IncrementalLoadingBehavior.cs

public class IncrementalLoadingBehavior : Behavior<LongListSelector>
{
    public ICommand LoadCommand { get; set; }

    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.ItemRealized += OnItemRealized;
    }

    private void OnItemRealized(object sender, ItemRealizationEventArgs e)
    {
        var longListSelector = sender as LongListSelector;
        if (longListSelector == null)
        {
            return;
        }

        var item = e.Container.Content;
        var items = longListSelector.ItemsSource;
        var index = items.IndexOf(item);

        if ((items.Count - index <= 1) && (LoadCommand != null) && (LoadCommand.CanExecute(null)))
        {
            LoadCommand.Execute(null);
        }
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        AssociatedObject.ItemRealized -= OnItemRealized;
    }
}