MyAvaloniaDemo/AuroraDesk.Presentation/Views/Pages/TcpClientPageView.axaml.cs

using AuroraDesk.Presentation.ViewModels.Pages;
using System;
using System.Collections.Specialized;
using System.Linq;
using System.Reactive.Disposables;
using System.Reactive.Linq;
using Avalonia.Controls;
using Avalonia.Markup.Xaml;
using Avalonia.Threading;
using Avalonia.VisualTree;
using ReactiveUI;
using ReactiveUI.Avalonia;

namespace AuroraDesk.Presentation.Views.Pages;

public partial class TcpClientPageView : ReactiveUserControl<TcpClientPageViewModel>
{
    private ContentControl? _sessionDetailContent;
    private ListBox? _receivedMessagesListBox;
    private TcpClientSessionViewModel? _currentSession;
    private IDisposable? _messagesSubscription;
    private IDisposable? _autoScrollSubscription;

    public TcpClientPageView()
    {
        InitializeComponent();

        this.WhenActivated(disposables =>
        {
            _sessionDetailContent ??= this.FindControl<ContentControl>("SessionDetailContent");

            var sessionSubscription = this.WhenAnyValue(x => x.ViewModel)
                .WhereNotNull()
                .Select(vm => vm.WhenAnyValue(v => v.SelectedSession))
                .Switch()
                .Subscribe(OnSessionChanged);

            disposables.Add(sessionSubscription);
            disposables.Add(Disposable.Create(ClearSessionState));
        });
    }

    private void InitializeComponent()
    {
        AvaloniaXamlLoader.Load(this);
    }

    private void OnSessionChanged(TcpClientSessionViewModel? session)
    {
        if (ReferenceEquals(_currentSession, session))
        {
            UpdateReceivedMessagesListBox();
            return;
        }

        ClearSessionState();

        _currentSession = session;
        UpdateReceivedMessagesListBox();

        if (session is null)
        {
            return;
        }

        _messagesSubscription = Observable.FromEventPattern<NotifyCollectionChangedEventHandler, NotifyCollectionChangedEventArgs>(
                h => session.ReceivedMessages.CollectionChanged += h,
                h => session.ReceivedMessages.CollectionChanged -= h)
            .Subscribe(_ => CheckAutoScroll());

        _autoScrollSubscription = session.WhenAnyValue(x => x.IsAutoScrollToBottom)
            .Subscribe(_ => CheckAutoScroll());

        CheckAutoScroll();
    }

    private void CheckAutoScroll()
    {
        if (_currentSession?.IsAutoScrollToBottom == true)
        {
            ScrollToBottom();
        }
    }

    private void ClearSessionState()
    {
        _messagesSubscription?.Dispose();
        _messagesSubscription = null;

        _autoScrollSubscription?.Dispose();
        _autoScrollSubscription = null;

        _currentSession = null;
    }

    private void UpdateReceivedMessagesListBox()
    {
        if (_sessionDetailContent is null)
        {
            return;
        }

        Dispatcher.UIThread.Post(() =>
        {
            _receivedMessagesListBox = _sessionDetailContent
                .GetVisualDescendants()
                .OfType<ListBox>()
                .FirstOrDefault(x => x.Name == "SessionReceivedMessagesListBox");

            CheckAutoScroll();
        }, DispatcherPriority.Background);
    }

    private void ScrollToBottom()
    {
        if (_receivedMessagesListBox is null || _currentSession is null)
        {
            return;
        }

        if (_currentSession.ReceivedMessages.Count == 0)
        {
            return;
        }

        var lastItem = _currentSession.ReceivedMessages[^1];

        Dispatcher.UIThread.Post(() =>
        {
            if (_receivedMessagesListBox is null)
            {
                return;
            }

            _receivedMessagesListBox.ScrollIntoView(lastItem);
            _receivedMessagesListBox.SelectedIndex = -1;
        }, DispatcherPriority.Background);
    }
}