ListPicker within LongListSelector not retaining value after scrolling - windows-phone-7

Question

I am developing a WP7 app that contains a LongListSelector. Within the ItemTemplate there is a ListPicker. I select a value from the ListPicker for the first item in the LongListSelector, then select a value from the ListPicker for the 2nd item. If I then scroll down the page and back to the top again, the value I selected in the ListPicker for the 1st item will be reset (SelectedIndex=0).
I have put code in the Link and Unlink events of the LongListSelector to write to the output window and I have found that when the 1st item unlinks (due to scrolling down the page), the ListPicker value is what I selected but as soon as the link event fires (due to scrolling back up the page) for the 1st item the value is reset.
Im using ObserableCollection and implement the INPC interface for my objects and the ViewModel updates when the ListPicker selection is changed.
How can I ensure that the value in the ListPickers are retaining during scrolling of the LongListSelector?
MainPage.xaml
<phone:PhoneApplicationPage
x:Class="LongListSelector.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d" d:DesignWidth="480" d:DesignHeight="5000"
FontFamily="{StaticResource PhoneFontFamilyNormal}"
FontSize="{StaticResource PhoneFontSizeNormal}"
Foreground="{StaticResource PhoneForegroundBrush}"
SupportedOrientations="Portrait" Orientation="Portrait"
shell:SystemTray.IsVisible="True"
xmlns:toolkit="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone.Controls.Toolkit">
<phone:PhoneApplicationPage.Resources>
<DataTemplate x:Name="occurrenceItemTemplate">
<Grid>
<StackPanel Orientation="Vertical" Margin="5,0,0,0" >
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<TextBlock Name="tbkEventDateTime" Grid.Row="0" Grid.Column="0"
Text="{Binding ItemNumber}"
Style="{StaticResource PhoneTextSmallStyle}" />
<!--<TextBox Name="txtEventDateTime" Grid.Row="1" Grid.Column="0"
Text="{Binding Result}" />-->
<toolkit:ListPicker Name="lpkResult" Margin="12,0,12,12"
Grid.Row="1" Grid.Column="0"
ItemsSource="{Binding TestItemResultList, Mode=OneTime}"
SelectedIndex="{Binding Result, Mode=TwoWay}"
CacheMode="BitmapCache">
<toolkit:ListPicker.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Text}" />
</DataTemplate>
</toolkit:ListPicker.ItemTemplate>
</toolkit:ListPicker>
</Grid>
</StackPanel>
</Grid>
</DataTemplate>
</phone:PhoneApplicationPage.Resources>
<!--LayoutRoot is the root grid where all page content is placed-->
<Grid x:Name="LayoutRoot" Background="Transparent">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<!--TitlePanel contains the name of the application and page title-->
<StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="12,17,0,28">
<TextBlock x:Name="ApplicationTitle" Text="MY APPLICATION" Style="{StaticResource PhoneTextNormalStyle}"/>
<TextBlock x:Name="PageTitle" Text="page name" Margin="9,-7,0,0" Style="{StaticResource PhoneTextTitle1Style}"/>
</StackPanel>
<!--ContentPanel - place additional content here-->
<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0" VerticalAlignment="Stretch">
<toolkit:LongListSelector x:Name="lstOutstandingOccurrences"
Margin="0,12,0,0" Padding="0,0,0,24"
ItemTemplate="{StaticResource occurrenceItemTemplate}"
ItemsSource="{Binding TestItemCollection, Mode=TwoWay}"
IsFlatList="True" ShowListHeader="False" >
</toolkit:LongListSelector>
</Grid>
</Grid>
MainPage.xaml.cs
public partial class MainPage : PhoneApplicationPage
{
private TestItemViewModel _vm;
public TestItemViewModel ViewModel
{
get
{
if (_vm == null)
_vm = new TestItemViewModel();
return _vm;
}
private set
{
_vm = value;
}
}
// Constructor
public MainPage()
{
InitializeComponent();
this.lstOutstandingOccurrences.Link += new EventHandler<LinkUnlinkEventArgs>(lstOutstandingOccurrences_Link);
this.lstOutstandingOccurrences.Unlink += new EventHandler<LinkUnlinkEventArgs>(lstOutstandingOccurrences_Unlink);
this.DataContext = this.ViewModel;
}
void lstOutstandingOccurrences_Link(object sender, LinkUnlinkEventArgs e)
{
var item = e.ContentPresenter.Content as TestItem;
Debug.WriteLine("Link event for ItemNumber {0} = {1}", item.ItemNumber, item.Result);
}
void lstOutstandingOccurrences_Unlink(object sender, LinkUnlinkEventArgs e)
{
var item = e.ContentPresenter.Content as TestItem;
Debug.WriteLine("Unlink event for ItemNumber {0} = {1}", item.ItemNumber, item.Result);
}
}
TestItemViewModel.cs
public class TestItemViewModel : BaseINPC
{
public ObservableCollection<TestItem> TestItemCollection
{
get;
private set;
}
// Constructor
public TestItemViewModel()
{
this.TestItemCollection = new ObservableCollection<TestItem>();
CreateTestData(20);
}
public void CreateTestData(int totalItems)
{
//create test data for long list selector.
for (int i = 1; i <= totalItems; i++)
{
this.TestItemCollection.Add(new TestItem(i, 0));
}
}
}
Models
public class ListHelperListItem
{
public int Value { get; set; }
public string Text { get; set; }
}
public class TestItem : BaseINPC
{
public TestItem(int ItemNumber, int Result)
{
this.ItemNumber = ItemNumber;
this.Result = Result;
}
public int ItemNumber { get; set; }
private int _Result;
public int Result
{
get
{
return _Result;
}
set
{
//if statement is for debugging purposes only.
if (this.ItemNumber == 1)
{
_Result = value;
}
_Result = value;
RaisePropertyChanged("Result");
}
}
private List<ListHelperListItem> _TestItemResultList;
public List<ListHelperListItem> TestItemResultList
{
get
{
_TestItemResultList = new List<ListHelperListItem>();
_TestItemResultList.Add(new ListHelperListItem { Value = 0, Text = " " });
_TestItemResultList.Add(new ListHelperListItem { Value = 1, Text = "Yes" });
_TestItemResultList.Add(new ListHelperListItem { Value = 2, Text = "No" });
_TestItemResultList.Add(new ListHelperListItem { Value = 3, Text = "Ignore" });
return _TestItemResultList;
}
}
}
Thanks for your help!