Centering selected item in a scroll viewer

Question

I am trying to center a selected item in a ListView inside a ScrollViewer and struggling to calculate the vertical offset that I should be setting the ScrollViewer relative to the ListView.

The following links set me on the right track, but because of the limitation of the WinRT API, was not able to use them:

• Make ListView.ScrollIntoView Scroll the Item into the Center of the ListView (C#)
• http://blogs.msdn.com/b/delay/archive/2009/04/19/fewer-gotchas-to-getcha-enhancing-the-scrollintoviewcentered-method-for-wpf-s-listbox.aspx

The desired effect is as follows:

This is a sample setup in my XAML:

<ScrollViewer x:Name="MyScrollViewer">
    <ListView x:Name="MyView" VerticalAlignment="Center"
                      SelectionChanged="Selector_OnSelectionChanged">
        <ListView.ItemTemplate>
            <DataTemplate>
                <Grid Width="80" Height="80" Margin="0">
                    <TextBlock Text="{Binding}" />
                </Grid>
            </DataTemplate>
        </ListView.ItemTemplate>
        <ListView.Items>
            <x:String>1</x:String>
            <x:String>2</x:String>
            <x:String>3</x:String>
            <x:String>4</x:String>
            <x:String>5</x:String>
            <x:String>6</x:String>
            <x:String>7</x:String>
            <x:String>8</x:String>
            <x:String>9</x:String>
        </ListView.Items>
    </ListView>
</ScrollViewer>

Knowing the index of the selected item, how do I calculate the vertical offset that I can use in my method:

private void Selector_OnSelectionChanged(object sender, SelectionChangedEventArgs e)
{
    double maxVerticalOffset = MyScrollViewer.ExtentHeight - MyScrollViewer.ViewportHeight;

    int selectedItemIndex = MyView.SelectedIndex;

    double verticalOffset = ...

    MyScrollViewer.ChangeView(null, verticalOffset, null);
}

Toffic

Answer 1

Try ListView.ScrollIntoView() or ListView.MakeVisible first to scroll the container of the item into view and work around it being possibly virtualized out of the UI. Then use ListView.ItemContainerGenerator.ContainerFromIndex() to get the container of the item and then the VisualTreeHelper to get its position relative to the ScrollViewer. Then scroll the scrollviewer by the calculated offset.

*EDIT - Example positioning logic:

Get the VisualTreeHelperExtensions from WinRT XAML Toolkit to get access to the ScrollViewer easily with GetFirstDescendantOfType() extension method that wraps some calls to the VisualTreeHelper.

XAML

<Page
    x:Class="ListViewItemCentering.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:ListViewItemCentering"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid
        Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <ListView
            x:Name="listView">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <Border
                        Width="400"
                        Height="100">
                        <ContentControl
                            Content="{Binding}"
                            FontSize="48"
                            Padding="20,10"/>
                    </Border>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>

        <Button
            Content="Skip"
            Width="200"
            Height="100"
            HorizontalAlignment="Right"
            VerticalAlignment="Bottom"
            Click="ButtonBase_OnClick"/>
    </Grid>
</Page>

C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Windows.Foundation;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using WinRTXamlToolkit.Controls.Extensions;

namespace ListViewItemCentering
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class MainPage : Page
    {
        private Random random = new Random();
        public MainPage()
        {
            this.InitializeComponent();
            this.listView.ItemsSource = Enumerable.Range(1, 1000);
            this.listView.SelectionChanged += OnListViewSelectionChanged;
        }

        private async void OnListViewSelectionChanged(object sender, SelectionChangedEventArgs selectionChangedEventArgs)
        {
            if (listView.SelectedItem == null)
            {
                return;
            }

            var item = listView.SelectedItem;

            // Calculations relative to screen or ListView
            var listViewItem = (FrameworkElement)listView.ContainerFromItem(item);

            if (listViewItem == null)
            {
                listView.ScrollIntoView(item);
            }

            while (listViewItem == null)
            {
                await Task.Delay(1); // wait for scrolling to complete - it takes a moment
                listViewItem = (FrameworkElement)listView.ContainerFromItem(item);
            }

            var topLeft =
                listViewItem
                    .TransformToVisual(listView)
                    .TransformPoint(new Point()).Y;
            var lvih = listViewItem.ActualHeight;
            var lvh = listView.ActualHeight;
            var desiredTopLeft = (lvh - lvih) / 2.0;
            var desiredDelta = topLeft - desiredTopLeft;

            // Calculations relative to the ScrollViewer within the ListView
            var scrollViewer = listView.GetFirstDescendantOfType<ScrollViewer>();
            var currentOffset = scrollViewer.VerticalOffset;
            var desiredOffset = currentOffset + desiredDelta;
            scrollViewer.ScrollToVerticalOffset(desiredOffset);

            // better yet if building for Windows 8.1 to make the scrolling smoother use:
            // scrollViewer.ChangeView(null, desiredOffset, null);
        }

        private async void ButtonBase_OnClick(object sender, RoutedEventArgs e)
        {
            this.listView.SelectedIndex = random.Next(0, ((IEnumerable<int>)this.listView.ItemsSource).Count());
        }
    }
}

Scutari

Are you able to expand on the logic pertaining to using the VisualTreeHelper to position the selected listview item in the middle of the scroll viewer? – Toffic

No luck with the above logic. Given that its a windows store app, I was able to initially get the listViewItem as follows: ListViewItem listViewItem = (ListViewItem)listView.ContainerFromItem(listView.SelectedItem);. However, your calculation which I didn't quite understand its logic is very off from being centered in the middle of the ScrollViewer. Perhaps you can post a tested example? – Toffic

Right, my desiredDelta was the wrong way round. Let me update with a solution. – Scutari

Fantastic. Your sample worked. I think the key mistake that I was making is that I was wrapping my ListView with a ScrollViewer control. I didn't know that the ListView presenter itself contained a ScrollViewer built in that I should be using to map coordinates to and from. – Toffic

@FilipSkakun, I can not find GetFirstDescendantOfType() for ListView. Why ? Thanks for your help. – Cronin

Yes, this is an extension method in VisualTreeHelperExtensions. You can grab the file or just the method or get the entire toolkit from NuGet. – Scutari

If I want to do the same feature on a ListView who has an Horizontal Orientation, what should I change in your code? @FilipSkakun – Truc

Probably Y to X and Height to Width – Scutari

Answer 2

Yup, this worked for me out of the box with ScrollIntoView

    private void CountKeyBoard_NavigationEvent(object sender, Controls.UserControls.NavigationEventArgs e)
    {
        if (lvInvDetails.SelectedItem != null && lvInvDetails.SelectedIndex != -1)
        {
            switch (e.Direction)
            {
                case Controls.UserControls.NavigationEventArgs.DirectionEnum.NEXT:
                    if (lvInvDetails.SelectedIndex >= (lvInvDetails.Items.Count - 1))
                        lvInvDetails.SelectedIndex = 0;
                    else
                        lvInvDetails.SelectedIndex++;
                    break;

                case Controls.UserControls.NavigationEventArgs.DirectionEnum.PREVIOUS:
                    if (lvInvDetails.SelectedIndex > 0)
                        lvInvDetails.SelectedIndex--;
                    else
                        lvInvDetails.SelectedIndex = lvInvDetails.Items.Count - 1;
                    break;
            }
            lvInvDetails.ScrollIntoView(lvInvDetails.SelectedItem);
        }
    }

Anteater