Question

Windows Phone 7.1: How to add/delete items from LongListSelector control?

I am using a LongListSelector control from ‘Windows Phone Toolkit’. The control is data bound to a ViewModel inherited from an ObservableCollection. When I try the following code:

MyObject mo = new MyObject("Name", "Description", "Value");
App.MyObjectsViewModel.Add(mo);

The ViewModel does seem to get updated but the LongListSelector does not update? What am I missing?

PS: I am new to Silverlight and WP7 development.

Following the XAML for the LongListSelector and the DataTemplates. The code is pretty much straight out of the Windows Phone Toolkit sample (removed some formatting related code to keep the post small)

<DataTemplate x:Key="groupHeader">
    <TextBlock Text="{Binding Key}"/>
</DataTemplate>

<DataTemplate x:Key="groupItemHeader">
    <Border>
        <TextBlock Text="{Binding Key}" 
           Foreground="#FFFFFF" 
           FontSize="{StaticResource PhoneFontSizeLarge}"/>
        </Border>
</DataTemplate>

<DataTemplate x:Key="myobjectItemTemplate">
    <Grid>     
    <StackPanel VerticalAlignment="Top" Orientation="Vertical">
        <TextBlock Text="{Binding Symbol}"/>
        <TextBlock Text="{Binding Value}"/>
        <TextBlock Text="{Binding Description}" TextWrapping="Wrap"/>
    </StackPanel>
    </Grid>
</DataTemplate>

<controls:PivotItem Header="myobjects">
<toolkit:LongListSelector x:Name="myobjectsList" 
    Background="Transparent"
    GroupHeaderTemplate="{StaticResource groupHeader}"
    GroupItemTemplate="{StaticResource groupItemHeader}"
    ItemTemplate="{StaticResource myobjectItemTemplate}"
    GroupViewOpened="LongListSelector_GroupViewOpened"
    GroupViewClosing="LongListSelector_GroupViewClosing"/>
</controls:PivotItem>

C# code behind for setting the ItemSource

var myobjectsByClassification = from myobjects in App.MyObjectsLibrary
        group myobjects by myobjects.Classification into c
        orderby c.Key
        select new PublicGrouping<string, MyObject>(c);

this.myobjectsList.ItemsSource = myobjectsByClassification;

Answer 1

My guess is that the grouping code is only being called once somewhere in code behind. So the grouped collection is not updated when you add something to your ViewModel collection. The easiest way to handle this (but maybe not the most elegant) is to create your own AddItem() method for the ViewModel collection.

class MyViewModelObject
{
  void AddItem( MyObject obj )
  {
    App.MyObjectsLibrary.Add( obj );
    MyObjectsByClassification = from myobjects in App.MyObjectsLibrary
                                group myobjects by myobjects.Classification into c
                                orderby c.Key
                                select new PublicGrouping<string, MyObject>(c);
  }
}

Bind MyObjectsByClassification to LongListSelector.ItemsSource in XAML, and make sure you notify the LongListSelector of changes to the property by using INotifyPropertyChanged.