Replacing a ListView's ItemsSource Without Loosing SelectedItem

Question

Hello,

I'm data-binding a ListView to a collection that comes from a service layer. In response to events, the view model associated with the ListView refreshes the ListView's data. In order to retrieve updated data, the vm retrieves a new collection instance from the service layer. Items in this collection are Equals() but not ReferenceEquals() to the equivalent items in the previously-returned collection.

Can you recommend an approach that avoids the downsides of the approaches below while still allowing the vm to plug this data into the ListView without causing the ListView to lose its SelectedItem?

Thank you,
Ben

---

A simple approach (view model code; ListView's SelectedItem & ItemsSource are bound to identically-bound properties on the vm):

var selectedItem = SelectedItem;
ItemsSource = service.GetData();
SelectedItem = Accounts.SingleOrDefault(x => x.Equals(selectedItem));

This approach seems ugly. It also involves SelectedItem being reset--a potential problem if changing the SelectedItem changes the detail edit from on a master-detail form. (The setting of ItemsSource results in SelectedItem being cleared which is why it's re-set on the last line.)

Another approach is writing an adapter that loads an ObservableCollection<> with proxy objects pointing to the data returned from the initial service layer call. Any time updated data is retrieved from the service layer, the proxy objects are updated to point to the newly-retrieved objects. This way, the ListView's ItemsSource does not need to be reset to update the ListView (it stays bound to the same ObservableCollection<>) which means that SelectedItem won't be reset. A downside to this approach is the amount of code involved.

Answer 1

hmm... how about that:

Register event handlers for SelectionChanged and DataContextChanged on the ListView:

<ListView Name="listView" SelectionChanged="listView_SelectionChanged" DataContextChanged="listView_DataContextChanged" />

In the SelectionChanged handler save the SelectedItem in the Tag of the ListView

private void listView_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    (sender as ListView).Tag = (sender as ListView).SelectedItem;
}

Once the DataContext for the ListView changed, evaluate the last SelectedItem that was saved in the Tag against the new collection and set the new item from that collection:

    private void listView_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
    {
        List<CustomClass> newList = e.NewValue as List<CustomClass>;
        if (newList != null && (sender as ListView).Tag != null)
        {
            foreach (CustomClass cClass in newList)
                if (cClass.Equals((sender as ListView).Tag as CustomClass))
                    (sender as ListView).SelectedItem = cClass;
        }
    }

Answer 2

I ended up writing mapper code that loaded an ObservableCollection with proxy objects pointing to the real objects retrieved from the service layer. Each time an update was fetched from the service layer, the mapper code would add/remove/update the proxy objects, as appropriate.