Loading Screen Animation Stutter - wpf

Question

I use a background worker to load a very large set of items while displaying a loading animation. On the run worker completed event, I set all those loaded items to an observable collection and then toggle off the loading screen. The problem is the progress bar is animating smoothly until the items are set to the observable collection, then it just stops animating. I imagine this is because the rendering of those items interfere with the animation. Is there any way to make the animation smooth during the rendering phase? When running the application the times I get are:
Loading = 1000 ms
Set properties = 43 ms
Rendering = 5083 ms
The rendering improves to 19 ms when I turn virtualizing on. I leave it turned off to demonstrate a scenario of long rendering times.
public ObservableCollection<string> Items { get; set; }
private IEnumerable<string> _items;
private BackgroundWorker _worker;
private long _loadTime;
private long _renderTime;
private long _setPropertiesTime;
public MainWindow()
{
InitializeComponent();
Items = new ObservableCollection<string>();
_itemsGrid.DataContext = this;
_worker = new BackgroundWorker();
_worker.DoWork += DoWork;
_worker.RunWorkerCompleted += RunWorkerCompleted;
}
private void Button_Click_1(object sender, RoutedEventArgs e)
{
_renderLabel.Content = string.Empty;
_loadLabel.Content = string.Empty;
_loadingBorder.Visibility = Visibility.Visible;
_worker.RunWorkerAsync();
}
void DoWork(object sender, DoWorkEventArgs e)
{
_items = LoadItems();
}
void RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
var setItemsTimer = Stopwatch.StartNew();
Items.Clear();
foreach (var item in _items)
{
Items.Add(item);
}
setItemsTimer.Stop();
_setPropertiesTime = setItemsTimer.ElapsedMilliseconds;
var timer = Stopwatch.StartNew();
Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Loaded, new Action(() =>
{
timer.Stop();
_loadLabel.Content = string.Format("Loading took {0} ms, Set properties took {1} ms", _loadTime, _setPropertiesTime);
_renderLabel.Content = string.Format("Rendering took {0} ms", timer.ElapsedMilliseconds);
_loadingBorder.Visibility = Visibility.Collapsed;
}));
}
IEnumerable<string> LoadItems()
{
var timer = Stopwatch.StartNew();
var items = new List<string>();
for (int i = 0; i < 5000; i++)
{
items.Add("Testing");
}
Thread.Sleep(1000);
timer.Stop();
_loadTime = timer.ElapsedMilliseconds;
return items;
}

Answer 1

One way is to create an overlay Window on top of your current Window running under a different UI thread and have the loading animation displayed in this Window and set the appropriate properties to make the Window look part of the loading Window.
This is in essence a splash screen.