WPF event when adding items to ObservableCollection

1
votes

I'm facing an issue with ObservableCollection CollectionChanged event. I have a MainWindow.xaml file that contains a listView and I use the code behing to focus on my new added (or modified) elements in my listView.

<ListView x:Name="recordListView" Grid.Row="0" Grid.Column="0" Height="Auto" VerticalAlignment="Stretch" HorizontalAlignment="Stretch"
                 ScrollViewer.VerticalScrollBarVisibility="Visible" ScrollViewer.HorizontalScrollBarVisibility="Visible" Margin="20,20,20,10"
                 AlternationCount="2" 
                 ItemsSource="{Binding Path=SessionRecords}" FontSize="14" > 
…
</ListView>

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new MainWindowViewModel();
        }

        private void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            var mainWindowViewModel = (MainWindowViewModel)DataContext;
            mainWindowViewModel.SessionRecords.CollectionChanged += SessionRecords_CollectionChanged;
        }

        private void SessionRecords_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
            if (e.NewItems?[0] != null)
            {
                recordListView.ScrollIntoView(e.NewItems[0]);
            }
        }
    }

You probably notice I use a ViewModel.

    public partial class MainWindowViewModel : BaseViewModel
    {
        private ObservableCollection<Record> sessionRecords;

        public ObservableCollection<Record> SessionRecords
        {
            get
            {
                if (sessionRecords == null)
                {
                    sessionRecords = new ObservableCollection<Record>();
                    sessionRecords.CollectionChanged += new NotifyCollectionChangedEventHandler(SessionRecordsCollectionChangedMethod);
                }
                return sessionRecords;
            }
        }
    }

On runtime when I add a new item to my observable collection the collection changed event is raised before the item appears on the screen. How can I make sure the event is raised after the element appears in my screen? Or what should I do to make sure my grid always scroll and focus en new added or existing modified item? With MVVM or not, I don't care.

2
c#wpfevent-handling
e can't be null, so e?. is redundant. There is also e.Action property that you might be interested in. - vasily.sib
Be aware that there is no guarantee that the approach from the answer you've accepted always works. There may theoretically be an arbitrarily long timespan between updating the source collection and a UI update. The UI may even queue multiple updates. All theoretically of course. The only safe way to get notified about an updated view is to handle some event in the view, e.g. the Loaded event of an element in the ListView's ItemTemplate. - Clemens

2 Answers

2
votes

Try this. It will execute only after the UI update which has a higher priority.

Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Input, new Action(() =>
{
    recordListView.ScrollIntoView(e.NewItems[0]);
}));

More info about Dispatcher.BeginInvoke

Example:

        private void SessionRecords_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
            if (e.NewItems?[0] != null)
            {
                Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Input, new Action(() =>
                {
                    recordListView.ScrollIntoView(e.NewItems[0]);
                }));
            };
        }
1
votes

You could also use async/await and do something like this, it should also help avoiding excessively redundant scrolling (error handling skipped for brevity):

    private bool _pendingScroll = false;

    private async void SessionRecords_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        if (_pendingScroll) 
            return;

        _pendingScroll = true;
        try 
        {
            await System.Windows.Threading.Dispatcher.Yield(DispatcherPriority.ApplicationIdle);
            if (e.NewItems?[0] != null)
            {
                recordListView.ScrollIntoView(e.NewItems[0]);
            }
        }
        finally 
        {
            _pendingScroll = false;                        
        }
    }