Adding Items to a DataGrid (MVVM) - wpf

Question

Goal
To add a list of a custom class object (DamagedItems) to a DataGrid using the Model, View, ViewModel (MVVM) way of doing things.
I want the user to be able to create entries of damaged parts (deemed improper during inspection of a machine).
What I have done
I have created:
A window: wDamagedItems.xaml in which it's DataContext is set to DamagedItemViewModel
A Model: DamagedItemModel.vb which implements INotifyPropertyChanged
A ViewModel: DamagedItemViewModel.vb where I set properties of classes such as my DamagedItemModel
An ObservableCollection: DamagedItemList.vb which inherits an ObservableCollection(Of DamagedItemModel)
Since my DataContext is set to the DamagedItemViewModel, here is how I setup the properties:
Public Class DamagedItemViewModel
Private _DamagedItem As DamagedItemModel
Private _Add As ICommand
Private _DamagedItems As DamagedItemList
Public Property DamagedItem As DamagedItemModel
Get
Return _DamagedItem
End Get
Set(value As DamagedItemModel)
_DamagedItem = value
End Set
End Property
Public Property DamagedItems As DamagedItemList
Get
Return _DamagedItems
End Get
Set(value As DamagedItemList)
_DamagedItems = value
End Set
End Property
Public Property Add As ICommand
Get
Return _Add
End Get
Set(value As ICommand)
_Add = value
End Set
End Property
Public Sub New()
DamagedItem = New DamagedItemModel("", "", "")
DamagedItems = New DamagedItemList
Add = New DamagedItemAddEntryCommand(Me)
End Sub
Public Function CanUpdate() As Boolean
If DamagedItem.Description = "" Then Return False
If DamagedItem.Initiales = "" Then Return False
Return True
End Function
Public Sub AddEntry()
DamagedItems.Add(DamagedItem) 'Items get added to the datagrid
DamagedItem = New DamagedItemModel 'Does not seem to clear textboxes
End Sub
End Class
Here is how my XAML is set up:
<DataGrid ItemsSource="{Binding Path=DamagedItems}" AutoGenerateColumns="True" HorizontalAlignment="Stretch" Margin="12,90,12,0" Name="DataGrid1" VerticalAlignment="Top" Height="229" / >
<TextBox Text="{Binding DamagedItem.Description, UpdateSourceTrigger=PropertyChanged}" Height="23" HorizontalAlignment="Left" Margin="88,24,0,0" VerticalAlignment="Top" Width="249" />
<TextBox Text="{Binding DamagedItem.Initiales, UpdateSourceTrigger=PropertyChanged}" Height="23" HorizontalAlignment="Left" Margin="88,58,0,0" VerticalAlignment="Top" Width="249" />
As you can see, my textboxes are bound to my Model (which is contained in my ViewModel, which is bound to that Window's DataContext). Whenever I click on my "Add" button, whatever is in the textbox gets added to the DataGrid, but the content in the text boxes stay there.
This step is fine, I write in what I want to add and click on "Add"
After clicking on "Add" i get the following results in the DataGrid, which is fine. The issue is my text boxes are still filled with data yet the Model was cleared (see code after DamagedItemViewModel AddEntry method).
Now when I try to add the following text:
Description: "Part is bent"
Initiales: "A.C"
I get the following result:
The first letter typed in the description gets inputted in the first entry of the DataGrid, then it erases the text in the description textbox. Only then can I keep typing what I want. The same thing occurs for the initiales text box.
Any ideas? If you wish to see more of my code, suggest which portion I should add.
Thank you in advance!

Answer 1

Yup, I remember running into this one. You have to implement iNotifyPropertyCHnaged. This is how the viewmodel class "notifies" the user interface that there has been a change to the underlying property of a binding:
look here:
http://msdn.microsoft.com/en-us/library/ms743695.aspx
You will have to implement this for every property you want reflected back to the view. SO what I do is have a base viewmodel class (ViewModelBase which exposes method RasiePropertyChanged) which implements iNotifyPropertyChanged and then my viewmodles inherit from it. Then I notify the property changed in the property set of the property:
ie:
Public Property Selection As job
Get
Return Me._Selection
End Get
Set(ByVal value As job)
If _Selection Is value Then
Return
End If
_PreviousJob = _Selection
_Selection = value
RaisePropertyChanged(SelectionPropertyName)
End Set
End Property
This seems frustrating at first but is needed to keep the decoupling that MVVM supports. Its easy to implement.