Auto load more messages when scrolling up

src/Decho/ViewModels/ChannelViewModel.cs

@@ -111,4 +111,20 @@ public sealed class ChannelViewModel(ChannelModel model) : ViewModelBase
         Model.Messages.Add(message);
         Messages.Add(new MessageViewModel(message));
     }
+
+    public void InsertMessages(IList<MessageModel> olderMessages)
+    {
+        HashSet<string> existingIds = Messages.Select(m => m.Model.Id).ToHashSet();
+        List<MessageModel> fresh = olderMessages.Where(m => !existingIds.Contains(m.Id)).ToList();
+        if (fresh.Count == 0)
+        {
+            return;
+        }
+
+        foreach (MessageModel msg in fresh)
+        {
+            Model.Messages.Insert(0, msg);
+            Messages.Insert(0, new MessageViewModel(msg));
+        }
+    }
 }

src/Decho/ViewModels/ChatViewModel.cs

@@ -7,6 +7,8 @@ namespace Decho.ViewModels;
 
 public sealed class ChatViewModel : ViewModelBase
 {
+    public Action? LoadMoreRequested;
+
     public ObservableCollection<MessageViewModel> Messages
     {
         get;
@@ -57,6 +59,17 @@ public sealed class ChatViewModel : ViewModelBase
 
     public string CurrentChannelName { get; private set; } = string.Empty;
 
+    public bool IsLoadingMore
+    {
+        get;
+        set
+        {
+            if (field == value) return;
+            field = value;
+            this.RaisePropertyChanged();
+        }
+    }
+
     public ChatViewModel()
     {
         Composer = new MessageComposerViewModel();

src/Decho/ViewModels/MainWindowViewModel.cs

@@ -57,6 +57,7 @@ public sealed class MainWindowViewModel : ViewModelBase
         Chat.Composer.SendRequested += HandleSendRequested;
         Chat.Composer.CommandRequested += HandleCommandAsync;
         Chat.Composer.FileUploadRequested += HandleFileUploadRequested;
+        Chat.LoadMoreRequested += HandleLoadMoreRequested;
 
         WireCommandHandlerEvents();
         WireConnectionServiceEvents();
@@ -879,6 +880,43 @@ public sealed class MainWindowViewModel : ViewModelBase
         }
     }
 
+    private async void HandleLoadMoreRequested()
+    {
+        string serverUrl = Chat.CurrentServerUrl;
+        string channelName = Chat.CurrentChannelName;
+        if (string.IsNullOrEmpty(serverUrl) || string.IsNullOrEmpty(channelName))
+        {
+            return;
+        }
+
+        ChannelViewModel? channel = FindChannelViewModel(serverUrl, channelName);
+        if (channel is null)
+        {
+            return;
+        }
+
+        Chat.IsLoadingMore = true;
+
+        try
+        {
+            int offset = channel.Messages.Count;
+            List<MessageModel> older = await ConnectionService.GetHistoryAsync(serverUrl, channelName, HubConstants.DefaultHistoryCount, offset);
+
+            if (older.Count > 0)
+            {
+                channel.InsertMessages(older);
+            }
+        }
+        catch
+        {
+            // silently ignore
+        }
+        finally
+        {
+            Chat.IsLoadingMore = false;
+        }
+    }
+
     private void HandleChannelSelected(ChannelViewModel? channel)
     {
         if (channel is null)

src/Decho/Views/MessageListView.axaml.cs

@@ -1,3 +1,4 @@
+using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Threading;
 
@@ -12,6 +13,8 @@ public partial class MessageListView : UserControl
     private ChatViewModel? _chat;
     private bool _hasPendingScroll;
     private bool _wasAtBottom;
+    private bool _loadingMore;
+    private double _loadingMoreExtent;
 
     public MessageListView()
     {
@@ -25,6 +28,7 @@ public partial class MessageListView : UserControl
         {
             _chat.Messages.CollectionChanged -= OnMessagesChanged;
             _chat.PropertyChanged -= OnViewModelPropertyChanged;
+            _chat.LoadMoreRequested -= OnLoadMoreRequested;
         }
 
         ScrollViewer? scroll = this.FindControl<ScrollViewer>("MessageScrollViewer");
@@ -35,10 +39,20 @@ public partial class MessageListView : UserControl
         {
             _chat.Messages.CollectionChanged += OnMessagesChanged;
             _chat.PropertyChanged += OnViewModelPropertyChanged;
+            _chat.LoadMoreRequested += OnLoadMoreRequested;
             scroll?.ScrollChanged += OnScrollChanged;
         }
     }
 
+    private void OnLoadMoreRequested()
+    {
+        ScrollViewer? scroll = this.FindControl<ScrollViewer>("MessageScrollViewer");
+        if (scroll is null) return;
+
+        _loadingMore = true;
+        _loadingMoreExtent = scroll.Extent.Height;
+    }
+
     private void OnScrollChanged(object? sender, ScrollChangedEventArgs e)
     {
         if (e.ExtentDelta.Y > 0 && _wasAtBottom)
@@ -48,6 +62,14 @@ public partial class MessageListView : UserControl
         }
 
         ScrollViewer scrollViewer = (ScrollViewer)sender!;
+
+        if (!_loadingMore && _chat is not null && !_chat.IsLoadingMore
+            && scrollViewer.Offset.Y <= 0
+            && scrollViewer.Extent.Height > scrollViewer.Viewport.Height)
+        {
+            _chat.LoadMoreRequested?.Invoke();
+        }
+
         _wasAtBottom = scrollViewer.Offset.Y + scrollViewer.Viewport.Height >= scrollViewer.Extent.Height - 30;
     }
 
@@ -58,6 +80,10 @@ public partial class MessageListView : UserControl
             _chat!.Messages.CollectionChanged += OnMessagesChanged;
             ScrollToBottom();
         }
+        else if (e.PropertyName == nameof(ChatViewModel.IsLoadingMore) && !_chat!.IsLoadingMore && _loadingMore)
+        {
+            RestoreScrollPosition();
+        }
     }
 
     private void OnMessagesChanged(object? sender, NotifyCollectionChangedEventArgs e)
@@ -68,6 +94,19 @@ public partial class MessageListView : UserControl
         }
     }
 
+    private void RestoreScrollPosition()
+    {
+        ScrollViewer? scroll = this.FindControl<ScrollViewer>("MessageScrollViewer");
+        if (scroll is null) return;
+
+        Dispatcher.UIThread.Post(() =>
+        {
+            double delta = scroll.Extent.Height - _loadingMoreExtent;
+            scroll.Offset = new Vector(scroll.Offset.X, scroll.Offset.Y + delta);
+            _loadingMore = false;
+        }, DispatcherPriority.Background);
+    }
+
     private void TryAutoScroll()
     {
         ScrollViewer? scroll = this.FindControl<ScrollViewer>("MessageScrollViewer");