How to control the scroll position of a ListBox in a MVVM WPF app

Question

I have got a big ListBox with vertical scrolling enabled, my MVVM has New and Edit ICommands. I am adding new item to the end of the collection but I want the scrollbar also to auto position to the End when I call my MVVM-AddCommand. I am also making an item editable(By calling EditCommand with a particular row item) from some other part of the application so that my ListBoxItem getting in to edit mode using DataTrigger, but how will I bring that particular row(ListBoxItem) to the view by adjusting the scroll position.

If I am doing it in the View side I can call listBox.ScrollInToView(lstBoxItem). But what is the best way to solve this common Scroll issue from an MVVM perspective.

Answer 1

I typically set IsSynchronizedWithCurrentItem="True" on the ListBox. Then I add a SelectionChanged handler and always bring the selected item into view, with code like this:

    private void BringSelectionIntoView(object sender, SelectionChangedEventArgs e)
    {
        Selector selector = sender as Selector;
        if (selector is ListBox)
        {
            (selector as ListBox).ScrollIntoView(selector.SelectedItem);
        }
    }

From my VM I can get the default collection view and use one of the MoveCurrent*() methods to ensure that the item being edited is the current item.

CollectionViewSource.GetDefaultView(_myCollection).MoveCurrentTo(thisItem);

NOTE: Edited to use ListBox.ScrollIntoView() to accomodate virtualization

Answer 2

Using this in MVVM can be easily accomplished via an attached behavior like so:

using System.Windows.Controls;
using System.Windows.Interactivity;

namespace Jarloo.Sojurn.Behaviors
{
    public sealed class ScrollIntoViewBehavior : Behavior<ListBox>
    {
        protected override void OnAttached()
        {
            base.OnAttached();
            AssociatedObject.SelectionChanged += ScrollIntoView;
        }

        protected override void OnDetaching()
        {
            AssociatedObject.SelectionChanged -= ScrollIntoView;
            base.OnDetaching();
        }

        private void ScrollIntoView(object o, SelectionChangedEventArgs e)
        {
            ListBox b = (ListBox) o;
            if (b == null)
                return;
            if (b.SelectedItem == null)
                return;

            ListBoxItem item = (ListBoxItem) ((ListBox) o).ItemContainerGenerator.ContainerFromItem(((ListBox) o).SelectedItem);
            if (item != null) item.BringIntoView();
        }
    }
}

Then in the View ad this reference at the top:

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"

And do this:

<ListBox ItemsSource="{Binding MyData}" SelectedItem="{Binding MySelectedItem}">
         <i:Interaction.Behaviors>
             <behaviors:ScrollIntoViewBehavior />
         </i:Interaction.Behaviors>
</ListBox>

Now when the SelectedItem changes the behavior will do the BringIntoView() call for you.

Answer 3

This is the attached property form of the accepted answer:

using System.Windows;
using System.Windows.Controls;

namespace CommonBehaviors
{
    public static class ScrollCurrentItemIntoViewBehavior
    {
        public static readonly DependencyProperty AutoScrollToCurrentItemProperty =
            DependencyProperty.RegisterAttached("AutoScrollToCurrentItem",
                typeof(bool), typeof(ScrollCurrentItemIntoViewBehavior),
                new UIPropertyMetadata(default(bool), OnAutoScrollToCurrentItemChanged));

        public static bool GetAutoScrollToCurrentItem(DependencyObject obj)
        {
            return (bool)obj.GetValue(AutoScrollToCurrentItemProperty);
        }

        public static void OnAutoScrollToCurrentItemChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {
            var listBox = obj as ListBox;
            if (listBox == null) return;

            var newValue = (bool)e.NewValue;
            if (newValue)
                listBox.SelectionChanged += listBoxSelectionChanged;
            else
                listBox.SelectionChanged -= listBoxSelectionChanged;
        }

        static void listBoxSelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            var listBox = sender as ListBox;
            if (listBox == null || listBox.SelectedItem == null || listBox.Items == null) return;

            listBox.Items.MoveCurrentTo(listBox.SelectedItem);
            listBox.ScrollIntoView(listBox.SelectedItem);
        }

        public static void SetAutoScrollToCurrentItem(DependencyObject obj, bool value)
        {
            obj.SetValue(AutoScrollToCurrentItemProperty, value);
        }
    }
}

Usage:

<ListBox ItemsSource="{Binding}"
          IsSynchronizedWithCurrentItem="True"
          behaviors:ScrollCurrentItemIntoViewBehavior.AutoScrollToCurrentItem="True">

Answer 4

If the above code doesn't work for you, give this a try

public class ListBoxExtenders : DependencyObject
{
    public static readonly DependencyProperty AutoScrollToCurrentItemProperty = DependencyProperty.RegisterAttached("AutoScrollToCurrentItem", typeof(bool), typeof(ListBoxExtenders), new UIPropertyMetadata(default(bool), OnAutoScrollToCurrentItemChanged));

    public static bool GetAutoScrollToCurrentItem(DependencyObject obj)
    {
        return (bool)obj.GetValue(AutoScrollToSelectedItemProperty);
    }

    public static void SetAutoScrollToCurrentItem(DependencyObject obj, bool value)
    {
        obj.SetValue(AutoScrollToSelectedItemProperty, value);
    }

    public static void OnAutoScrollToCurrentItemChanged(DependencyObject s, DependencyPropertyChangedEventArgs e)
    {
        var listBox = s as ListBox;
        if (listBox != null)
        {
            var listBoxItems = listBox.Items;
            if (listBoxItems != null)
            {
                var newValue = (bool)e.NewValue;

                var autoScrollToCurrentItemWorker = new EventHandler((s1, e2) => OnAutoScrollToCurrentItem(listBox, listBox.Items.CurrentPosition));

                if (newValue)
                    listBoxItems.CurrentChanged += autoScrollToCurrentItemWorker;
                else
                    listBoxItems.CurrentChanged -= autoScrollToCurrentItemWorker;
            }
        }
    }

    public static void OnAutoScrollToCurrentItem(ListBox listBox, int index)
    {
        if (listBox != null && listBox.Items != null && listBox.Items.Count > index && index >= 0)
            listBox.ScrollIntoView(listBox.Items[index]);
    }

}

Usage in XAML

<ListBox IsSynchronizedWithCurrentItem="True" extenders:ListBoxExtenders.AutoScrollToCurrentItem="True" ..../>