LongListSelector and DataTemplateSelector

Question

I\'m using the LongListSelector to realize List or Grid display for my items. For this, I created a DataTemplateSelector and I change the LayoutMode property at runtime. This is

Answer 1

I've done something similar with my app, but allowed the user to choose the LayoutMode of LLS using an Appbar button. I basically change the LongListSelector.LayoutMode and then it's ItemTemplate in code and the LLS automatically refreshes itself. I'm not sure if this will help, but here's my code.

private void layoutModeButton_Click(object sender, EventArgs e)
    {
        ApplicationBarIconButton layoutModeButton = (ApplicationBarIconButton)ApplicationBar.Buttons[0];

        if (MainLongListSelector.LayoutMode == LongListSelectorLayoutMode.Grid)
        {
            MainLongListSelector.LayoutMode = LongListSelectorLayoutMode.List;
            MainLongListSelector.ItemTemplate = this.Resources["ListListLayout"] as DataTemplate;
            layoutModeButton.IconUri = _gridButtonUri;
            layoutModeButton.Text = "grid";
        }
        else
        {
            MainLongListSelector.LayoutMode = LongListSelectorLayoutMode.Grid;
            MainLongListSelector.ItemTemplate = this.Resources["GridListLayout"] as DataTemplate;
            layoutModeButton.IconUri = _listButtonUri;
            layoutModeButton.Text = "list";
        }
    }

You might have figured out the answer already, but just to add to the conversation: this gives me really good performance for a fairly large amount of data. Maybe you can do something similar when navigating back to the page after changing the layout in settings?

Answer 2

I haven't got the solution but maybe a clue for someone who will solve the problem.
I think the problem is with LongListSelector.UpdateLayout() method - when it's fired for the first time there are no items to which LLS was bound - OnChangeMethod is called that many times as the Itemsource.Count. But when we leave the page and go back - LLS is Updated and method is called ommiting the middle element.
It means it works for even number of items - OnChangeMethod is called correct number of times, But for odd number of items - it's called numer of items - 1.
The second thing is why it's called at all - when there are no changes.

I also add a code to work on which (very simple).

Answer 3

Here is one walk around. (Maybe the problem will be corrected with WP 8.1 Update, along with others I've spotted working with LLS. I know - this idea is ugly, hard and so on, but maybe it will be enough for your purpose:

Becouse the problem concerns 'reloading' LLS, I've forced it to initialize it every time I navigate to the page (in fact I need to initialize the whole Page - it won't work only with LLS). I've moved InitializeComponent() and buttons events and so on to OnNavigatedTo():

 protected override void OnNavigatedTo(NavigationEventArgs e)
  {
     base.OnNavigatedTo(e);

     this._contentLoaded = false;
     InitializeComponent();

     first.Click += first_Click;
     second.Click += second_Click;
     ItemLongListSelector.ItemsSource = Items;
  }

At least OnContentChanged() is fired that many Times its needed. Code you can find here.