Infinite scroll collection view don't work.

I am trying to load data incrementally but the RemainingItemsThresholdReachedCommand is triggered one time, then again and again, continually, until all items are loaded in the Source Item Collection View. Then nothing happens on the ui screen.

I want him triggered only when I have 2 item not showing on my screen according to : ItemTreshold = 2;

Here the doc related to this : https://docs.microsoft.com/fr-fr/xamarin/xamarin-forms/user-interface/collectionview/populate-data

here my collection view page :

`<ContentPage.Content>
<StackLayout BackgroundColor="#b4b4b4">
    <RefreshView Command="{Binding RefreshItemsCommand}" IsRefreshing="{Binding IsRefreshing}" >
        <CollectionView x:Name="CollectionViewThumbnails" SelectionMode="Multiple" Margin="13" ItemsSource="{Binding Items}" 
                          RemainingItemsThresholdReachedCommand="{Binding ItemTresholdReachedCommand}"
                          RemainingItemsThreshold="{Binding ItemTreshold}">
            <CollectionView.ItemsLayout>
                <GridItemsLayout Orientation="Vertical" Span="1" VerticalItemSpacing="13" HorizontalItemSpacing="13"/>
            </CollectionView.ItemsLayout>
            <CollectionView.ItemTemplate>
                <DataTemplate x:Name="data">
                    <Frame >
                        <StackLayout>
                            <Image Source="{Binding EmplacementThumbnails}" Aspect="AspectFill"/>
                        </StackLayout>
                    </Frame>
                </DataTemplate>
            </CollectionView.ItemTemplate>
        </CollectionView>
    </RefreshView>
    <ActivityIndicator IsRunning="{Binding IsBusy}"
                   HeightRequest="30"
                   HorizontalOptions="Center"
                   VerticalOptions="Center"
                   WidthRequest="30"/>
    <StackLayout>
        <Button Text="Go" Clicked="StartProcessConcatePages"/>
    </StackLayout>
</StackLayout>

</ContentPage.Content>`

My code behind :

` public partial class ConcatePageThumbnails : ContentPage
{
FileEndpoint fileEndpoint = new FileEndpoint();
FileInfo fileInfo;
BitmapDataStore viewModel;

protected async override void OnAppearing()
{
    base.OnAppearing();

    if (viewModel.Items.Count == 0)
        viewModel.LoadItemsCommand.Execute(null);

    MessagingCenter.Subscribe<object, ThumbnailsModel>(this, BitmapDataStore.ScrollToPreviousLastItem, (sender, item) =>
    {
        CollectionViewThumbnails.ScrollTo(item, ScrollToPosition.End);
    });
}

public ConcatePageThumbnails(FileInfo fileInfo)
  {
    InitializeComponent();

    this.fileInfo = fileInfo;
    BindingContext = viewModel = new BitmapDataStore(fileInfo);

    }

}`

And my view model :

`public class BitmapDataStore
{
IGetThumbnails getThumbnails;
FileInfo fileInfo;
public List<ThumbnailsModel> Items { get; set; }
public Command LoadItemsCommand { get; set; }
public Command ItemTresholdReachedCommand { get; set; }
public Command RefreshItemsCommand { get; set; }
public const string ScrollToPreviousLastItem = "Scroll_ToPrevious";
private int _itemTreshold;
private bool _isRefreshing;
bool isBusy = false;
public bool IsBusy
{
    get { return isBusy; }
    set { isBusy = value; }
}
public bool IsRefreshing
{
    get { return _isRefreshing; }
    set { _isRefreshing = value; }
}

public int ItemTreshold
{
    get { return _itemTreshold; }
    set { _itemTreshold = value; }
}

public BitmapDataStore(FileInfo fileInfo)
{
    ItemTreshold = 2;
    Items = new List<ThumbnailsModel>();
    this.fileInfo = fileInfo;
    getThumbnails = DependencyService.Get<IGetThumbnails>();

    LoadItemsCommand = new Command(async () => await ExecuteLoadItemsCommand());
    ItemTresholdReachedCommand = new Command(async () => await ItemsTresholdReached());
    RefreshItemsCommand = new Command(async () =>
    {
        await ExecuteLoadItemsCommand();
        IsRefreshing = false;
    });
}

async Task ItemsTresholdReached()
{
    if (IsBusy)
        return;

    IsBusy = true;

    try
    {
        var items = await getThumbnails.GetBitmaps(fileInfo.FullName, Items.Count);

        var previousLastItem = Items.Last();
        foreach (var item in items)
        {
            Items.Add(item);
        }
        Debug.WriteLine($"{items.Count()} {Items.Count} ");
        if (items.Count() == 0)
        {
            ItemTreshold = -1;
            return;
        }
        MessagingCenter.Send<object, ThumbnailsModel>(this, ScrollToPreviousLastItem, previousLastItem);
    }
    catch (Exception ex)
    {
        Debug.WriteLine(ex);
    }
    finally
    {
        IsBusy = false;
    }
}

async Task ExecuteLoadItemsCommand()
{
    if (IsBusy)
        return;

    IsBusy = true;

    try
    {
        ItemTreshold = 2;
        Items.Clear();
        var items = await getThumbnails.GetBitmaps(fileInfo.FullName, Items.Count);
        foreach (var item in items)
        {
            Items.Add(item);
        }
    }
    catch (Exception ex)
    {
        Debug.WriteLine(ex);
    }
    finally
    {
        IsBusy = false;
    }
}

}`

My Dependencyservice class where I get the thumbnails and The items for my Collection view :

`public async Task<List<ThumbnailsModel>> GetBitmaps(string filePath, int lastIndex = 0)
 {
    var sw = new Stopwatch();
    sw.Start();

    int numberOfItemsPerPage = 6;
    PdfRenderer pdfRenderer = new PdfRenderer(GetSeekableFileDescriptor(filePath));

    var appDirectory = System.Environment.GetFolderPath(System.Environment.SpecialFolder.MyDocuments);
    string fileName = System.IO.Path.GetFileNameWithoutExtension(filePath);
    string directoryPath = System.IO.Path.Combine(appDirectory, "thumbnailsTemp", System.IO.Path.GetFileNameWithoutExtension(fileName));
    List<ThumbnailsModel> thumbnailsModels = new List<ThumbnailsModel>();

    if (!Directory.Exists(directoryPath))
    {
        Directory.CreateDirectory(directoryPath);
    }
        //int pageCount = pdfRenderer.PageCount;

        for (int i = lastIndex; i < lastIndex + numberOfItemsPerPage; i++)
        {
            PdfRenderer.Page page = pdfRenderer.OpenPage(i);
            Android.Graphics.Bitmap bmp = Android.Graphics.Bitmap.CreateBitmap(page.Width, page.Height, Android.Graphics.Bitmap.Config.Argb4444);
            page.Render(bmp, null, null, PdfRenderMode.ForDisplay);

            try
            {
                using (FileStream output = new FileStream(System.IO.Path.Combine(directoryPath, fileName + "Thumbnails" + i + ".png"), FileMode.Create))
                {
                    bmp.Compress(Android.Graphics.Bitmap.CompressFormat.Png, 0, output);
                }

                page.Close();
            }
            catch (Exception ex)
            {
                //TODO -- GERER CETTE EXPEXPTION
                throw new Exception();
            }
        }



    int y = 1;
    Directory.GetFiles(directoryPath).ToList<string>().Skip(lastIndex).Take(numberOfItemsPerPage).ForEach(delegate (string thumbnailsEmplacement)
    {
        thumbnailsModels.Add(new ThumbnailsModel(y, thumbnailsEmplacement));
        y++;
    });

    sw.Stop();





    return await Task.FromResult(thumbnailsModels);
}`