Programmatically scrolling a CollectionView to end causes exception "Invalid Target Position"

Question

The above header refers.

I am trying to implement a chatting app. There are no compilation errors but whenever it starts running and gets to this line:

MessageList.ScrollTo(viewModel.Messages.Last(), null, ScrollToPosition.End, true);

it triggers this error:

"Invalid target position".

This is the stack trace:

"Ex = {Java.Lang.IllegalArgumentException: Invalid target position at Java.Interop.JniEnvironment+InstanceMethods.CallVoidMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniMethodInfo method, Java.Interop.JniArgumentValue* args) [0x00068] in /Use..."

This is the ChatView.xaml file where MessageList is defined:

This is the ChatViewModel.cs file:

using Acr.UserDialogs;
using Chat.Services;
using Chat.Messages;
using Plugin.Media;
using Plugin.Media.Abstractions;
using System;
using System.Collections.ObjectModel;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using System.Windows.Input;
using Xamarin.Forms;

namespace Chat.ViewModels
{
    public class ChatViewModel : BaseViewModel
    {
        private readonly IChatService chatService;
        public ObservableCollection Messages { get; private set; }

        public ChatViewModel(IChatService chatService)
        {
            this.chatService = chatService;

            Messages = new ObservableCollection();

            chatService.NewMessage += ChatService_NewMessage;

            Task.Run(async () =>
            {
                if (!chatService.IsConnected)
                {
                    await chatService.CreateConnection();
                }

                await chatService.SendMessage(new UserConnectedMessage(User));
            });
        }

        private string text;
        public string Text
        {
            get => text;
            set => Set(ref text, value);
        }

        public ICommand Send => new Command(async () =>
        {
            var message = new SimpleTextMessage(User)
            {
                Text = this.Text
            };

            Messages.Add(new LocalSimpleTextMessage(message));

            await chatService.SendMessage(message);

            Text = string.Empty;
        });

        public ICommand Photo => new Command(async () =>
        {
            var options = new PickMediaOptions
            {
                CompressionQuality = 50
            };

            var photo = await CrossMedia.Current.PickPhotoAsync();

            UserDialogs.Instance.ShowLoading("Uploading photo");

            var stream = photo.GetStream();
            var bytes = ReadFully(stream);

            var base64photo = Convert.ToBase64String(bytes);

            var message = new PhotoMessage(User)
            {
                Base64Photo = base64photo,
                FileEnding = photo.Path.Split('.').Last()
            };

            Messages.Add(message);
            await chatService.SendMessage(message);

            UserDialogs.Instance.HideLoading();
        });

        private void ChatService_NewMessage(object sender, Events.NewMessageEventArgs e)
        {
            Device.BeginInvokeOnMainThread(() =>
            {
                if (!Messages.Any(x => x.Id == e.Message.Id))
                {
                    Messages.Add(e.Message);
                }
            });
        }

        private byte[] ReadFully(Stream input)
        {
            using (MemoryStream ms = new MemoryStream())
            {
                ms.CopyTo(input);

                return ms.ToArray();
            }
        }

        private string username;

        public string Username { get => username; set => Set(ref username, value); }

        private ImageSource url;

        public ImageSource Url { get => url; set => Set(ref url, value); }

        private ImageSource base64Photo;

        public ImageSource Base64Photo { get => base64Photo; set => Set(ref base64Photo, value); }
    }
}

This is the ChatView.xaml.cs file:

using Chat.ViewModels;
using System;
using System.Linq;
using Xamarin.Forms;
using Xamarin.Forms.PlatformConfiguration.iOSSpecific;
using Xamarin.Forms.Xaml;

namespace Chat.Views
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class ChatView : ContentPage
    {
        private ChatViewModel viewModel;

        public ChatView(ChatViewModel viewModel)
        {
            this.viewModel = viewModel;

            InitializeComponent();

            On().SetUseSafeArea(true);

            viewModel.Messages.CollectionChanged += Messages_CollectionChanged;
            BindingContext = viewModel; 
        }

        protected override void OnAppearing()
        {
            base.OnAppearing();
            var safeArea = On().SafeAreaInsets();
            MainGrid.HeightRequest = this.Height - safeArea.Top - safeArea.Bottom;
        }

        private void Messages_CollectionChanged(object sender,
            System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
            try
            {
                int Kounter = viewModel.Messages.Count;

                //if (Kounter == 1)
                //{
                    //MessageList.ScrollTo(viewModel.Messages.First(), null, ScrollToPosition.Start, true);
                //}                    
                //else
                //{
                    MessageList.ScrollTo(viewModel.Messages.Last(), null, ScrollToPosition.End, true);
                //}
                
            }
            catch (Exception Ex)
            {
                Console.WriteLine(Ex);
            }            
        }
    }
}

I am using Xamarin.forms version 5.0.0.2515 on Visual Studio version 17.3.

So, what is the issue and how do I sort it out?

ToolmakerSteve · Accepted Answer

Instead of manually scrolling, set CollectionView's ItemsUpdatingScrollMode property.

From Control scroll position when new items are added:

Programmatically scrolling a CollectionView to end causes exception "Invalid Target Position"

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Programmatically scrolling a CollectionView to end causes exception &quot;Invalid Target Position&quot;

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Programmatically scrolling a CollectionView to end causes exception "Invalid Target Position"