How can I open a LongListSelector with the groups collapsed in WP7?

Question

I have a LongListSelector that displays lots of items. When the longListSelector is opened, I see the groups expanded i.e. the items are displayed within the groups. I want the longList Selector to display the collapsed Panel showing only the group names at start. Like an index. When you tap a group, its items expand. How can this be done?

Answer 1

Just needed to implement this myself - did it as follows:

In the item (not header!) template definition in XAML, bind the Visibility property of the containing item (in my case, a Grid):

<DataTemplate x:Key="itemTemplate">
  <Grid Visibility="{Binding FolderVisibility}">
  ...

Derive the item group from ObservableCollection and create a suitable property for handling the expand/collapse status:

public class MyGroup : ObservableCollection<MyItem>
{
    ...
    private bool m_expanded = true;
    public bool Expanded
    {
        get { return m_expanded; }
        set
        {
            m_expanded = value;
            OnPropertyChanged( new PropertyChangedEventArgs( "Expanded" ));
            foreach( var i in this )
            {
                i.OnFolderCollapsedExpanded();
            }
        }
    }
    ...

Finally, you need the FolderVisibility property on each list item:

public class MyItem : INotifyPropertyChanged
{
    ...
    public event PropertyChangedEventHandler PropertyChanged;
    ...
    public Visibility FolderVisibility
    {
        get { return MyFolder.Expanded ? Visibility.Visible : Visibility.Collapsed; }
    }
    public void OnFolderCollapsedExpanded()
    {
        var h = PropertyChanged;
        if( h != null ) h( this, new PropertyChangedEventArgs( "FolderVisibility" ));
    }
    ...

Then just toggle the folder's Expanded property in a suitable place (e.g. a Click handler for the folder's header template).