ItemsControl.ItemsSource MVVM performance

Question

I have an (non-virtualized) ItemsControl that binds its ItemsSource to a ObeservableCollection of ViewModel instances. Now once the large amount Model instances is loaded all the ViewModel complemnents needs to be added to that ObservableCollection. How can I add a large amount of ViewModels without making the UI Thread hang?

I suppose the UI Thread hangs because each time a new item is added the ItemsControl needs to update itself and does layout etc. over and over again.

• Should I suspend the binding add all items and then resume? If so, how?
• Should I override the ObservableCollection to implement an AddRange so only 1 CollectionChanged Event is fired for adding multiple items? Or alternatively just replace the whole collection?
• Or is it better to add each items separately and call Dispatcher.Invoke for each item separately? So I would unblock frequently.

How do you handle large dynamic lists that can not be virtualized?

Answer 1

You can create a a class derived from ObservableCollection which allows you to temporarily suspend CollectionChanged events like this:

public class SuspendableObservableCollection : ObservableCollection
{
    private bool suspended;

    public bool Suspended 
    {
        get
        {
            return this.suspended;
        }
        set
        {
            this.suspended = value;
            OnCollectionChanged(new NotifyCollectionChangedEventArgs(
                NotifyCollectionChangedAction.Reset));
        }
    }

    protected override void OnCollectionChanged(
        NotifyCollectionChangedEventArgs args)
    {
       if (!Suspended)
       {
           base.OnCollectionChanged(args);
       }
    }
}

Answer 2

<ItemsControl IsAsync="True" ... />