChanged event is triggered again.

    The problem would go away if the combobox set the value of the bound object BEFORE it calls ValueChanged.

    Sorta sounds like a bug to me!?

     

     

     

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