I have a list with a lot of items that are expensive to exhibit. However, VirtualizingStackPanel will only take care of this by visualizing the visible elements. I want to override the control pattern for ScrollViewer, because by default it has a gray rectangle between horizontal and vertical scrollbars. I just copied a file provided by Microsoft ( ScrollViewer ControlTemplate Example ) which does not have a gray rectangle problem.
However, this controltemplate parameter disables virtualization, providing an infinite height for the VirtualizationStackPanel. This means that VirtualizingStackPanel will display all the elements as it considers all the elements to be visible.
In the demo code below, I'm showing 10,000 items in a list. I test the problem by comparing it with and without the ScrollViewer style. The demo works very slowly with it, resizing takes many seconds. Without style, it is very fast. I was outputting some information about VirtualizingStackPanel to prove my point:
Without ScrollViewer style (comment style in XAML):
ViewportHeight: 8
ExtentHeight: 10000
ActualHeight: 245
IsVirtualizing: True
Virtualization: Standard
With ScrollViewer style:
ViewportHeight: 0
ExtentHeight: 0
ActualHeight: 272766.666666707
IsVirtualizing: True
:
ScrollViewer, ?
XAML:
<Window x:Class="VirtualTest.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300">
<Window.Resources>
<Style x:Key="{x:Type ScrollViewer}" TargetType="{x:Type ScrollViewer}">
<Setter Property="OverridesDefaultStyle" Value="true" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ScrollViewer}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<ScrollContentPresenter Grid.Row="0" Grid.Column="0" />
<ScrollBar
Name="PART_VerticalScrollBar"
Grid.Row="0" Grid.Column="1"
Value="{TemplateBinding VerticalOffset}"
Maximum="{TemplateBinding ScrollableHeight}"
ViewportSize="{TemplateBinding ViewportHeight}"
Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}" />
<ScrollBar
Name="PART_HorizontalScrollBar"
Orientation="Horizontal"
Grid.Row="1" Grid.Column="0"
Value="{TemplateBinding HorizontalOffset}"
Maximum="{TemplateBinding ScrollableWidth}"
ViewportSize="{TemplateBinding ViewportWidth}"
Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}" />
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
<Grid>
<ListBox
ItemsSource="{Binding Numbers}"
ScrollViewer.ScrollChanged="ListBox_ScrollChanged"
Background="Orange">
<ListBox.ItemTemplate>
<DataTemplate>
<Border BorderBrush="Red" BorderThickness="2" Margin="5">
<TextBlock Text="{Binding .}" Width="400"/>
</Border>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>
</Window>
:
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
namespace VirtualTest
{
public partial class Window1 : Window
{
public Window1()
{
InitializeComponent();
DataContext = this;
}
public IEnumerable<double> Numbers
{
get
{
for (int i = 0; i < 10000; i++)
{
yield return i;
}
}
}
private void ListBox_ScrollChanged(object sender, ScrollChangedEventArgs e)
{
ListBox listBox = sender as ListBox;
VirtualizingStackPanel virtualizingStackPanel = FindVirtualizingStackPanel(listBox);
Debug.WriteLine("ViewportHeight: " + virtualizingStackPanel.ViewportHeight);
Debug.WriteLine("ExtentHeight: " + virtualizingStackPanel.ExtentHeight);
Debug.WriteLine("ActualHeight: " + virtualizingStackPanel.ActualHeight);
Debug.WriteLine("IsVirtualizing: " + VirtualizingStackPanel.GetIsVirtualizing(virtualizingStackPanel));
Debug.WriteLine("VirtualizationMode: " + VirtualizingStackPanel.GetVirtualizationMode(virtualizingStackPanel));
}
private VirtualizingStackPanel FindVirtualizingStackPanel(Visual visual)
{
for (int i = 0; i < VisualTreeHelper.GetChildrenCount(visual); i++)
{
Visual child = VisualTreeHelper.GetChild(visual, i) as Visual;
if (child != null)
{
if (child is VirtualizingStackPanel)
{
return child as VirtualizingStackPanel;
}
VirtualizingStackPanel found = FindVirtualizingStackPanel(child);
if (found != null)
{
return found;
}
}
}
return null;
}
}
}