I have an ItemsControl
containing a list of data that I would like to virtualize, however VirtualizingStackPanel.IsVirtualizing="True"
does not seem to work with an ItemsControl
.
Is this really the case or is there another way of doing this that I am not aware of?
To test I have been using the following block of code:
<ItemsControl ItemsSource="{Binding Path=AccountViews.Tables[0]}"
VirtualizingStackPanel.IsVirtualizing="True">
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBlock Initialized="TextBlock_Initialized"
Margin="5,50,5,50" Text="{Binding Path=Name}" />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
If I change the ItemsControl
to a ListBox
, I can see that the Initialized
event only runs a handful of times (the huge margins are just so I only have to go through a few records), however as an ItemsControl
every item gets initialized.
I have tried setting the ItemsControlPanelTemplate
to a VirtualizingStackPanel
but that doesn't seem to help.
There's actually much more to it than just making the ItemsPanelTemplate
use VirtualizingStackPanel
. The default ControlTemplate
for ItemsControl
does not have a ScrollViewer
, which is the key to virtualization. Adding to the the default control template for ItemsControl
(using the control template for ListBox
as a template) gives us the following:
<ItemsControl ItemsSource="{Binding AccountViews.Tables[0]}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBlock Initialized="TextBlock_Initialized"
Text="{Binding Name}" />
</DataTemplate>
</ItemsControl.ItemTemplate>
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<VirtualizingStackPanel IsVirtualizing="True"
VirtualizationMode="Recycling" />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
<ItemsControl.Template>
<ControlTemplate TargetType="ItemsControl">
<Border BorderThickness="{TemplateBinding BorderThickness}"
BorderBrush="{TemplateBinding BorderBrush}"
Background="{TemplateBinding Background}">
<ScrollViewer CanContentScroll="True"
Padding="{TemplateBinding Padding}"
Focusable="False">
<ItemsPresenter />
</ScrollViewer>
</Border>
</ControlTemplate>
</ItemsControl.Template>
</ItemsControl>
(BTW, a great tool for looking at default control templates is Show Me The Template)
Things to notice:
You have to set ScrollViewer.CanContentScroll="True"
, see here for why.
Also notice that I put VirtualizingStackPanel.VirtualizationMode="Recycling"
. This will reduce the numbers of times TextBlock_Initialized
is called to however many TextBlocks are visible on the screen. You can read more on UI virtualization here
.
EDIT: Forgot to state the obvious: as an alternate solution, you can just replace ItemsControl
with ListBox
:)
Also, check out this Optimizing Performance on MSDN page and notice that ItemsControl
isn't in the "Controls That Implement Performance Features" table, which is why we need to edit the control template.