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

using Avalonia.Controls;
using Avalonia.Controls.Primitives;
using Avalonia.LogicalTree;
using Avalonia.Markup.Xaml;
using ReactiveUI.Avalonia;
using ReactiveUI;
using System.Reactive.Disposables;
using AuroraDesk.Presentation.ViewModels.Pages;
using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Avalonia.Threading;
using Avalonia.VisualTree;

namespace AuroraDesk.Presentation.Views.Pages;

/// <summary>
/// 图片浏览页面视图
/// 
/// 核心功能：
/// - 实现滚动检测，当滚动接近底部时自动加载更多图片
/// - 距底部小于500px时触发加载
/// - 防止重复加载的保护逻辑
/// </summary>
public partial class ImageGalleryPageView : ReactiveUserControl<ImageGalleryPageViewModel>
{
    private ScrollViewer? _scrollViewer;
    private CancellationTokenSource? _scrollDetectionCancellationToken;
    private CancellationTokenSource? _layoutCheckCancellationToken;
    private bool _isLoadingMore = false; // 防止重复加载
    
    public ImageGalleryPageView()
    {
        InitializeComponent();
        this.WhenActivated(disposables =>
        {
            // 当视图激活时，设置滚动检测
            Dispatcher.UIThread.Post(() => SetupScrollDetection(), DispatcherPriority.Loaded);
            
            // 当视图停用时，清理资源
            disposables.Add(Disposable.Create(() =>
            {
                _scrollDetectionCancellationToken?.Cancel();
                _scrollDetectionCancellationToken?.Dispose();
                _layoutCheckCancellationToken?.Cancel();
                _layoutCheckCancellationToken?.Dispose();
            }));
        });
    }

    private void InitializeComponent()
    {
        AvaloniaXamlLoader.Load(this);
    }
    
    /// <summary>
    /// 设置滚动检测
    /// 监听 ScrollViewer 的滚动事件，当接近底部时加载更多
    /// </summary>
    private void SetupScrollDetection()
    {
        // 查找 ScrollViewer
        _scrollViewer = this.FindControl<ScrollViewer>("MainScrollViewer");
        if (_scrollViewer == null)
        {
            _scrollViewer = this.GetLogicalDescendants().OfType<ScrollViewer>().FirstOrDefault();
        }
        
        if (_scrollViewer != null)
        {
            // 绑定滚动事件
            _scrollViewer.ScrollChanged += OnScrollChanged;
            
            // 监听布局完成事件，确保在布局完成后检查是否需要加载更多
            _scrollViewer.LayoutUpdated += OnLayoutUpdated;
            
            // 延迟检查一次，确保在初始加载完成后能自动加载更多
            _ = Task.Run(async () =>
            {
                await Task.Delay(800);
                await Dispatcher.UIThread.InvokeAsync(async () =>
                {
                    if (!_isLoadingMore && ViewModel != null)
                    {
                        await CheckAndLoadMoreAsync();
                    }
                }, DispatcherPriority.Background);
            });
        }
    }
    
    /// <summary>
    /// 布局更新时检查是否需要加载更多
    /// </summary>
    private void OnLayoutUpdated(object? sender, EventArgs e)
    {
        // 只在布局稳定后检查一次，避免频繁触发
        if (!_isLoadingMore && ViewModel != null && _scrollViewer != null)
        {
            // 防抖：取消之前的布局检查任务
            _layoutCheckCancellationToken?.Cancel();
            _layoutCheckCancellationToken?.Dispose();
            _layoutCheckCancellationToken = new CancellationTokenSource();
            
            var token = _layoutCheckCancellationToken.Token;
            
            // 检查是否需要加载更多（延迟检查，避免在布局过程中频繁触发）
            _ = Task.Run(async () =>
            {
                await Task.Delay(200, token);
                if (token.IsCancellationRequested)
                    return;
                    
                await Dispatcher.UIThread.InvokeAsync(async () =>
                {
                    if (!token.IsCancellationRequested && !_isLoadingMore && ViewModel != null)
                    {
                        await CheckAndLoadMoreAsync();
                    }
                }, DispatcherPriority.Background);
            }, token);
        }
    }
    
    /// <summary>
    /// 滚动时检查是否需要加载更多
    /// </summary>
    private void OnScrollChanged(object? sender, ScrollChangedEventArgs e)
    {
        // 防抖：取消之前的检测任务
        _scrollDetectionCancellationToken?.Cancel();
        _scrollDetectionCancellationToken?.Dispose();
        _scrollDetectionCancellationToken = new CancellationTokenSource();
        
        var token = _scrollDetectionCancellationToken.Token;
        
        // 延迟200ms检测，避免频繁触发
        _ = Task.Delay(200, token).ContinueWith(async _ =>
        {
            if (!token.IsCancellationRequested && ViewModel != null && _scrollViewer != null)
            {
                await CheckAndLoadMoreAsync();
            }
        }, token);
    }
    
    /// <summary>
    /// 检查是否需要加载更多图片
    /// 当滚动位置距底部小于500px时，触发加载
    /// </summary>
    private async Task CheckAndLoadMoreAsync()
    {
        if (ViewModel == null || _scrollViewer == null || _isLoadingMore)
            return;
        
        try
        {
            // 如果正在加载初始数据，不触发加载更多
            if (ViewModel.IsLoading)
                return;
            
            // 获取滚动信息
            var scrollOffset = _scrollViewer.Offset.Y;
            var viewportHeight = _scrollViewer.Viewport.Height;
            var extentHeight = _scrollViewer.Extent.Height;
            
            if (extentHeight <= 0 || viewportHeight <= 0)
                return;
            
            // 计算距离底部的距离
            var maxScrollOffset = extentHeight - viewportHeight;
            var distanceToBottom = maxScrollOffset - scrollOffset;
            
            // 如果距离底部小于500px，且还有更多内容未加载，则触发加载
            if (distanceToBottom < 500 && ViewModel.Images.Count < ViewModel.TotalImageCount)
            {
                _isLoadingMore = true;
                
                try
                {
                    // 调用 ViewModel 加载更多
                    await ViewModel.LoadMoreImagesAsync();
                }
                finally
                {
                    // 延迟重置标志，避免短时间内重复触发
                    await Task.Delay(300);
                    _isLoadingMore = false;
                }
            }
        }
        catch (Exception ex)
        {
            // 忽略错误，避免影响UI
            System.Diagnostics.Debug.WriteLine($"检查加载更多时出错: {ex.Message}");
            _isLoadingMore = false;
        }
    }
}