How can I have a ListBox auto-scroll when a new item is added?

Question

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 th

Answer 1

I was not happy with proposed solutions.

• I didn't want to use "leaky" property descriptors.
• I didn't want to add Rx dependency and 8-line query for seemingly trivial task. Neither did I want a constantly running timer.
• I did like shawnpfiore's idea though, so I've built an attached behavior on top of it, which so far works well in my case.

Here is what I ended up with. Maybe it will save somebody some time.

public class AutoScroll : Behavior
{
    public static readonly DependencyProperty ModeProperty = DependencyProperty.Register(
        "Mode", typeof(AutoScrollMode), typeof(AutoScroll), new PropertyMetadata(AutoScrollMode.VerticalWhenInactive));
    public AutoScrollMode Mode
    {
        get => (AutoScrollMode) GetValue(ModeProperty);
        set => SetValue(ModeProperty, value);
    }

    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.Loaded += OnLoaded;
        AssociatedObject.Unloaded += OnUnloaded;
    }

    protected override void OnDetaching()
    {
        Clear();
        AssociatedObject.Loaded -= OnLoaded;
        AssociatedObject.Unloaded -= OnUnloaded;
        base.OnDetaching();
    }

    private static readonly DependencyProperty ItemsCountProperty = DependencyProperty.Register(
        "ItemsCount", typeof(int), typeof(AutoScroll), new PropertyMetadata(0, (s, e) => ((AutoScroll)s).OnCountChanged()));
    private ScrollViewer _scroll;

    private void OnLoaded(object sender, RoutedEventArgs e)
    {
        var binding = new Binding("ItemsSource.Count")
        {
            Source = AssociatedObject,
            Mode = BindingMode.OneWay
        };
        BindingOperations.SetBinding(this, ItemsCountProperty, binding);
        _scroll = AssociatedObject.FindVisualChild() ?? throw new NotSupportedException("ScrollViewer was not found!");
    }

    private void OnUnloaded(object sender, RoutedEventArgs e)
    {
        Clear();
    }

    private void Clear()
    {
        BindingOperations.ClearBinding(this, ItemsCountProperty);
    }

    private void OnCountChanged()
    {
        var mode = Mode;
        if (mode == AutoScrollMode.Vertical)
        {
            _scroll.ScrollToBottom();
        }
        else if (mode == AutoScrollMode.Horizontal)
        {
            _scroll.ScrollToRightEnd();
        }
        else if (mode == AutoScrollMode.VerticalWhenInactive)
        {
            if (_scroll.IsKeyboardFocusWithin) return;
            _scroll.ScrollToBottom();
        }
        else if (mode == AutoScrollMode.HorizontalWhenInactive)
        {
            if (_scroll.IsKeyboardFocusWithin) return;
            _scroll.ScrollToRightEnd();
        }
    }
}

public enum AutoScrollMode
{
    /// 

    /// No auto scroll
    /// 
    Disabled,
    /// 

    /// Automatically scrolls horizontally, but only if items control has no keyboard focus
    /// 
    HorizontalWhenInactive,
    /// 

    /// Automatically scrolls vertically, but only if itmes control has no keyboard focus
    /// 
    VerticalWhenInactive,
    /// 

    /// Automatically scrolls horizontally regardless of where the focus is
    /// 
    Horizontal,
    /// 

    /// Automatically scrolls vertically regardless of where the focus is
    /// 
    Vertical
}