Slow performance with WPF DataGrid and ScrollViewer

Question

I have this style for a data grid:

<Setter Property="Template">
    <Setter.Value>
        <ControlTemplate TargetType="{x:Type DataGrid}">
            <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="True">
                <ScrollViewer x:Name="DG_ScrollViewer" Focusable="false">
                    <ScrollViewer.Template>
                        <ControlTemplate TargetType="{x:Type ScrollViewer}">
                            <Grid>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="Auto"/>
                                    <ColumnDefinition Width="*"/>
                                    <ColumnDefinition Width="Auto"/>
                                </Grid.ColumnDefinitions>
                                <Grid.RowDefinitions>
                                    <RowDefinition Height="Auto"/>
                                    <RowDefinition Height="*"/>
                                    <RowDefinition Height="Auto"/>
                                </Grid.RowDefinitions>

                                <DataGridColumnHeadersPresenter x:Name="PART_ColumnHeadersPresenter"
                                                                Grid.Column="1"
                                                                Visibility="{Binding HeadersVisibility, ConverterParameter={x:Static DataGridHeadersVisibility.Column}, Converter={x:Static DataGrid.HeadersVisibilityConverter}, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"/>

                                <ScrollContentPresenter x:Name="PART_ScrollContentPresenter"
                                                        CanContentScroll="{TemplateBinding CanContentScroll}"
                                                        Grid.ColumnSpan="2"
                                                        Grid.Row="1" />

                                <ScrollBar x:Name="PART_VerticalScrollBar"
                                           Grid.Column="2"
                                           Maximum="{TemplateBinding ScrollableHeight}"
                                           Orientation="Vertical"
                                           Grid.Row="1"
                                           Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}" Value="{Binding VerticalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}"
                                           ViewportSize="{TemplateBinding ViewportHeight}"/>

                                <Grid Grid.Column="1" Grid.Row="2">
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="{Binding NonFrozenColumnsViewportHorizontalOffset, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"/>
                                        <ColumnDefinition Width="*"/>
                                    </Grid.ColumnDefinitions>

                                    <ScrollBar x:Name="PART_HorizontalScrollBar"
                                               Grid.Column="1"
                                               Maximum="{TemplateBinding ScrollableWidth}"
                                               Orientation="Horizontal"
                                               Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}" Value="{Binding HorizontalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportWidth}"/>
                                </Grid>
                            </Grid>
                        </ControlTemplate>
                    </ScrollViewer.Template>

                    <Grid>
                        <ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                                        Grid.Row="0" />

                        <Canvas Width="128"
                                VerticalAlignment="Stretch"
                                HorizontalAlignment="Left"
                                Grid.Row="0"
                                x:Name="Image" />
                    </Grid>
                </ScrollViewer>
            </Border>
        </ControlTemplate>
    </Setter.Value>
</Setter>

I know that if you load a lot of data on a data grid, the performance suffers. I can use virtualization to mitigate that performance hit, however, as soon as I throw the grid in a custom scroll viewer, the virtualization is lost.

I am trying to get it back, but I'm not sure how -- while still retain the element named Image in my XAML.

Basically, I want to have an image scrolling with the data grid contents and the above code works fine, it's just that I don't know how to enable virtualization. Is it even possible?

Update: Looks like I've found a problem. The last Grid in the template causes a problem:

<Grid>
    <ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                    Grid.Row="0" />

    <Canvas Width="128"
            VerticalAlignment="Stretch"
            HorizontalAlignment="Left"
            Grid.Row="0"
            x:Name="Image" />
</Grid>

As soon as I take the Canvas and Grid out, leaving only the ItemsPresenter, then it's fast again. How can I get it fast and still retain this Canvas?

Update 2: How can I apply this (ScrollViewer slow perfomance with DataGrid) strategy for my Grid shown above? I tried this:

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="*" />
    </Grid.RowDefinitions>

    <Rectangle Name="sizingElement" Grid.Row="0" Fill="Transparent" Margin="1"/>

    <ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                    Grid.Row="0"
                    Height="{Binding ElementName=sizingElement, Path=ActualHeight, FallbackValue=1}" />

    <Canvas Width="128"
            VerticalAlignment="Stretch"
            HorizontalAlignment="Left"
            Grid.Row="0"
            x:Name="Image" />
</Grid>

However, now the scrollbars have disappeared?

I realize that I can't virtualize a Canvas, and I don't need to. In fact, the whole Canvas gets drawn and I have no logic to separate it into smaller parts. It's completely fine to render the image at its entirety, as long as I can keep row virtualization.

Answer 1

I've got virtualization working for my custom control based on TreeView (.Net 4.0). I modified a little the style to match DataGrid, hope it is going to work for your case:

  <Style TargetType="{x:Type DataGrid}">
<Setter Property="OverridesDefaultStyle" Value="True" />
<Setter Property="VirtualizingStackPanel.IsVirtualizing" Value="True" />
<Setter Property="VirtualizingStackPanel.VirtualizationMode" Value="Recycling" />
<Setter Property="BorderThickness" Value="1" />
<Setter Property="ItemsPanel">
  <Setter.Value>
    <ItemsPanelTemplate>
      <VirtualizingStackPanel IsItemsHost="True" VirtualizingStackPanel.IsVirtualizing="True"
                              VirtualizingStackPanel.VirtualizationMode="Recycling" />
    </ItemsPanelTemplate>
  </Setter.Value>
</Setter>
<Setter Property="Template">
  <Setter.Value>
            <ControlTemplate TargetType="{x:Type DataGrid}">
      <Border x:Name="Border" Grid.Column="0" Background="{TemplateBinding Background}"
              BorderBrush="{StaticResource SolidBorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="2">
        <DockPanel>

            <ScrollViewer x:Name="PART_Body_Scroll" Background="White" HorizontalScrollBarVisibility="Auto"
                        VerticalScrollBarVisibility="Auto" CanContentScroll="True">

              <ItemsPresenter x:Name="ItemsHost" VirtualizingStackPanel.IsVirtualizing="True"
                            VirtualizingStackPanel.VirtualizationMode="Recycling" />

          </ScrollViewer>

        </DockPanel>
      </Border>
    </ControlTemplate>
  </Setter.Value>
</Setter>

Answer 2

the problem is that virtualization only works if the contents of the scrollviewer supports IScrollInfo / VirtualizingPanel.

As far as I can see you want to have your items with a canvas with something below it - all inside your scrolling area. Is it actually a special kind of row you want? If so you could go that way and insert a special row instead? Or you could just move that outside the datagrid - or try using RowDetails - it is not the same apperance I know - but far easiere to use.

To get virtualization to work with what you have your canvas have to be inside the virtualizing panel.