WPF-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，其ItemsSource集合绑定到我的视图模型上的IEnumerable类型的属性.当该preoprty的引用发生更改时，ListBox会按预期更新，但是，我有一个问题，如果我有大量项，并滚动到ListBox的底部，然后将引用更改为另一个包含例如1个项的集合，列表框视图为空白，并且不显示滚动条.然后，我必须使用鼠标滚轮向上滚动列表框，直到显示1个项目为止.

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，我想我要想的是一种在ItemsSource属性更改时将ListBox的滚动位置重置到顶部的方法，以便无论集合有多大或少，总会显示一些内容. /p>

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，更改ItemsSource时，ListBox会滚动到新集合中的最后一项).无论如何，每次ItemsSource更改时，要将ListBox滚动到顶部，您都可以在后面使用一些代码.首先听ItemsSourceProperty中的更改，然后在生成ListBox项后将其滚动到顶部

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

进行附加行为，以防止代码落后.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;
    }
}

以及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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