WP7: When does the ListBox use VirtualizingStackPanel?

Question

Everybody says the default ItemsPanel for a ListBox is a VirtualizingStackPanel. I created a ListBox-derived class (call it MyListBox) and it defaults to StackPanel instead.

I mean I have to force the virtualization, for example this way:

const string itemsPanelTemplateString = @"
<ItemsPanelTemplate
xmlns=""http://schemas.microsoft.com/winfx/2006/xaml/presentation""
xmlns:x=""http://schemas.microsoft.com/winfx/2006/xaml"" >
<VirtualizingStackPanel/>
</ItemsPanelTemplate>";

MyListBox {
    this.ItemsPanel = (ItemsPanelTemplate)
        System.Windows.Markup.XamlReader.Load(itemsPanelTemplateString);
}

I could reprint here my class, but that's not the point. I would like to know general answer.

The class does not change predefined ListBox style, but it uses own ListBoxItem-derived class.

I am pretty sure there are some conditions for using virtualization, as my colleague said he saw respective ListBox code in the past. Unfortunately right now we don't have access to the debug versions of MS dll's.

Answer 1

You can find good recommendations on improving performance of the Listbox at http://blogs.msdn.com/b/slmperf/archive/2010/10/06/silverlight-for-windows-phone-7-listbox-scroll-performance.aspx

THer's also a good alternative documented at http://blogs.msdn.com/b/delay/archive/2010/09/08/never-do-today-what-you-can-put-off-till-tomorrow-deferredloadlistbox-and-stackpanel-help-windows-phone-7-lists-scroll-smoothly-and-consistently.aspx

Another thing to note (apparently) is that you'll also only get virtualization if the collection you're binding to implements IList.

Answer 2

ListBox and controls derived from ListBox will have VirtualizedStackPanel as the ItemsPanel by default, unless user code changes it explicitely.

However, if your custom ListBox happens to derive directly from ItemsControl (as opossed to actually deriving from ListBox) then you will get StackPanel as the default ItemsPanel.

Could that be the case in your code? If not, please share your control code.

Answer 3

The default style for a ListBox does not assign the ItemsPanel template.

According to the internal code I can see in reflector OnApplyTemplate will assign a VirtualizingStackPanel to the internal ItemsHost if a ItemsPanel template is not supplied.

Perhaps including your class code might be a good idea after all.

Answer 4

Solved. It was my bug:

When overriding ListBox.OnApplyTemplate() (for the purpose of time measurement), I forgot to call base.OnApplyTemplate(). Apparently the selection of the item panel is done there.

Dangerous bug because everything seemingly worked.

Thank you to all who tried to help.