Question

I have a ListView which i am updating as soon as i get data . ListView gets updated if i add items to observableCollection in the ViewModel`s constructor but not if i do it in some method after i get callback .

I have spend a lot of time on Stackoverflow and i know there are many related questions but i could not find answer . Whats bugging me is that , if it works when i add values in the constructor then why it doesnt work if i add values in the some method .

• I am using ObservableCollection
• Setting DataContext
• Binding ObservableCollection to ListView in XAML

here is the XAML code

<UserControl x:Class="WFP_Illustrated.UserControls.WeatherForcastControl"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300"
         >
<UserControl.Resources>
    <Style TargetType="TextBlock" x:Key="TextStyle">
        <Setter Property="FontFamily" Value="Adobe Caslon Pro"/>
        <Setter Property="FontSize" Value="18"/>
        <Setter Property="MinWidth" Value="60"/>
    </Style>
    <DataTemplate x:Key="ForcastTemplate">
        <Border BorderBrush="Aqua" BorderThickness="1" Padding="5" Margin="5">
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition/>
                    <RowDefinition/>
                    <RowDefinition/>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition/>
                    <ColumnDefinition/>
                </Grid.ColumnDefinitions>

                <TextBlock Grid.Row="0" Grid.ColumnSpan="2" Text="{Binding Path= Day}" Style="{StaticResource TextStyle}"/>
                <TextBlock Grid.Row="1" Grid.Column="0" Text="{Binding Path= Condition}"   Style="{StaticResource TextStyle}"/>
                <Image Grid.Row="1" Grid.Column="1" />
                <TextBlock Grid.Row="2" Grid.Column="0" Text="{Binding Path= Low}"  Style="{StaticResource TextStyle}"/>
                <TextBlock Grid.Row="2" Grid.Column="1" Text="{Binding Path= High}"  Style="{StaticResource TextStyle}"/>
            </Grid>
        </Border>
    </DataTemplate>
</UserControl.Resources>
<Grid>
    <ListView Name="ForcastList"
             ItemTemplate="{StaticResource ForcastTemplate}"
             ItemsSource="{Binding ForcastList}"
             Background="SlateGray"
             >

    </ListView>
</Grid>

XAML CodeBehind

public partial class WeatherForcastControl : UserControl
{
    private WeatherForcastViewModel _viewModel;
    public WeatherForcastControl()
    {
        InitializeComponent();
        _viewModel = new WeatherForcastViewModel();
        this.DataContext = _viewModel;
    }
}

Here is ModelView Code

  private ObservableCollection<ForcastInfo> _forcastList;
    public ObservableCollection<ForcastInfo> ForcastList
    {
        get { return _forcastList; }
        //set
        //{
        //    _forcastList = value;
        //    OnPropertyChanged("ForcastList");
        //}
    }

    public WeatherForcastViewModel()
    {
        //If this is uncommented , I will see values in ListView
        //ForcastInfo forcast = new ForcastInfo();
        //forcast.Condition = "clear";
        //forcast.Day = "Sunday";
        //forcast.High = "70";
        //forcast.Low = "50";
        //_forcastList = new ObservableCollection<ForcastInfo>();
        //ForcastList.Add(forcast);
        //ForcastList.Add(forcast);
        //ForcastList.Add(forcast);
        //ForcastList.Add(forcast);
        //ForcastList.Add(forcast);
        //ForcastList.Add(forcast);

        //Callback from MainView ,forcast data is available
        Messenger.Default.Register<GoToForcast>
       (
            this, (action) => MessageFromMain(action)
       );
    }

    private void MessageFromMain(GoToForcast message)
    {
        //This should work but its not working
        ForcastInfo forcast = new ForcastInfo();
        forcast.Condition = "clear";
        forcast.Day = "Sunday";
        forcast.High = "70";
        forcast.Low = "50";
        _forcastList = new ObservableCollection<ForcastInfo>();
        ForcastList.Add(forcast);
        ForcastList.Add(forcast);
        ForcastList.Add(forcast);
        ForcastList.Add(forcast);
        ForcastList.Add(forcast);
        ForcastList.Add(forcast);

        //ForcastList = message.ForcastList;
    }

Answer 1

The problem is that you’re setting the _forecastList to a new instance of an ObservableCollection. Just do this:

public WeatherForcastViewModel()
{

    _forcastList = new ObservableCollection<ForcastInfo>();
    //Callback from MainView ,forcast data is available
    Messenger.Default.Register<GoToForcast>
   (
        this, (action) => MessageFromMain(action)
   );
}


private void MessageFromMain(GoToForcast message)
{
    //This should work but its not working
    ForcastInfo forcast = new ForcastInfo();
    forcast.Condition = "clear";
    forcast.Day = "Sunday";
    forcast.High = "70";
    forcast.Low = "50";
    //_forcastList = new ObservableCollection<ForcastInfo>(); <---- this is messing you up
    ForcastList.Add(forcast);
    ForcastList.Add(forcast);
    ForcastList.Add(forcast);
    ForcastList.Add(forcast);
    ForcastList.Add(forcast);
    ForcastList.Add(forcast);

    //ForcastList = message.ForcastList;
}