WPF - 当 ItemsSource 更改时重置 ListBox 滚动位置 [英] WPF - reset ListBox scroll position when ItemsSource changes

问题描述

我目前有一个 ListBox，它的 ItemsSource 集合绑定到我的视图模型上的 IEnumerable 类型的属性.当 preoprty 的引用更改时，ListBox 会按预期更新，但是我有一个问题，如果我有大量项目并滚动到 ListBox 的底部，然后将引用更改为另一个包含 1 个项目的集合，ListBox 视图为空白且不显示滚动条.然后我必须用鼠标滚轮向上滚动列表框，直到出现 1 项.

I currently have a ListBox whose ItemsSource collection is bound to a property on my viewmodel, of type IEnumerable. When that preoprty's reference changes, the ListBox updates as expected, however I have a problem in that if I have a large collection of items and scroll to the bottom of the ListBox, and then change the reference to another collection containing, say, 1 item, the ListBox view is blank and no scrollbar is displayed. I have to then scroll the listbox up with the mouse wheel, until the 1 item comes into view.

所以，我想我追求的是一种将 ListBox 的滚动位置重置到顶部的方法，只要 ItemsSource 属性发生变化，这样无论集合有多大或多小，都会始终显示某些内容.

So, what I think I'm after, is a way of resetting the scroll position of the ListBox to the top, whenever the ItemsSource property changes, so that something is always displayed no matter how large or small the collection.

推荐答案

我无法重现您的问题(对我来说，ListBox 在更改时会滚动到新集合中的最后一项ItemsSource).无论如何，要在每次 ItemsSource 更改时将 ListBox 滚动到顶部，您可以在后面使用一些代码.首先监听 ItemsSourceProperty 中的变化，然后在生成项目后将 ListBox 滚动到顶部

I'm unable to reproduce your problem (for me, the ListBox is scrolled to the last item in the new collection when changing ItemsSource). Anyway, to scroll the ListBox to the top every time its ItemsSource changes you can use some code behind. First listen to changes in the ItemsSourceProperty and then scroll the ListBox to the top once its items has been generated

更新

做了一个附加的行为来避免代码落后.可以这样使用

Made an attached behavior that does this instead to avoid code behind. It can be used like this

<ListBox ...
         behaviors:ScrollToTopBehavior.ScrollToTop="True"/>

ScrollToTopBehavior

public static class ScrollToTopBehavior 
{
    public static readonly DependencyProperty ScrollToTopProperty = 
        DependencyProperty.RegisterAttached 
        (
            "ScrollToTop", 
            typeof(bool),
            typeof(ScrollToTopBehavior),
            new UIPropertyMetadata(false, OnScrollToTopPropertyChanged) 
        );
    public static bool GetScrollToTop(DependencyObject obj) 
    {
        return (bool)obj.GetValue(ScrollToTopProperty); 
    }
    public static void SetScrollToTop(DependencyObject obj, bool value) 
    {
        obj.SetValue(ScrollToTopProperty, value); 
    }
    private static void OnScrollToTopPropertyChanged(DependencyObject dpo, 
                                                     DependencyPropertyChangedEventArgs e) 
    {
        ItemsControl itemsControl = dpo as ItemsControl;
        if (itemsControl != null) 
        {
            DependencyPropertyDescriptor dependencyPropertyDescriptor =
                    DependencyPropertyDescriptor.FromProperty(ItemsControl.ItemsSourceProperty, typeof(ItemsControl));
            if (dependencyPropertyDescriptor != null)
            {
                if ((bool)e.NewValue == true) 
                {
                    dependencyPropertyDescriptor.AddValueChanged(itemsControl, ItemsSourceChanged);
                }
                else 
                {
                    dependencyPropertyDescriptor.RemoveValueChanged(itemsControl, ItemsSourceChanged);
                }
            } 
        } 
    }
    static void ItemsSourceChanged(object sender, EventArgs e)
    {
        ItemsControl itemsControl = sender as ItemsControl;
        EventHandler eventHandler = null;
        eventHandler = new EventHandler(delegate
        {
            if (itemsControl.ItemContainerGenerator.Status == GeneratorStatus.ContainersGenerated)
            {
                ScrollViewer scrollViewer = GetVisualChild<ScrollViewer>(itemsControl) as ScrollViewer;
                scrollViewer.ScrollToTop();
                itemsControl.ItemContainerGenerator.StatusChanged -= eventHandler;
            }
        });
        itemsControl.ItemContainerGenerator.StatusChanged += eventHandler;
    }
}

还有一个 GetVisualChild 的实现

And an implementation of GetVisualChild

private T GetVisualChild<T>(DependencyObject parent) where T : Visual
{
    T child = default(T);
    int numVisuals = VisualTreeHelper.GetChildrenCount(parent);
    for (int i = 0; i < numVisuals; i++)
    {
        Visual v = (Visual)VisualTreeHelper.GetChild(parent, i);
        child = v as T;
        if (child == null)
        {
            child = GetVisualChild<T>(v);
        }
        if (child != null)
        {
            break;
        }
    }
    return child;
}

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