添加新项目时如何让 ListBox 自动滚动? [英] How can I have a ListBox auto-scroll when a new item is added?

问题描述

我有一个设置为水平滚动的 WPF 列表框.ItemsSource 绑定到我的 ViewModel 类中的 ObservableCollection.每次添加新项目时，我都希望 ListBox 向右滚动以便可以查看新项目.

I have a WPF ListBox that is set to scroll horizontally. The ItemsSource is bound to an ObservableCollection in my ViewModel class. Every time a new item is added, I want the ListBox to scroll to the right so that the new item is viewable.

ListBox 是在 DataTemplate 中定义的，因此我无法在我的代码隐藏文件中按名称访问 ListBox.

The ListBox is defined in a DataTemplate, so I am unable to access the ListBox by name in my code behind file.

如何让 ListBox 始终滚动以显示最新添加的项目?

How can I get a ListBox to always scroll to show a latest added item?

我想知道 ListBox 何时添加了新项目，但我没有看到执行此操作的事件.

I would like a way to know when the ListBox has a new item added to it, but I do not see an event that does this.

推荐答案

您可以通过使用附加属性来扩展 ListBox 的行为.在您的情况下，我将定义一个名为 ScrollOnNewItem 的附加属性，当设置为 true 时，它会钩入列表框项目源的 INotifyCollectionChanged 事件，然后检测到一个新项目，将列表框滚动到它.

You can extend the behavior of the ListBox by using attached properties. In your case I would define an attached property called ScrollOnNewItem that when set to true hooks into the INotifyCollectionChanged events of the list box items source and upon detecting a new item, scrolls the list box to it.

例子:

class ListBoxBehavior
{
    static readonly Dictionary<ListBox, Capture> Associations =
           new Dictionary<ListBox, Capture>();

    public static bool GetScrollOnNewItem(DependencyObject obj)
    {
        return (bool)obj.GetValue(ScrollOnNewItemProperty);
    }

    public static void SetScrollOnNewItem(DependencyObject obj, bool value)
    {
        obj.SetValue(ScrollOnNewItemProperty, value);
    }

    public static readonly DependencyProperty ScrollOnNewItemProperty =
        DependencyProperty.RegisterAttached(
            "ScrollOnNewItem",
            typeof(bool),
            typeof(ListBoxBehavior),
            new UIPropertyMetadata(false, OnScrollOnNewItemChanged));

    public static void OnScrollOnNewItemChanged(
        DependencyObject d,
        DependencyPropertyChangedEventArgs e)
    {
        var listBox = d as ListBox;
        if (listBox == null) return;
        bool oldValue = (bool)e.OldValue, newValue = (bool)e.NewValue;
        if (newValue == oldValue) return;
        if (newValue)
        {
            listBox.Loaded += ListBox_Loaded;
            listBox.Unloaded += ListBox_Unloaded;
            var itemsSourcePropertyDescriptor = TypeDescriptor.GetProperties(listBox)["ItemsSource"];
            itemsSourcePropertyDescriptor.AddValueChanged(listBox, ListBox_ItemsSourceChanged);
        }
        else
        {
            listBox.Loaded -= ListBox_Loaded;
            listBox.Unloaded -= ListBox_Unloaded;
            if (Associations.ContainsKey(listBox))
                Associations[listBox].Dispose();
            var itemsSourcePropertyDescriptor = TypeDescriptor.GetProperties(listBox)["ItemsSource"];
            itemsSourcePropertyDescriptor.RemoveValueChanged(listBox, ListBox_ItemsSourceChanged);
        }
    }

    private static void ListBox_ItemsSourceChanged(object sender, EventArgs e)
    {
        var listBox = (ListBox)sender;
        if (Associations.ContainsKey(listBox))
            Associations[listBox].Dispose();
        Associations[listBox] = new Capture(listBox);
    }

    static void ListBox_Unloaded(object sender, RoutedEventArgs e)
    {
        var listBox = (ListBox)sender;
        if (Associations.ContainsKey(listBox))
            Associations[listBox].Dispose();
        listBox.Unloaded -= ListBox_Unloaded;
    }

    static void ListBox_Loaded(object sender, RoutedEventArgs e)
    {
        var listBox = (ListBox)sender;
        var incc = listBox.Items as INotifyCollectionChanged;
        if (incc == null) return;
        listBox.Loaded -= ListBox_Loaded;
        Associations[listBox] = new Capture(listBox);
    }

    class Capture : IDisposable
    {
        private readonly ListBox listBox;
        private readonly INotifyCollectionChanged incc;

        public Capture(ListBox listBox)
        {
            this.listBox = listBox;
            incc = listBox.ItemsSource as INotifyCollectionChanged;
            if (incc != null)
            {
                incc.CollectionChanged += incc_CollectionChanged;
            }
        }

        void incc_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            if (e.Action == NotifyCollectionChangedAction.Add)
            {
                listBox.ScrollIntoView(e.NewItems[0]);
                listBox.SelectedItem = e.NewItems[0];
            }
        }

        public void Dispose()
        {
            if (incc != null)
                incc.CollectionChanged -= incc_CollectionChanged;
        }
    }
}

用法:

<ListBox ItemsSource="{Binding SourceCollection}" 
         lb:ListBoxBehavior.ScrollOnNewItem="true"/>

UPDATE 根据 Andrej 在下面评论中的建议，我添加了钩子来检测 ListBox 的 ItemsSource 的变化.

UPDATE As per Andrej's suggestion in the comments below, I added hooks to detect a change in the ItemsSource of the ListBox.

这篇关于添加新项目时如何让 ListBox 自动滚动?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！