从VirtualizingStackPanel派生不起作用

Question

我正在尝试创建虚拟化的统一网格，所以我从here中获取了一些代码，它创建了一个面板，我可以在其中设置列数，它工作得很好，它比标准的统一网格快3-4倍(我在一个包含许多记录的列表框的复杂项目模板上测试了它)。所以我想我把基本控件从面板改成了VirtualizingStackPanel，但是一旦我这样做了，就没有记录显示。你知道为什么吗？以下是我的工作代码：

在下面更改为从VirtualizingPanel或VirtualizingStackPanel派生，它将不再起作用:(

public class MyUniformGrid : Panel // VirtualizingStackPanel
{
    public static readonly DependencyProperty ColumnsProperty = DependencyProperty.Register("Columns", typeof(int), typeof(MyUniformGrid), new FrameworkPropertyMetadata(1, FrameworkPropertyMetadataOptions.AffectsMeasure));

    public int Columns
    {
        set { SetValue(ColumnsProperty, value); }
        get { return (int)GetValue(ColumnsProperty); }
    }

    private int Rows => (InternalChildren.Count + Columns - 1) / Columns;

    protected override Size MeasureOverride(Size sizeAvailable)
    {
        var sizeChild = new Size(sizeAvailable.Width / Columns, sizeAvailable.Height / Rows);

        double maxwidth = 0;
        double maxheight = 0;

        foreach (UIElement child in InternalChildren)
        {
            child.Measure(sizeChild);

            maxwidth = Math.Max(maxwidth, child.DesiredSize.Width);
            maxheight = Math.Max(maxheight, child.DesiredSize.Height);
        }
        return new Size(Columns * maxwidth, Rows * maxheight);
    }

    protected override Size ArrangeOverride(Size sizeFinal)
    {
        var sizeChild = new Size(sizeFinal.Width / Columns, sizeFinal.Height / Rows);
        for (var index = 0; index < InternalChildren.Count; index++)
        {
            var row = index / Columns;
            var col = index % Columns;
            var rectChild = new Rect(new Point(col * sizeChild.Width, row * sizeChild.Height), sizeChild);

            InternalChildren[index].Arrange(rectChild);
        }
        return sizeFinal;
    }
}

xaml

<ListBox x:Name="MyListBox">
    <ListBox.ItemsPanel>
        <ItemsPanelTemplate>
            <uniformGridDemo:MyUniformGrid Columns="2" />
        </ItemsPanelTemplate>
    </ListBox.ItemsPanel>
</ListBox>

代码隐藏

    public MainWindow()
    {
        InitializeComponent();
        var list = new List<int>();
        for (int i = 0; i < 1000; i++)
        {
            list.Add(i);
        }
        MyListBox.ItemsSource = list;
    }

谢谢

Answer 1

我不是VirtualizingStackPanel方面的专家，但如果你只是想解决项目显示问题，下面的方法就行了。

 public class MyUniformGrid :  VirtualizingStackPanel
{
    public static readonly DependencyProperty ColumnsProperty = DependencyProperty.Register("Columns", typeof(int), typeof(MyUniformGrid), new FrameworkPropertyMetadata(1, FrameworkPropertyMetadataOptions.AffectsMeasure));

    public int Columns
    {
        set { SetValue(ColumnsProperty, value); }
        get { return (int)GetValue(ColumnsProperty); }
    }

    private int Rows
    {
        get
        {                
            return (InternalChildren.Count + Columns - 1) / Columns;
        }
    }

    protected override Size MeasureOverride(Size sizeAvailable)
    {
        base.MeasureOverride(sizeAvailable);
        var sizeChild = new Size(sizeAvailable.Width / Columns, sizeAvailable.Height / Rows);

        double maxwidth = 0;
        double maxheight = 0;

        foreach (UIElement child in this.InternalChildren)
        {
            child.Measure(sizeChild);

            maxwidth = Math.Max(maxwidth, child.DesiredSize.Width);
            maxheight = Math.Max(maxheight, child.DesiredSize.Height);
        }
        return new Size(Columns * maxwidth, Rows * maxheight);

    }

    protected override Size ArrangeOverride(Size sizeFinal)
    {
        base.ArrangeOverride(sizeFinal);
        var sizeChild = new Size(sizeFinal.Width / Columns, sizeFinal.Height / Rows);
        for (var index = 0; index < InternalChildren.Count; index++)
        {
            var row = index / Columns;
            var col = index % Columns;
            var rectChild = new Rect(new Point(col * sizeChild.Width, row * sizeChild.Height), sizeChild);

            InternalChildren[index].Arrange(rectChild);
        }
        return sizeFinal;
    }
}

但是VirtualizingStackPanel的实现要比overriding、MeasureOverride和ArrangeOverride方法和安排项复杂得多。您需要处理滚动信息，并且需要处理面板中项目的添加和删除。因此，即使在完全显示项目之后，您也可能无法获得所需的性能。

Answer 2

我在Silverlight中实现了类似的功能，将样式应用到ListBox，并用WrapPanel替换了ItemsPanelTemplate。我已经在Windows Phone和WPF项目中使用了相同的方法，没有出现任何问题。

样式

<!--Wrapping ListBox Styles-->
<Style x:Key="StretchedItemContainerStyle" TargetType="ListBoxItem">
    <Setter Property="HorizontalContentAlignment" Value="Stretch" />
</Style>

<Style x:Key="ListBox_HorizontalWrapStyle" TargetType="ListBox">
    <Setter Property="ItemContainerStyle" Value="{StaticResource StretchedItemContainerStyle}"/>
    <Setter Property="ItemsPanel">
        <Setter.Value>
            <ItemsPanelTemplate>
                <WrapPanel Orientation="Horizontal" Margin="0"/>
            </ItemsPanelTemplate>
        </Setter.Value>
    </Setter>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ListBox">
                <ScrollViewer VerticalScrollBarVisibility="Auto" BorderBrush="{x:Null}" >
                    <ItemsPresenter />
                </ScrollViewer>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>
<!--End Wrapping ListBox Styles-->

Xaml

<ListBox x:Name="MyListBox" Style="StaticResource ListBox_HorizontalWrapStyle">

您还应该能够设置WrapPanel的样式，以获得任何其他性能收益。