// Emulates a long items getting process using some delay of getting of each item
public static class AsyncDataProvider
{    
    private const int _DefaultDelayTime = 300;
 
    public static ReadOnlyCollection<string> GetItems()
    {
        return GetItems(_DefaultDelayTime);
    }
 
    public static ReadOnlyCollection<string> GetItems(int delayTime)
    {            
        List<string> items = new List<string>();            
        foreach (string item in Enum.GetNames(typeof(AttributeTargets)).OrderBy(item => item.ToLower()))
        {
            items.Add(item);
            // Syntetic delay to emulate a long items getting process
            Thread.Sleep(delayTime);
        }
 
        return items.AsReadOnly();
    }        
}

<Window.Resources>
    <ObjectDataProvider x:Key="AsyncDataSource" 
        IsAsynchronous="True"
        ObjectType="Providers:AsyncDataProvider" 
        MethodName="GetItems" />
    <Converters:NullToBooleanConverter x:Key="NullToBooleanConverter" />
</Window.Resources>

<ListBox x:Name="ItemsListBox" 
            ItemsSource="{Binding Source={StaticResource AsyncDataSource}, IsAsync=True}">        
    <ListBox.Style>
        <Style TargetType="{x:Type ListBox}">
            <Style.Triggers>
                <Trigger Property="ItemsSource" Value="{x:Null}">
                    <Setter Property="Template" Value="{StaticResource WaitControlTemplate}" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </ListBox.Style>
</ListBox>