Need More Info
Last Updated: 22 May 2020 11:47 by ADMIN

Hi guys,

we have a messaging service that broadcasts a couple of messages every 1-5 seconds.

When our client module receives those messages, we want to append them at the bottom of a grid (RadGridView).

After appending them, we also want to scroll to the very bottom of that grid, so that the newest and therefore bottommost items come into view.

 

Documentation and forums suggest we go the AttachedBahaviour and ScrollIntoViewAsync way.

 

Our behaviour looks like this:


    public class ScrollToNewItemBehavior : Behavior<RadGridView>
    { 
        public static bool GetIsEnabled(DependencyObject obj) => (bool)obj.GetValue(IsEnabledProperty);
        public static void SetIsEnabled(DependencyObject obj, bool value) => obj.SetValue(IsEnabledProperty, value);
        public static readonly DependencyProperty IsEnabledProperty = DependencyProperty.RegisterAttached("IsEnabled", typeof(bool), typeof(ScrollToNewItemBehavior), new PropertyMetadata(false, OnIsEnabledChanged));
        private static void OnIsEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is RadGridView gridView)
                gridView.Items.CollectionChanged += (s, args) =>
                {
                    if (args.Action == NotifyCollectionChangedAction.Add)
                    {
                        gridView.ScrollIntoViewAsync(args.NewItems[0], gridView.Columns[0], null);

                        // exchanging args.NewItems[0] for gridView.Items[gridView.Items.Count-1] yields same result
                        //gridView.ScrollIntoViewAsync(gridView.Items[gridView.Items.Count-1], gridView.Columns[0], null);
                    }
                };
        } 
    }


 

Our Message class:


    public class Message : ModelBase<Message>
    {
        public string Text { get; set; }
    }


Our Messages collection in the ViewModel:


        private RadObservableCollection<Message> _messages;
        public RadObservableCollection<Message> Messages
        {
            get => _messages;
            set
            {
                _messages = value;
                NotifyPropertyChanged(m => m.Messages);
            }
        }


The collection is updated (in the ViewModel) like this:


            Task.Run(async () =>
            {
                for (int i = 0; ; i++)
                {
                    await Task.Delay(1000);
                    Messages.Add(new Message { Text = $"{i} - sftrvwj,erhvtwejhrfvtjlwehftrwejh" });
                }
            });


 

The grid is defined like this:


    <telerik:RadGridView
        x:Name="gridView"
        ItemsSource="{Binding Messages, Mode=OneWay}"
        IsSynchronizedWithCurrentItem="False"
        IsPropertyChangedAggregationEnabled="True"
        AutoGenerateColumns="False" 
        SelectionMode="Single"
        CanUserFreezeColumns="False"
        EnableColumnVirtualization="True"
        EnableRowVirtualization="True"
        CanUserDeleteRows="False"
        CanUserInsertRows="False"
        behaviours:ScrollToNewItemBehavior.IsEnabled="True"
        CanUserGroupColumns="False"
        IsReadOnly="True"
        IsManipulationEnabled="False"
        CanUserReorderColumns="False"
        CanUserSearch="False"
        ShowGroupPanel="False">
        <telerik:RadGridView.Columns>
            <telerik:GridViewDataColumn
                DataMemberBinding="{Binding Text}"
                Header="Text"
                ShowDistinctFilters="False"
                IsSortable="False"/>
        </telerik:RadGridView.Columns>
    </telerik:RadGridView>


Nothing too fancy.

 

What we observe:


 

When a message is added, the view and the scroll indicator randomly jump to the top or the bottom of the grid and stay there until the next message is added.

We tried AddRange, Suspend-/ResumeNotifications, ObservableCollection instead of RadObservableCollection.

We disabled many grid features.

We tried .NetCore

,

we tried .net Framework

To no avail.

This seems to be a bug.

 

Do you know any workarounds or a completely different approach to achieve the desired behaviour?

 

 

Side note:

When we set GroupRenderingMode to Flat,

        GroupRenderMode="Flat"

the view stays at the bottom. The scroll indicator stays at the bottom as well, but once in a while jumps a little bit up, as if by one row, and down to the bottom again without the view changing.

At one time disabling filtering on all columns seemd to work... but later didn't.

 

 

Thanks in advance

Thorsten