c# - 我如何知道 ListBoxItem 是否是 Wpf 的 ListBox 中的最后一项？

Question

我如何知道 a是否是WpfListBoxItem中集合的最后一项（在模板中ItemContainerStyle或模板中） ？ItemContainerListBox

这个问题是因为我需要知道一个项目是否是最后一个以其他方式显示的项目。例如：假设我想显示用分号分隔的项目，但最后一个：a;b;c

使用 ccs 选择器在 html 和 ccs 中很容易做到这一点。但是，我怎么能在 Wpf 中做到这一点？

Answer 1

由于似乎很难为ListBoxItem 实现一个“索引”附加属性来完成这项工作，我相信更简单的方法是在 MVVM 中实现。您可以将必要的逻辑（“IsLast”属性等）添加到列表的实体类型，并让 ViewModel 处理它，并在修改或替换集合时更新它。

编辑

经过一些尝试，我设法使用附加属性和继承 ListBox 的组合来实现 ListBoxItems 的索引（并因此检查最后一个）。看看这个：

public class IndexedListBox : System.Windows.Controls.ListBox
{
    public static int GetIndex(DependencyObject obj)
    {
        return (int)obj.GetValue(IndexProperty);
    }
    public static void SetIndex(DependencyObject obj, int value)
    {
        obj.SetValue(IndexProperty, value);
    }
    /// <summary>
    /// Keeps track of the index of a ListBoxItem
    /// </summary>
    public static readonly DependencyProperty IndexProperty =
        DependencyProperty.RegisterAttached("Index", typeof(int), typeof(IndexedListBox), new UIPropertyMetadata(0));


    public static bool GetIsLast(DependencyObject obj)
    {
        return (bool)obj.GetValue(IsLastProperty);
    }
    public static void SetIsLast(DependencyObject obj, bool value)
    {
        obj.SetValue(IsLastProperty, value);
    }
    /// <summary>
    /// Informs if a ListBoxItem is the last in the collection.
    /// </summary>
    public static readonly DependencyProperty IsLastProperty =
        DependencyProperty.RegisterAttached("IsLast", typeof(bool), typeof(IndexedListBox), new UIPropertyMetadata(false));


    protected override void OnItemsSourceChanged(System.Collections.IEnumerable oldValue, System.Collections.IEnumerable newValue)
    {
        // We capture the ItemsSourceChanged to check if the new one is modifiable, so we can react to its changes.

        var oldSource = oldValue as INotifyCollectionChanged;
        if(oldSource != null)
            oldSource.CollectionChanged -= ItemsSource_CollectionChanged;

        var newSource = newValue as INotifyCollectionChanged;
        if (newSource != null)
            newSource.CollectionChanged += ItemsSource_CollectionChanged;

        base.OnItemsSourceChanged(oldValue, newValue);
    }

    void ItemsSource_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        this.ReindexItems();
    }

    protected override void PrepareContainerForItemOverride(System.Windows.DependencyObject element, object item)
    {
        // We set the index and other related properties when generating a ItemContainer
        var index = this.Items.IndexOf(item); 
        SetIsLast(element, index == this.Items.Count - 1);
        SetIndex(element, index);

        base.PrepareContainerForItemOverride(element, item);
    }

    private void ReindexItems()
    {
        // If the collection is modified, it may be necessary to reindex all ListBoxItems.
        foreach (var item in this.Items)
        {
            var itemContainer = this.ItemContainerGenerator.ContainerFromItem(item);
            if (itemContainer == null) continue;

            int index = this.Items.IndexOf(item);
            SetIsLast(itemContainer, index == this.Items.Count - 1);
            SetIndex(itemContainer, index);
        }
    }
}

为了测试它，我们设置了一个简单的 ViewModel 和一个 Item 类：

public class ViewModel : INotifyPropertyChanged
{
    #region INotifyPropertyChanged

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    #endregion

    private ObservableCollection<Item> items;
    public ObservableCollection<Item> Items
    {
        get { return this.items; }
        set
        {
            if (this.items != value)
            {
                this.items = value;
                this.OnPropertyChanged("Items");
            }
        }
    }

    public ViewModel()
    {
        this.InitItems(20);
    }

    public void InitItems(int count)
    {

        this.Items = new ObservableCollection<Item>();
        for (int i = 0; i < count; i++)
            this.Items.Add(new Item() { MyProperty = "Element" + i });
    }

}

public class Item
{
    public string MyProperty { get; set; }

    public override string ToString()
    {
        return this.MyProperty;
    }
}

风景：

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WpfApplication3"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d" x:Class="WpfApplication3.MainWindow"
    Title="MainWindow" Height="350" Width="525">
<Window.Resources>
    <DataTemplate x:Key="DataTemplate">
        <Border x:Name="border">
            <StackPanel Orientation="Horizontal">
                <TextBlock TextWrapping="Wrap" Text="{Binding (local:IndexedListBox.Index), RelativeSource={RelativeSource AncestorType={x:Type ListBoxItem}}}" Margin="0,0,8,0"/>
                <TextBlock TextWrapping="Wrap" Text="{Binding (local:IndexedListBox.IsLast), RelativeSource={RelativeSource AncestorType={x:Type ListBoxItem}}}" Margin="0,0,8,0"/>
                <ContentPresenter Content="{Binding}"/>
            </StackPanel>
        </Border>
        <DataTemplate.Triggers>
            <DataTrigger Binding="{Binding (local:IndexedListBox.IsLast), RelativeSource={RelativeSource AncestorType={x:Type ListBoxItem}}}" Value="True">
                <Setter Property="Background" TargetName="border" Value="Red"/>
            </DataTrigger>
        </DataTemplate.Triggers>
    </DataTemplate>
</Window.Resources>
<Window.DataContext>
    <local:ViewModel/>
</Window.DataContext>
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="0.949*"/>
    </Grid.RowDefinitions>

    <local:IndexedListBox ItemsSource="{Binding Items}" Grid.Row="1" ItemTemplate="{DynamicResource DataTemplate}"/>

    <Button Content="Button" HorizontalAlignment="Left" Width="75" d:LayoutOverrides="Height" Margin="8" Click="Button_Click"/>
    <Button Content="Button" HorizontalAlignment="Left" Width="75" Margin="110,8,0,8" Click="Button_Click_1"  d:LayoutOverrides="Height"/>
    <Button Content="Button" Margin="242,8,192,8" Click="Button_Click_2"  d:LayoutOverrides="Height"/>
</Grid>
</Window>

在后面的视图代码中，我放置了一些逻辑来测试更新集合时解决方案的行为：

public partial class MainWindow : Window
{
    public ViewModel ViewModel { get { return this.DataContext as ViewModel; } }

    public MainWindow()
    {
        InitializeComponent();
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        this.ViewModel.Items.Insert( 5, new Item() { MyProperty= "NewElement" });
    }

    private void Button_Click_1(object sender, RoutedEventArgs e)
    {
        this.ViewModel.Items.RemoveAt(5);
    }

    private void Button_Click_2(object sender, RoutedEventArgs e)
    {
        this.ViewModel.InitItems(new Random().Next(10,30));
    }
}

该解决方案可以处理静态列表以及 ObservableCollections 以及添加、删除、插入项目。希望你觉得它有用。

编辑

用 CollectionViews 对其进行了测试，它工作得很好。

在第一个测试中，我更改了 ListBox.Items 中的 Sort/GroupDescriptions。当其中一个发生更改时，ListBox 会重新创建容器，然后 PrepareContainerForItemOverride 命中。当它在 ListBox.Items 本身中寻找正确的索引时，顺序会正确更新。

在第二个中，我将 ViewModel 中的 Items 属性设置为 ListCollectionView。在这种情况下，当描述更改时，会引发 CollectionChanged 并且 ListBox 会按预期做出反应。