ComboBox selected item does not get set until SelectionChangedCommand's execute is finished

Question

I used this article to set up a SelectionChangedCommand that I can bind to from my view model.

Here's my view:

<ComboBox Height="20" 
          SelectedItem="{Binding SelectedName}" 
          ItemsSource="{Binding NameCollection}"
          commandBehaviors:SelectionChangedBehavior.Command="{Binding SelectionChangedCommand}">

In my view model's SelectionChangedCommand Executed method:

 private void SelectionChangedExecuted()
 {
   // The logic in this method can take up to 30 seconds at times.
 }

Now my problem is that when the user selects a new name from my comboBox, it can take up to 30 seconds until they see that their name is selected. It'll show the old name, until the SelectionChangedExecuted method finishes. Ideally, when they select a name, I want that name to show up immediately, THEN they can wait 30 seconds. Can I accomplish this with my current set up?

Current Behavior:
-Current Item in ComboBox: "Bob"
-User selects: "Steve"
-User waits 30 seconds, while Current Item in ComboxBox is still "Bob"
-30 seconds is over, Current Item in ComboBox: "Steve"

Wanted Behavior:
-Current Item in ComboBox: "Bob"
-User selects: "Steve", and Current Item in ComboBox is "Steve"
-User waits 30 seconds, while Current Item in ComboxBox is still "Steve"
-30 seconds is over, Current Item in ComboBox is still: "Steve"

Answer 1

The reason is that you're executing your SelectionChangedExecuted() method synchronously on the UI thread. You should do this asynchronously using async / await to execute your code on a background worker thread.

private static async void SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    Selector selector = (Selector)sender;
    if (selector != null)
    {
         ICommand command = selector.GetValue(CommandProperty) as ICommand;
         if (command != null)
         {
             await Task.Run(() => command.Execute(selector.SelectedItem));
         }
    }
}