Can I rollback collection changes on collection changed event?

Question

I have 2 list views...and add/remove buttons between them.

On collection changed event of a list-view-collection in viewmodel, can i rollback the changes for a particular condition ?

Answer 1

You could handle the CollectionChanged event of the ObservableCollection to backup (via the VM or whatever) the old values (see NotifyCollectionChangedEventArgs.OldItems property) and get them back when needed i.e. when user clicks 'Undo' etc.

Update In reference to comments bellow:

If you do want to rollback the collection from withing the CollectionChanged event-handler, create a flag where you escape the handler from a recursive call (not tested with multi-threaded application), here is a simple example, you can easily tweak it to fit in your V/VM.

private void Window_Loaded(object sender, RoutedEventArgs e)
{
  var x = new ObservableCollection<string>();
  x.CollectionChanged += 
    new NotifyCollectionChangedEventHandler(x_CollectionChanged);
  x.Add("asdf");
  x.Remove("asdf");
}

bool rollingBack = false;
void x_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
  if (rollingBack) return;

  if (e.Action == NotifyCollectionChangedAction.Remove)
  {
    if (e.OldItems.Contains("asdf"))
    {
      var oc = (ObservableCollection<string>)sender;
      rollingBack = true;
      oc.Add("asdf");
      rollingBack = false;
    }
  }
}

Answer 2

Considering you get the sender of the event as an object (ie. the first parameter of the event) and the list of objects that were modified, yes you can do that. I wouldn't advise that though. If you encounter such a condition, provide a method on the ViewModel which is provided with the EventArgs, and let it do the work. The view isn't the place to do logic.

Even better: check for the condition in the ViewModel itself (ie in the commands that are responseable for adding/removing)! The viewmodel is responseable for the state of the information, so keep your logic there. The view is just there to display the data.

Answer 3

Shimmy's answer didn't work for me on a Windows Store application, you'll still run into re-entrancy issues and get an InvalidOperationException saying "Cannot change ObservableCollection during a CollectionChanged event."

I had to use the UI's dispatcher and disable/enable the event handler to avoid these issues.

Be warned: this is a hack, and the framework designers went to great lengths to prevent you from doings this. So if you want to ignore their warning, be careful not to shoot yourself in the foot.

Items.CollectionChanged += ItemsChanged;

private async void ItemsChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    if(condition)
    {
        //rollback
        await CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(
            CoreDispatcherPriority.Normal, () => {

                //disable/enable event handler
                Items.CollectionChanged -= ItemsChanged;

                Items.Remove(e.NewItems[0]);

                Items.CollectionChanged += ItemsChanged;
            })).AsTask();
    }
}

This will avoid the exception, avoid calling the handler recursively, and update the UI correctly.