Prism Windows 8.1 NotifyProperyChanged does not fire?

Question

The project: a small notice board with "post-it" messages. VB.Net, VS2013, Windows 8.1 Desktop. Prism / MVVM.

The problem: the NotifyPropertyChanged event seems not to fire at all when properties change. And the messages do not show up on the noticeboard.

The question: why is that so? or: what is wrong with the code below?

The code:

First, we have the View. It is defined like so:

<prism:VisualStateAwarePage 
    x:Name="pageRoot"
    x:Class="MemoBoard.Views.BoardPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:prism="using:Microsoft.Practices.Prism.StoreApps"
    prism:ViewModelLocator.AutoWireViewModel="True"
    mc:Ignorable="d">

Furthermore, the notes are presented using a "Post-it" template, defined as:

<prism:VisualStateAwarePage.Resources>
    <DataTemplate x:Name="PostIt">
        <Grid Width="250" Height="250" >
            <TextBlock 
                FontFamily="Buxton Sketch" 
                Text="{Binding Text}" 
                FontSize="20" 
                HorizontalAlignment="Left" 
                VerticalAlignment="Top" 
                Margin="20,40,50,0" 
                TextWrapping="WrapWholeWords">
            </TextBlock>
       </Grid>
   </DataTemplate>
</prism:VisualStateAwarePage.Resources>

And finally the GridView with the properties for the ItemsSource and the Template:

<GridView x:Name="grdMessages"
   ItemsSource="{Binding Messages}"
   ItemTemplate="{StaticResource PostIt}"/>

So far for the XAML for the presentation in the View. The codebehind is nothing but a simple constructor with Initialization, it looks like:

Namespace Views
    Public NotInheritable Class BoardPage
        Inherits VisualStateAwarePage
        Public Sub New()
            InitializeComponent()
        End Sub
    End Class
End Namespace

This is all pretty straightforward, thankfully using the Prism library to get richer functionality. The next part is the ViewModel, and also the Viewmodel is straightforward, out of the book. It looks like:

Namespace ViewModels
    Public Class BoardPageViewModel
    Inherits ViewModel

    Private _MessageRepository As IMessageRepository
    Private Property _Messages As ObservableCollection(Of Message)
    Public Property Messages As ObservableCollection(Of Message)
        Get
            Return _Messages
        End Get
        Set(value As ObservableCollection(Of Message))
                SetProperty(_Messages, value)
            End Set
    End Property

    Public Overrides Async Sub OnNavigatedTo(navigationParameter As Object, navigationMode As NavigationMode, viewModelState As Dictionary(Of String, Object))
        MyBase.OnNavigatedTo(navigationParameter, navigationMode, viewModelState)
        _MessageRepository = New MessageRepository
        Messages = Await _MessageRepository.GetMessagesAsync()
    End Sub

    End Class
End Namespace

The messages are retrieved via a WebAPI that is part of the Repository and I'm sure that it is working fine. Items are read correctly and passed to the ViewModel.

When I step through the code, I see the Messages arriving and the Set, and thus the SetProperty are called.

But the Messages do not show up on the View. It seems to me that the View is not notified of the change, when the Messages have been retrieved.

What is wrong with this approach?

Answer 1

The problem is not the approach, but the fact that the local version of the _Messages collection should not be declared as a private (property), but as a local variable. So the complete statement for the declaration of the property and the local version should be like this:

Dim _Messages As ObservableCollection(Of Message)
Public Property Messages As ObservableCollection(Of Message)
    Get
        Return _Messages
    End Get
    Set(value As ObservableCollection(Of Message))
            SetProperty(_Messages, value)
        End Set
End Property

Now the SetProperty will fire the event and the right values will be passed through to the ViewModel's binding objects.