Windows Phone Toolkit ListPicker Throws an Unhandled Exception

问题

I'm working on a Windows Phone 8 app. My app uses the ListPicker from the Tookit. My code in question looks like the following:

<toolkit:ListPicker x:Name="myListPicker" Margin="12,-6,12,-2" Loaded="myListPicker_Loaded">
  <toolkit:ListPicker.Items>
    <!-- Items are defined here -->
  </toolkit:ListPicker.Items>
</toolkit:ListPicker>


private void myListPicker_Loaded(object sender, RoutedEventArgs e)
{
  if ((myListPicker != null) && (viewModel != null))
  {

  }
}

Whenever the total number of items passes a certain threshold, my app throws an System.ArgumentException. I know this, because I have the following code:

    private void Application_UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)
    {
        MessageBox.Show(e.ExceptionObject.Message + "\n\nException\n" + e.ExceptionObject.GetType().FullName + "\n" + e.ExceptionObject.StackTrace);
        if (Debugger.IsAttached)
        {
            // An unhandled exception has occurred; break into the debugger
            Debugger.Break();
        }
    }

The message says "Value does not fall within the expected range.". From what I can tell, this happens when the ListPicker needs to go into full screen mode. I can't figure out why this happens though.

Does anyone have any insights?

回答1:

Seemingly, with full screen mode you cannot set the ListPicker's items to specific UI Elements within the xaml page. You must bind them or use templating.

After having this exact problem, I found an explanation here: http://silverlight.codeplex.com/workitem/9412

ListPickerItems are UIElements, and the ListPicker renders them in its presenter. When there are 5 or less items, the expanded mode opens on the current page, and you can see all the items in the presenter. When 6 or more items are present, opening the list picker goes to full mode which opens a new page. This new page has a listbox, which gets its items property set to the listpicker's items. This is where it breaks. By setting the listbox's items to the listpicker's items (in this case a list of listpickeritems), the listbox will put those UIElements into its view. Now a single listboxitem is included in two places on the visual tree.

Because of this issue, ListPicker only supports databinding and templating. DO NOT set the ListPicker's items to specific UIElements.

I managed to get my solution working doing something like this:

<toolkit:ListPicker x:Name="myListPicker" Margin="12,-6,12,-2" Loaded="myListPicker_Loaded">
  <toolkit:ListPicker.ItemTemplate>
      <DataTemplate>
          <TextBlock Text="{Binding Name}" Tag="{Binding ID}"/>
      </DataTemplate>
   </toolkit:ListPicker.ItemTemplate>
   <toolkit:ListPicker.FullModeItemTemplate>
      <DataTemplate>
          <TextBlock Text="{Binding Name}" Tag="{Binding ID}"/>
      </DataTemplate>
   </toolkit:ListPicker.FullModeItemTemplate>
</toolkit:ListPicker>


private void myListPicker_Loaded(object sender, RoutedEventArgs e)
{
  if ((myListPicker != null) && (viewModel != null))
  {
     myListPicker.ItemsSource = _Data; //_data is an array of objects with 2 properties named ID & Name
  }
}

回答2:

ListPicker has a predefined number of elements it can display in the standard list. In older versions of the toolkit, you could change this threshold, but they have since removed it.

However, since the toolkit is open source, you can always do what I did - go into the source code and change it to allow larger lists.

回答3:

Use this version of the toolkit with a limit of 500 for the list picker (make sure you right-click and unblock the .dll so you can use it) Windows Phone Toolkit Hacked For ListPicker To Support 500 Items - Fall 2013 WP8 Toolkit - By Jeremiah Isaacson

来源：https://stackoverflow.com/questions/17065970/windows-phone-toolkit-listpicker-throws-an-unhandled-exception

标签

xaml

windows-phone