Windows Phone 7 Back Button issue - windows-phone-7

Question

In my app I have several pages. When I click on the Windows "Back" Button everything goes back as expected.
However I have 2 pages that are causing me grief. Page "A" is doing some binding in the XAML:
<ListBox x:Name="lbPrograms" ItemsSource="{Binding Items}" SelectionChanged="lbPrograms_SelectionChanged">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel x:Name="DataTemplateStackPanel" Orientation="Horizontal">
<Image x:Name="ItemImage" Source="/images/ArrowImg.png" Height="43" Width="43" VerticalAlignment="Top" Margin="10,0,20,0"/>
<StackPanel>
<TextBlock x:Name="ItemText" Text="{Binding programName}" Margin="-2,-13,0,0" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
<TextBlock x:Name="DetailsText" Text="{Binding createDate}" Margin="0,-6,0,3" Style="{StaticResource PhoneTextSubtleStyle}"/>
</StackPanel>
<Image x:Name="ItemFavs" Source="/images/favs.png" Height="43" Width="43" VerticalAlignment="Top" Margin="10,0,20,0"/>
<Image x:Name="ItemDelete" Source="/images/delete.png" Height="43" Width="43" VerticalAlignment="Top" Margin="10,0,20,0"/>
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
The code behind for Page A is fairly simple:
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
// Set the data context of the listbox control to the sample data
if (DataContext == null)
DataContext = App.ViewModel;
App.ViewModel.Refresh();
lbPrograms.ItemsSource = App.ViewModel.Items;
}
private void lbPrograms_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
NavigationService.Navigate(new Uri("/DisplayProgram.xaml?selectedItem=" + lbPrograms.SelectedIndex, UriKind.Relative));
}
private void BackBtn_Click(object sender, EventArgs e)
{
NavigationService.Navigate(new Uri("/MainPage.xaml", UriKind.Relative));
}
Page B has no binding in the XAML as I am taking the data from the ModelView a drawing it out dynamically on the screen. Like so:
private int index;
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
string selectedIndex = "";
if (NavigationContext.QueryString.TryGetValue("selectedItem", out selectedIndex))
{
//prevents error
if (int.Parse(selectedIndex) == -1)
{
return;
}
if ((DataContext == null))
{
index = int.Parse(selectedIndex);
App.ViewModel.Refresh();
DataContext = App.ViewModel.Items[index].nValDictionary;
int i = 0;
foreach (KeyValuePair<string, string> kvp in (((System.Collections.Generic.Dictionary<string, string>)(DataContext))))
{
StackPanel sp = new StackPanel();
sp.Name = "sp" + i;
sp.Background = new SolidColorBrush(Colors.Black);
sp.Width = 460;
WrapPanel wp = new WrapPanel();
wp.Name = "test" + i;
wp.Width = 300;
wp.Height = 200;
TextBlock txt = new TextBlock();
txt.Text = kvp.Key.ToString();
txt.Foreground = new SolidColorBrush(Colors.White);
sp.Children.Add(txt);
int chkBoxesVal = 0;
if (kvp.Value == "")
{
chkBoxesVal = 0;
}else{
chkBoxesVal = Convert.ToInt32(kvp.Value.ToString());
}
int iCount = 0;
for (iCount = 0; iCount <= chkBoxesVal - 1; iCount++)
{
CheckBox chk = new CheckBox();
chk.Name = i.ToString();
chk.Width = 56;
chk.Height = 70;
chk.Content = "";
//chk.Background = new SolidColorBrush(Colors.Black);
//chk.BorderBrush = new SolidColorBrush(Colors.White);
chk.Style = (Style)Application.Current.Resources["checkBoxNG"];
wp.Children.Add(chk);
}
sp.Children.Add(wp);
lbItems.Items.Add(sp);
i += 1;
}
}
}
}
}
So when I'm going forward everything works fine, but when I hit the Windows "Back" button on Page B I get an error. I stepped through my code and when I hit the "Back" button it does go back to Page A, but then it is also going to Page B, which then throws the error and stops. So can anyone tell me why this behavior is happening? I would expect that it would go back to Page A and just stop there. Not to go back to Page B. is there something in my code that is causing it to reload Page B? Any resources that you can provide that might explain this behavior is also welcome!
Thanks!

Answer 1

It looks like SelectionChanged on Page A is firing as a result of the ItemsSource initialisation you're doing in OnNavigatedTo.
You could verify the SelectedIndex is -1 before taking any action in the SelectionChanged event.
Alternatively you could remove any existing event handler on SelectionChanged while doing this initialisation and restore that event handler on completion.

Answer 2

My assumption is that lbPrograms_SelectionChanged event occurs when you press back button and page A is loaded again.
Change your navigation design. For your DataTemplateStackPanel you could use ManipulationStarted event
and inside add
NavigationService.Navigate(new Uri("/DisplayProgram.xaml?selectedItem=" + lbPrograms.SelectedIndex, UriKind.Relative));