<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        Height="300" Width="300"
        x:Class="AvaloniaUI.DirectoryTreeView"
        Title="DirectoryTreeView">
    <Grid Margin="3">
        <TreeView Name="treeFileSystem"/>
    </Grid>
</Window>

using Avalonia;
using Avalonia.Controls;
using Avalonia.Interactivity;
using System;
using System.Collections.Generic;
using System.IO;

namespace AvaloniaUI;

public partial class DirectoryTreeView : Window
{
    private static readonly object Placeholder = new();

    public DirectoryTreeView()
    {
        InitializeComponent();
        BuildTree();
    }
    private void BuildTree()
    {
        treeFileSystem.Items.Clear();

        foreach (var drive in DriveInfo.GetDrives())
        {
            if (!drive.IsReady)
                continue;

            var item = CreateItem(drive.Name, drive);
            AddPlaceholder(item);

            treeFileSystem.Items.Add(item);
        }
    }

    private TreeViewItem CreateItem(string header, object tag)
    {
        var item = new TreeViewItem
        {
            Header = header,
            Tag = tag
        };

        item.Expanded += ItemExpanded;
        return item;
    }

    private void ItemExpanded(object? sender, RoutedEventArgs e)
    {
        if (sender is not TreeViewItem item)
            return;

        // 只在第一次展开（且存在占位符）时加载
        if (item.Items.Count != 1 || !ReferenceEquals(item.Items[0], Placeholder))
            return;

        item.Items.Clear();

        var dir = GetDirectoryInfo(item.Tag);
        if (dir is null)
            return;

        foreach (var subDir in SafeEnumerateDirectories(dir))
        {
            var child = CreateItem(subDir.Name, subDir);

            // 只有确实还有子目录，才显示展开箭头（放占位符）
            if (HasSubdirectories(subDir))
                AddPlaceholder(child);

            item.Items.Add(child);
        }
    }

    private static DirectoryInfo? GetDirectoryInfo(object? tag)
    {
        if (tag is DriveInfo drive)
            return drive.RootDirectory;

        if (tag is DirectoryInfo dir)
            return dir;

        return null;
    }

    private static void AddPlaceholder(TreeViewItem item)
    {
        item.Items.Add(Placeholder);
    }

    private static IEnumerable<DirectoryInfo> SafeEnumerateDirectories(DirectoryInfo dir)
    {
        IEnumerator<DirectoryInfo>? enumerator = null;

        try
        {
            enumerator = dir.EnumerateDirectories().GetEnumerator();
        }
        catch
        {
            yield break;
        }

        using (enumerator as IDisposable)
        {
            while (true)
            {
                DirectoryInfo current;

                try
                {
                    if (!enumerator.MoveNext())
                        break;

                    current = enumerator.Current;
                }
                catch
                {
                    // 某个子目录读不了：跳过，不影响整层
                    continue;
                }

                yield return current;
            }
        }
    }

    private static bool HasSubdirectories(DirectoryInfo dir)
    {
        try
        {
            using var e = dir.EnumerateDirectories().GetEnumerator();
            return e.MoveNext();
        }
        catch
        {
            return false;
        }
    }
}