绑定{RelativeSource PreviousData}在特定情况下中断绑定

Question

我尝试在{RelativeSource PreviousData}中使用ListBox.ItemTemplate，但它运行正常。

但是，当使用下面提供的特定代码时，当向下滚动几次并且Rectangle的某些缺失时，绑定会停止工作。

即使使用单个DataTrigger，问题也会重现，但当ListBox.Height超过 178 时，它会不重建。

示例GIF - 缺少绿线！：

MainWindow.Xaml来源：

<Window
    x:Class="PreviousDataBindingWheelIssue.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:local="clr-namespace:PreviousDataBindingWheelIssue"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Title="PreviousData Issue"
    d:DataContext="{d:DesignInstance Type=local:MyModel}"
    SizeToContent="WidthAndHeight"
    mc:Ignorable="d">
    <StackPanel>

        <!--  Height must be less or equal to 178  -->
        <ListBox
            Width="300"
            Height="178"
            HorizontalContentAlignment="Stretch"
            ItemsSource="{Binding MyData}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <DockPanel Background="#FFFFFFED">
                        <Rectangle
                            Height="2"
                            Margin="0"
                            DockPanel.Dock="Top">
                            <Rectangle.Style>
                                <Style TargetType="Rectangle">
                                    <Setter Property="Fill" Value="#FF63605C" />
                                    <Style.Triggers>

                                        <!--
                                            Hide our magnificent separator if this is the first item on the list
                                            see http://stackoverflow.com/a/22705507/426315
                                            but, it seems to have some issues when using mouse wheel
                                            some of the rows does NOT have the rectangle even when PreviousData SHOULD not be null
                                        -->
                                        <DataTrigger Binding="{Binding RelativeSource={RelativeSource PreviousData}}" Value="{x:Null}">
                                            <Setter Property="Visibility" Value="Collapsed" />
                                        </DataTrigger>

                                        <DataTrigger Binding="{Binding}" Value="Fun Item">
                                            <Setter Property="Fill" Value="SpringGreen" />
                                        </DataTrigger>
                                    </Style.Triggers>
                                </Style>
                            </Rectangle.Style>
                        </Rectangle>

                        <TextBlock
                            Margin="5,7,5,7"
                            VerticalAlignment="Center"
                            FontSize="12"
                            Text="{Binding}" />
                    </DockPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </StackPanel>
</Window>

主窗口代码背后：

using System.Windows;
namespace PreviousDataBindingWheelIssue
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new MyModel();
        }
    }
}

MyModel.cs来源：

using System.Collections.ObjectModel;

namespace PreviousDataBindingWheelIssue
{
    public class MyModel
    {
        public ObservableCollection<string> MyData { get; set; }

        public MyModel()
        {
            MyData = new ObservableCollection<string>()
            {
                "Lorem ipsum dolor", "sit amet, consectetur", "adipiscing elit. Sed",
                "Fun Item",
                "rhoncus leo convallis", "pulvinar tellus at",
                "Fun Item",
                "porta metus. Mauris", "sed mauris quis", "neque congue semper",
                "Fun Item",
                "vitae non leo", "Donec aliquet feugiat", "massa vitae luctus",
                "Fun Item",
                "Duis pharetra velit", "et lorem blandit"
            };
        }
    }
}

Answer 1

由于PreviousData绑定在虚拟化方面不可靠，您可以通过在VirtualizingPanel.IsVirtualizing="False"上设置ListBox来禁用虚拟化，或者使绑定虚拟化准备就绪。

处理此类问题的一种方法是在我的示例代码中创建自定义列表框（ListBox2），覆盖PrepareContainerForItemOverride并设置一些可用于进一步操作的属性。为此，我创建了一个附加属性ItemIndex。

public class ListBox2 : ListBox
{
    protected override void PrepareContainerForItemOverride(DependencyObject element, object item)
    {
        base.PrepareContainerForItemOverride(element, item);
        SetItemIndex(element, ItemContainerGenerator.IndexFromContainer(element));
    }


    // helper attached property to indicate the index of listbox items

    public static int GetItemIndex(DependencyObject obj)
    {
        return (int)obj.GetValue(ItemIndexProperty);
    }

    protected static void SetItemIndex(DependencyObject obj, int value)
    {
        obj.SetValue(ItemIndexPropertyKey, value);
    }

    private static readonly DependencyPropertyKey ItemIndexPropertyKey =
        DependencyProperty.RegisterAttachedReadOnly("ItemIndex", typeof(int), typeof(ListBox2), new PropertyMetadata(-1));

    public static readonly DependencyProperty ItemIndexProperty = ItemIndexPropertyKey.DependencyProperty;
}

然后更改xaml以使用ListBox2并依赖ItemIndex代替PreviousData：

<local:ListBox2
    Width="300"
    Height="178"
    HorizontalContentAlignment="Stretch"
    ItemsSource="{Binding MyData}">
    <local:ListBox2.ItemTemplate>
        <DataTemplate>
            <DockPanel Background="#FFFFFFED">
                <Rectangle
                    Height="2"
                    Margin="0"
                    DockPanel.Dock="Top">
                    <Rectangle.Style>
                        <Style TargetType="Rectangle">
                            <Setter Property="Fill" Value="#FF63605C" />
                            <Style.Triggers>
                                <DataTrigger Binding="{Binding Path=(local:ListBox2.ItemIndex),RelativeSource={RelativeSource AncestorType=ListBoxItem}}" Value="0">
                                    <Setter Property="Visibility" Value="Collapsed" />
                                </DataTrigger>

                                <DataTrigger Binding="{Binding}" Value="Fun Item">
                                    <Setter Property="Fill" Value="SpringGreen" />
                                </DataTrigger>
                            </Style.Triggers>
                        </Style>
                    </Rectangle.Style>
                </Rectangle>

                <TextBlock
                    Margin="5,7,5,7"
                    VerticalAlignment="Center"
                    FontSize="12"
                    Text="{Binding}" />
            </DockPanel>
        </DataTemplate>
    </local:ListBox2.ItemTemplate>
</local:ListBox2>