VirtualizingStackPanel Is not working

Question

I have the following ListBox:

<ScrollViewer>
    <!--Spec Definitions-->
    <ListBox DataContext="{Binding SpecPackageSpecGroupListViewModel}" 
         VirtualizingStackPanel.IsVirtualizing="True" 
         VirtualizingStackPanel.VirtualizationMode="Recycling" 
         ScrollViewer.IsDeferredScrollingEnabled="True"
         ItemContainerStyle="{StaticResource SpecPackageSpecGroupListBoxStyle}" 
         ItemsSource="{Binding SortedChildren}" 
         Background="Transparent"
         BorderThickness="0" SelectionMode="Extended"
         Margin="5,5,5,5">
        <ListBox.ItemTemplate>
            <DataTemplate>
                <Controls:SpecPackageSpecGroupControl/>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>
</ScrollViewer>

This list box is supposed to host ~1000 items, but complex ones. I want it to work with the VirtualizingStackPanel, so I have set the visualizing XAML configuration to:

VirtualizingStackPanel.IsVirtualizing="True"
VirtualizingStackPanel.VirtualizationMode="Recycling" 

My problem is that I think it doesn't work - first - it takes a very long time to load ~700 items, and secondly, when I hit a breakpoint on my control constructor - I can see it is called 700 times:

public static int Counter = 0;

public SpecPackageSpecGroupControl()
{
    InitializeComponent();

    Counter++;
    if (Counter%100 == 0)
        Console.WriteLine("Hi");
}

I break point on the Console.WriteLine("Hi") and I can see that the static counter reached 700.

So basically the UIElements are being created although this is a virtual mode.

Am I misunderstanding the virtualization mode, or is there something I'm doing wrong?

Answer 1

Don't put it in a ScrollViewer. The XAML as you pasted indeed bypasses virtualization but for a different reason: the ListBox extends fully (without scrolling) because the scrollViewer allows it to. Because it is fully 'extended', the ListBox doesn't use virtualization. It will use its built-in scroll viewer if you place it in a regular container - Border, Grid etc.