我有一个列表框,其中有很多项需要进行昂贵的渲染。但是VirtualizingStackPanel通过仅呈现可见项来解决了这个问题。我想覆盖ScrollViewer的控件模板,因为默认模板在水平和垂直滚动条之间有灰色矩形。我只是复制了Microsoft提供的样例(ScrollViewer ControlTemplate Example),它没有灰色矩形的问题。
然而,这个控件模板通过给VirtualizingStackPanel无限高度来禁用虚拟化。这意味着VirtualizingStackPanel将呈现所有项,因为它认为所有项都是可见的。
在下面的演示代码中,我在列表框中显示了10000个项。我通过比较应用ScrollViewer样式和不应用ScrollViewer样式的运行情况来验证此问题。使用它会导致演示运行非常缓慢,调整大小需要很多秒钟。而没有使用该样式时则非常快。我输出了一些有关VirtualizingStackPanel的信息来证明我的观点:
不使用ScrollViewer样式(在XAML中注释掉样式):
ViewportHeight:8 ExtentHeight:10000 ActualHeight:245 IsVirtualizing:True VirtualizationMode:Standard
使用ScrollViewer样式:
ViewportHeight:0 ExtentHeight:0 ActualHeight:272766.666666707 IsVirtualizing:True VirtualizationMode:Standard
有没有什么办法编写一个不会影响虚拟化的ScrollViewer控件模板?
XAML:
然而,这个控件模板通过给VirtualizingStackPanel无限高度来禁用虚拟化。这意味着VirtualizingStackPanel将呈现所有项,因为它认为所有项都是可见的。
在下面的演示代码中,我在列表框中显示了10000个项。我通过比较应用ScrollViewer样式和不应用ScrollViewer样式的运行情况来验证此问题。使用它会导致演示运行非常缓慢,调整大小需要很多秒钟。而没有使用该样式时则非常快。我输出了一些有关VirtualizingStackPanel的信息来证明我的观点:
不使用ScrollViewer样式(在XAML中注释掉样式):
ViewportHeight:8 ExtentHeight:10000 ActualHeight:245 IsVirtualizing:True VirtualizationMode:Standard
使用ScrollViewer样式:
ViewportHeight:0 ExtentHeight:0 ActualHeight:272766.666666707 IsVirtualizing:True VirtualizationMode:Standard
有没有什么办法编写一个不会影响虚拟化的ScrollViewer控件模板?
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>
后台代码:
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;
}
}
}