file_system_entity.dart

// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

part of dart.io;

/// The type of an entity on the file system,
/// such as a file, directory, or link.
///
/// These constants are used by the [FileSystemEntity] class
/// to indicate the object's type.
final class FileSystemEntityType {
  static const file = const FileSystemEntityType._internal(0);

  static const directory = const FileSystemEntityType._internal(1);

  static const link = const FileSystemEntityType._internal(2);

  static const unixDomainSock = const FileSystemEntityType._internal(3);

  static const pipe = const FileSystemEntityType._internal(4);

  static const notFound = const FileSystemEntityType._internal(5);
  @Deprecated("Use notFound instead")
  static const NOT_FOUND = notFound;

  static const _typeList = const [
    FileSystemEntityType.file,
    FileSystemEntityType.directory,
    FileSystemEntityType.link,
    FileSystemEntityType.unixDomainSock,
    FileSystemEntityType.pipe,
    FileSystemEntityType.notFound,
  ];
  final int _type;

  const FileSystemEntityType._internal(this._type);

  static FileSystemEntityType _lookup(int type) => _typeList[type];
  String toString() => const [
        'file',
        'directory',
        'link',
        'unixDomainSock',
        'pipe',
        'notFound'
      ][_type];
}

/// The result of calling the POSIX `stat()` function on a file system object.
///
/// This is an immutable object, representing the snapshotted values returned
/// by the `stat()` call.
interface class FileStat {
  // These must agree with enum FileStat in file.h.
  static const _type = 0;
  static const _changedTime = 1;
  static const _modifiedTime = 2;
  static const _accessedTime = 3;
  static const _mode = 4;
  static const _size = 5;

  static final _epoch = DateTime.fromMillisecondsSinceEpoch(0, isUtc: true);
  static final _notFound = new FileStat._internal(
      _epoch, _epoch, _epoch, FileSystemEntityType.notFound, 0, -1);

  /// The time of the last change to the data or metadata of the file system
  /// object.
  ///
  /// On Windows platforms, this is instead the file creation time.
  final DateTime changed;

  /// The time of the last change to the data of the file system object.
  final DateTime modified;

  /// The time of the last access to the data of the file system object.
  ///
  /// On Windows platforms, this may have 1 day granularity, and be
  /// out of date by an hour.
  final DateTime accessed;

  /// The type of the underlying file system object.
  ///
  /// [FileSystemEntityType.notFound] if [stat] or [statSync] failed.
  final FileSystemEntityType type;

  /// The mode of the file system object.
  ///
  /// Permissions are encoded in the lower 16 bits of this number, and can be
  /// decoded using the [modeString] getter.
  final int mode;

  /// The size of the file system object.
  final int size;

  FileStat._internal(this.changed, this.modified, this.accessed, this.type,
      this.mode, this.size);

  external static _statSync(_Namespace namespace, String path);

  /// Calls the operating system's `stat()` function (or equivalent) on [path].
  ///
  /// If [path] is a symbolic link then it is resolved and results for the
  /// resulting file are returned.
  ///
  /// Returns a [FileStat] object containing the data returned by `stat()`.
  /// If the call fails, returns a [FileStat] object with [FileStat.type] set to
  /// [FileSystemEntityType.notFound] and the other fields invalid.
  static FileStat statSync(String path) {
    final IOOverrides? overrides = IOOverrides.current;
    if (overrides == null) {
      return _statSyncInternal(path);
    }
    return overrides.statSync(path);
  }

  static FileStat _statSyncInternal(String path) {
    // Trailing path is not supported on Windows.
    if (Platform.isWindows) {
      path = FileSystemEntity._trimTrailingPathSeparators(path);
    }
    var data = _statSync(_Namespace._namespace, path);
    if (data is! Int64List) return FileStat._notFound;
    return FileStat._internal(
        DateTime.fromMillisecondsSinceEpoch(data[_changedTime]),
        DateTime.fromMillisecondsSinceEpoch(data[_modifiedTime]),
        DateTime.fromMillisecondsSinceEpoch(data[_accessedTime]),
        FileSystemEntityType._lookup(data[_type]),
        data[_mode],
        data[_size]);
  }

  /// Asynchronously calls the operating system's `stat()` function (or
  /// equivalent) on [path].
  ///
  /// If [path] is a symbolic link then it is resolved and results for the
  /// resulting file are returned.
  ///
  /// Returns a [Future] which completes with the same results as [statSync].
  static Future<FileStat> stat(String path) {
    final IOOverrides? overrides = IOOverrides.current;
    if (overrides == null) {
      return _stat(path);
    }
    return overrides.stat(path);
  }

  static Future<FileStat> _stat(String path) {
    // Trailing path is not supported on Windows.
    if (Platform.isWindows) {
      path = FileSystemEntity._trimTrailingPathSeparators(path);
    }
    return _File._dispatchWithNamespace(_IOService.fileStat, [null, path])
        .then((response) {
      if (response is List<Object?> && response[0] != _successResponse) {
        return FileStat._notFound;
      }
      // Unwrap the real list from the "I'm not an error" wrapper.
      var data = (response as List)[1] as List<Object?>;
      return FileStat._internal(
          DateTime.fromMillisecondsSinceEpoch(data[_changedTime] as int),
          DateTime.fromMillisecondsSinceEpoch(data[_modifiedTime] as int),
          DateTime.fromMillisecondsSinceEpoch(data[_accessedTime] as int),
          FileSystemEntityType._lookup(data[_type] as int),
          data[_mode] as int,
          data[_size] as int);
    });
  }

  String toString() => """
FileStat: type $type
          changed $changed
          modified $modified
          accessed $accessed
          mode ${modeString()}
          size $size""";

  /// The mode value as a human-readable string.
  ///
  /// The string is in the format "rwxrwxrwx", reflecting the user, group, and
  /// world permissions to read, write, and execute the file system object, with
  /// "-" replacing the letter for missing permissions. Extra permission bits
  /// may be represented by prepending "(suid)", "(guid)", and/or "(sticky)" to
  /// the mode string.
  String modeString() {
    var permissions = mode & 0xFFF;
    var codes = const ['---', '--x', '-w-', '-wx', 'r--', 'r-x', 'rw-', 'rwx'];
    var result = [];
    if ((permissions & 0x800) != 0) result.add("(suid) ");
    if ((permissions & 0x400) != 0) result.add("(guid) ");
    if ((permissions & 0x200) != 0) result.add("(sticky) ");
    result
      ..add(codes[(permissions >> 6) & 0x7])
      ..add(codes[(permissions >> 3) & 0x7])
      ..add(codes[permissions & 0x7]);
    return result.join();
  }
}

/// The common superclass of [File], [Directory], and [Link].
///
/// [FileSystemEntity] objects are returned from directory listing
/// operations. To determine whether a [FileSystemEntity] is a [File], a
/// [Directory], or a [Link] perform a type check:
/// ```dart
/// if (entity is File) (entity as File).readAsStringSync();
/// ```
/// You can also use the [type] or [typeSync] methods to determine
/// the type of a file system object.
///
/// Most methods in this class exist both in synchronous and asynchronous
/// versions, for example, [exists] and [existsSync].
/// Unless you have a specific reason for using the synchronous version
/// of a method, prefer the asynchronous version to avoid blocking your program.
///
/// Here's the exists method in action:
/// ```dart
/// var isThere = await entity.exists();
/// print(isThere ? 'exists' : 'nonexistent');
/// ```
///
/// ## Other resources
///
/// * The [Files and directories](https://dart.dev/guides/libraries/library-tour#files-and-directories)
///   section of the library tour.
///
/// * [Write Command-Line Apps](https://dart.dev/tutorials/server/cmdline),
///   a tutorial about writing command-line apps, includes information about
///   files and directories.
abstract class FileSystemEntity {
  static const _backslashChar = 0x5c;
  static const _slashChar = 0x2f;
  static const _colonChar = 0x3a;

  String get _path;
  Uint8List get _rawPath;

  String get path;

  /// A [Uri] representing the file system entity's location.
  ///
  /// The returned URI's scheme is always "file" if the entity's [path] is
  /// absolute, otherwise the scheme will be empty and the URI relative.
  Uri get uri => new Uri.file(path);

  /// Checks whether the file system entity with this path exists.
  ///
  /// Returns a `Future<bool>` that completes with the result.
  ///
  /// Since [FileSystemEntity] is abstract, every [FileSystemEntity] object
  /// is actually an instance of one of the subclasses [File],
  /// [Directory], and [Link]. Calling [exists] on an instance of one
  /// of these subclasses checks whether the object exists in the file
  /// system object exists *and* is of the correct type (file, directory,
  /// or link). To check whether a path points to an object on the
  /// file system, regardless of the object's type, use the [type]
  /// static method.
  Future<bool> exists();

  /// Synchronously checks whether the file system entity with this path
  /// exists.
  ///
  /// Since [FileSystemEntity] is abstract, every [FileSystemEntity] object
  /// is actually an instance of one of the subclasses [File],
  /// [Directory], and [Link].  Calling [existsSync] on an instance of
  /// one of these subclasses checks whether the object exists in the
  /// file system object exists and is of the correct type (file,
  /// directory, or link).  To check whether a path points to an object
  /// on the file system, regardless of the object's type, use the
  /// [typeSync] static method.
  bool existsSync();

  /// Renames this file system entity.
  ///
  /// Returns a `Future<FileSystemEntity>` that completes with a
  /// [FileSystemEntity] instance for the renamed file system entity.
  Future<FileSystemEntity> rename(String newPath);

  /// Synchronously renames this file system entity.
  ///
  /// Returns a [FileSystemEntity] instance for the renamed entity.
  FileSystemEntity renameSync(String newPath);

  /// Resolves the path of a file system object relative to the
  /// current working directory.
  ///
  /// Resolves all symbolic links on the path and resolves all `..` and `.` path
  /// segments.
  ///
  /// [resolveSymbolicLinks] uses the operating system's native
  /// file system API to resolve the path, using the `realpath` function
  /// on Linux and OS X, and the `GetFinalPathNameByHandle` function on
  /// Windows. If the path does not point to an existing file system object,
  /// `resolveSymbolicLinks` throws a `FileSystemException`.
  ///
  /// On Windows the `..` segments are resolved _before_ resolving the symbolic
  /// link, and on other platforms the symbolic links are _resolved to their
  /// target_ before applying a `..` that follows.
  ///
  /// To ensure the same behavior on all platforms resolve `..` segments before
  /// calling `resolveSymbolicLinks`. One way of doing this is with the [Uri]
  /// class:
  /// ```dart
  /// var path = Uri.parse('.').resolveUri(Uri.file(input)).toFilePath();
  /// if (path == '') path = '.';
  /// var resolved = await File(path).resolveSymbolicLinks();
  /// print(resolved);
  /// ```
  /// since `Uri.resolve` removes `..` segments. This will result in the Windows
  /// behavior.
  ///
  /// On Windows, attempting to resolve a symbolic link where the link type
  /// does not match the type of the resolved file system object will cause the
  /// Future to complete with a [PathAccessException] error.
  Future<String> resolveSymbolicLinks() {
    return _File._dispatchWithNamespace(
        _IOService.fileResolveSymbolicLinks, [null, _rawPath]).then((response) {
      _checkForErrorResponse(response, "Cannot resolve symbolic links", path);
      return response as String;
    });
  }

  /// Resolves the path of a file system object relative to the
  /// current working directory.
  ///
  /// Resolves all symbolic links on the path and resolves all `..` and `.` path
  /// segments.
  ///
  /// [resolveSymbolicLinksSync] uses the operating system's native
  /// file system API to resolve the path, using the `realpath` function
  /// on linux and OS X, and the `GetFinalPathNameByHandle` function on
  /// Windows. If the path does not point to an existing file system object,
  /// `resolveSymbolicLinksSync` throws a `FileSystemException`.
  ///
  /// On Windows the `..` segments are resolved _before_ resolving the symbolic
  /// link, and on other platforms the symbolic links are _resolved to their
  /// target_ before applying a `..` that follows.
  ///
  /// To ensure the same behavior on all platforms resolve `..` segments before
  /// calling `resolveSymbolicLinksSync`. One way of doing this is with the [Uri]
  /// class:
  /// ```dart
  /// var path = Uri.parse('.').resolveUri(Uri.file(input)).toFilePath();
  /// if (path == '') path = '.';
  /// var resolved = File(path).resolveSymbolicLinksSync();
  /// print(resolved);
  /// ```
  /// since `Uri.resolve` removes `..` segments. This will result in the Windows
  /// behavior.
  ///
  /// On Windows, a symbolic link is created as either a file link or a
  /// directory link. Attempting to resolve such a symbolic link requires the
  /// link type to match the type of the file system object that it points to,
  /// otherwise it throws a [PathAccessException].
  String resolveSymbolicLinksSync() {
    var result = _resolveSymbolicLinks(_Namespace._namespace, _rawPath);
    _throwIfError(result, "Cannot resolve symbolic links", path);
    return result;
  }

  /// Calls the operating system's `stat()` function on [path].
  ///
  /// Identical to `FileStat.stat(this.path)`.
  ///
  /// Returns a `Future<FileStat>` object containing the data returned by
  /// `stat()`.
  ///
  /// If [path] is a symbolic link then it is resolved and results for the
  /// resulting file are returned.
  ///
  /// If the call fails, completes the future with a [FileStat] object
  /// with `.type` set to [FileSystemEntityType.notFound] and the other fields
  /// invalid.
  Future<FileStat> stat() => FileStat.stat(path);

  /// Synchronously calls the operating system's `stat()` function on [path].
  ///
  /// Identical to `FileStat.statSync(this.path)`.
  ///
  /// Returns a [FileStat] object containing the data returned by `stat()`.
  ///
  /// If [path] is a symbolic link then it is resolved and results for the
  /// resulting file are returned.
  ///
  /// If the call fails, returns a [FileStat] object with `.type` set to
  /// [FileSystemEntityType.notFound] and the other fields invalid.
  FileStat statSync() => FileStat.statSync(path);

  /// Deletes this [FileSystemEntity].
  ///
  /// The exact details vary according to the [FileSystemEntity]:
  ///
  ///   * [Directory.delete]
  ///   * [File.delete]
  ///   * [Link.delete]
  Future<FileSystemEntity> delete({bool recursive = false}) =>
      _delete(recursive: recursive);

  /// Synchronously deletes this [FileSystemEntity].
  ///
  /// The exact details vary according to the [FileSystemEntity]:
  ///
  ///   * [Directory.deleteSync]
  ///   * [File.deleteSync]
  ///   * [Link.deleteSync]
  void deleteSync({bool recursive = false}) =>
      _deleteSync(recursive: recursive);

  /// Start watching the [FileSystemEntity] for changes.
  ///
  /// The implementation uses platform-dependent event-based APIs for receiving
  /// file-system notifications, thus behavior depends on the platform.
  ///
  ///   * `Windows`: Uses `ReadDirectoryChangesW`. The implementation only
  ///     supports watching directories. Recursive watching is supported.
  ///   * `Linux`: Uses `inotify`. The implementation supports watching both
  ///     files and directories. Recursive watching is not supported.
  ///     Note: When watching files directly, delete events might not happen
  ///     as expected.
  ///   * `OS X`: Uses the
  ///     [File System Events API](https://developer.apple.com/library/archive/documentation/Darwin/Conceptual/FSEvents_ProgGuide/TechnologyOverview/TechnologyOverview.html).
  ///     The implementation supports watching both files and directories.
  ///     Recursive watching is supported.
  ///     This API has several limitations:
  ///
  ///     * Changes that occurred shortly *before* the [watch] method was
  ///       called may still appear in the [Stream].
  ///     * Changes that occur in a short period of time may arrive
  ///       out-of-order.
  ///     * Multiple changes made in a single directory may be coalesced into
  ///       a single `FileSystemEvent`.
  ///
  /// The system will start listening for events once the returned [Stream] is
  /// being listened to, not when the call to [watch] is issued.
  ///
  /// The returned value is an endless broadcast [Stream], that only stops when
  /// one of the following happens:
  ///
  ///   * The [Stream] is canceled, e.g. by calling `cancel` on the
  ///      [StreamSubscription].
  ///   * The [FileSystemEntity] being watched is deleted.
  ///   * System Watcher exits unexpectedly. e.g. On `Windows` this happens when
  ///     buffer that receive events from `ReadDirectoryChangesW` overflows.
  ///
  /// Use `events` to specify what events to listen for. The constants in
  /// [FileSystemEvent] can be or'ed together to mix events. Default is
  /// [FileSystemEvent.all].
  ///
  /// A move event may be reported as separate delete and create events.
  Stream<FileSystemEvent> watch(
      {int events = FileSystemEvent.all, bool recursive = false}) {
    // FIXME(bkonyi): find a way to do this using the raw path.
    final String trimmedPath = _trimTrailingPathSeparators(path);
    final IOOverrides? overrides = IOOverrides.current;
    if (overrides == null) {
      return _FileSystemWatcher._watch(trimmedPath, events, recursive);
    }
    return overrides.fsWatch(trimmedPath, events, recursive);
  }

  Future<FileSystemEntity> _delete({bool recursive = false});
  void _deleteSync({bool recursive = false});

  static Future<bool> _identical(String path1, String path2) {
    return _File._dispatchWithNamespace(
        _IOService.fileIdentical, [null, path1, path2]).then((response) {
      _checkForErrorResponse(
          response, "Error in FileSystemEntity.identical($path1, $path2)", "");
      return response as bool;
    });
  }

  /// Checks whether two paths refer to the same object in the
  /// file system.
  ///
  /// Returns a `Future<bool>` that completes with the result.
  ///
  /// Comparing a link to its target returns `false`, as does comparing two links
  /// that point to the same target.  To check the target of a link, use
  /// Link.target explicitly to fetch it.  Directory links appearing
  /// inside a path are followed, though, to find the file system object.
  ///
  /// Completes the returned Future with an error if one of the paths points
  /// to an object that does not exist.
  static Future<bool> identical(String path1, String path2) {
    IOOverrides? overrides = IOOverrides.current;
    if (overrides == null) {
      return _identical(path1, path2);
    }
    return overrides.fseIdentical(path1, path2);
  }

  static final RegExp _absoluteWindowsPathPattern =
      new RegExp(r'^(?:\\\\|[a-zA-Z]:[/\\])');

  /// Whether this object's path is absolute.
  ///
  /// An absolute path is independent of the current working
  /// directory ([Directory.current]).
  /// A non-absolute path must be interpreted relative to
  /// the current working directory.
  ///
  /// On Windows, a path is absolute if it starts with `\\`
  /// (two backslashes or representing a UNC path) or with a drive letter
  /// between `a` and `z` (upper or lower case) followed by `:\` or `:/`.
  /// This makes, for example, `\file.ext` a non-absolute path
  /// because it depends on the current drive letter.
  ///
  /// On non-Windows, a path is absolute if it starts with `/`.
  ///
  /// If the path is not absolute, use [absolute] to get an entity
  /// with an absolute path referencing the same object in the file system,
  /// if possible.
  bool get isAbsolute => _isAbsolute(path);

  static bool _isAbsolute(String path) {
    if (Platform.isWindows) {
      return path.startsWith(_absoluteWindowsPathPattern);
    } else {
      return path.startsWith('/');
    }
  }

  /// A [FileSystemEntity] whose path is the absolute path of [path].
  ///
  /// The type of the returned instance is the same as the type of
  /// this entity.
  ///
  /// A file system entity with an already absolute path
  /// (as reported by [isAbsolute]) is returned directly.
  /// For a non-absolute path, the returned entity is absolute ([isAbsolute])
  /// *if possible*, but still refers to the same file system object.
  FileSystemEntity get absolute;

  String get _absolutePath {
    if (isAbsolute) return path;
    if (Platform.isWindows) return _absoluteWindowsPath(path);
    String current = Directory.current.path;
    if (current.endsWith('/')) {
      return '$current$path';
    } else {
      return '$current${Platform.pathSeparator}$path';
    }
  }

  /// The ASCII code of the Windows drive letter of [path], if any.
  ///
  /// Returns the ASCII code of the upper-cased drive letter of
  /// the path of [path], if it has a drive letter (starts with `[a-zA-z]:`),
  /// or `-1` if it has no drive letter.
  static int _windowsDriveLetter(String path) {
    if (path.isEmpty || !path.startsWith(':', 1)) return -1;
    var first = path.codeUnitAt(0) & ~0x20;
    if (first >= 0x41 && first <= 0x5b) return first;
    return -1;
  }

  /// The relative [path] converted to an absolute path.
  static String _absoluteWindowsPath(String path) {
    assert(Platform.isWindows);
    assert(!_isAbsolute(path));
    // Could perhaps use something like
    // https://docs.microsoft.com/en-us/windows/win32/api/pathcch/nf-pathcch-pathalloccombine
    var current = Directory.current.path;
    if (path.startsWith(r'\')) {
      assert(!path.startsWith(r'\', 1));
      // Absolute path, no drive letter.
      var currentDrive = _windowsDriveLetter(current);
      if (currentDrive >= 0) {
        return '${current[0]}:$path';
      }
      // If `current` is a UNC path \\server\share[...],
      // we make the absolute path relative to the share.
      // Also works with `\\?\c:\` paths.
      if (current.startsWith(r'\\')) {
        var serverEnd = current.indexOf(r'\', 2);
        if (serverEnd >= 0) {
          // We may want to recognize UNC paths of the form:
          //   \\?\UNC\Server\share\...
          // specially, and be relative to the *share* not to UNC\.
          var shareEnd = current.indexOf(r'\', serverEnd + 1);
          if (shareEnd < 0) shareEnd = current.length;
          return '${current.substring(0, shareEnd)}$path';
        }
      }
      // If `current` is not in the drive-letter:path format,
      // or not \\server\share[\path],
      // we ignore it and return a relative path.
      return path;
    }
    var entityDrive = _windowsDriveLetter(path);
    if (entityDrive >= 0) {
      if (entityDrive != _windowsDriveLetter(current)) {
        // Need to resolve relative to current directory of the drive.
        // Windows remembers the last CWD of each drive.
        // We currently don't have that information,
        // so we assume the root of that drive.
        return '${path[0]}:\\$path';
      }

      /// A `c:relative\path` path on the same drive as `current`.
      path = path.substring(2);
      assert(!path.startsWith(r'\\'));
    }
    if (current.endsWith(r'\') || current.endsWith('/')) {
      return '$current$path';
    }
    return '$current\\$path';
  }

  static bool _identicalSync(String path1, String path2) {
    var result = _identicalNative(_Namespace._namespace, path1, path2);
    _throwIfError(result, 'Error in FileSystemEntity.identicalSync');
    return result;
  }

  /// Synchronously checks whether two paths refer to the same object in the
  /// file system.
  ///
  /// Comparing a link to its target returns `false`, as does comparing two links
  /// that point to the same target.  To check the target of a link, use
  /// Link.target explicitly to fetch it.  Directory links appearing
  /// inside a path are followed, though, to find the file system object.
  ///
  /// Throws an error if one of the paths points to an object that does not
  /// exist.
  static bool identicalSync(String path1, String path2) {
    IOOverrides? overrides = IOOverrides.current;
    if (overrides == null) {
      return _identicalSync(path1, path2);
    }
    return overrides.fseIdenticalSync(path1, path2);
  }

  /// Test if [watch] is supported on the current system.
  ///
  /// OS X 10.6 and below is not supported.
  static bool get isWatchSupported {
    final IOOverrides? overrides = IOOverrides.current;
    if (overrides == null) {
      return _FileSystemWatcher.isSupported;
    }
    return overrides.fsWatchIsSupported();
  }

  // The native methods which determine type of the FileSystemEntity require
  // that the buffer provided is null terminated.
  static Uint8List _toUtf8Array(String s) =>
      _toNullTerminatedUtf8Array(utf8.encode(s));

  static Uint8List _toNullTerminatedUtf8Array(Uint8List l) {
    if (l.isEmpty || (l.isNotEmpty && l.last != 0)) {
      final tmp = new Uint8List(l.length + 1);
      tmp.setRange(0, l.length, l);
      return tmp;
    } else {
      return l;
    }
  }

  static String _toStringFromUtf8Array(Uint8List l) {
    Uint8List nonNullTerminated = l;
    if (l.last == 0) {
      nonNullTerminated =
          new Uint8List.view(l.buffer, l.offsetInBytes, l.length - 1);
    }
    return utf8.decode(nonNullTerminated, allowMalformed: true);
  }

  /// Finds the type of file system object that a path points to.
  ///
  /// Returns a `Future<FileSystemEntityType>` that completes with the same
  /// results as [typeSync].
  static Future<FileSystemEntityType> type(String path,
      {bool followLinks = true}) {
    return _getType(_toUtf8Array(path), followLinks);
  }

  /// Synchronously finds the type of file system object that a path points to.
  ///
  /// Returns [FileSystemEntityType.notFound] if [path] does not point to a file
  /// system object or if any other error occurs in looking up the path.
  ///
  /// If [path] points to a link and [followLinks] is `true` then the result
  /// will be for the file system object that the link points to. If that
  /// object does not exist then the result will be
  /// [FileSystemEntityType.notFound]. If [path] points to a link and
  /// [followLinks] is `false` then the result will be
  /// [FileSystemEntityType.link].
  static FileSystemEntityType typeSync(String path, {bool followLinks = true}) {
    return _getTypeSync(_toUtf8Array(path), followLinks);
  }

  /// Whether [path] refers to a link.
  ///
  /// Checks whether `type(path, followLinks: false)`
  /// returns [FileSystemEntityType.link].
  static Future<bool> isLink(String path) => _isLinkRaw(_toUtf8Array(path));

  static Future<bool> _isLinkRaw(Uint8List rawPath) => _getType(rawPath, false)
      .then((type) => (type == FileSystemEntityType.link));

  /// Whether [path] refers to a file.
  ///
  /// Checks whether `type(path)` returns [FileSystemEntityType.file].
  static Future<bool> isFile(String path) => _getType(_toUtf8Array(path), true)
      .then((type) => (type == FileSystemEntityType.file));

  /// Whether [path] refers to a directory.
  ///
  /// Checks whether `type(path)` returns [FileSystemEntityType.directory].
  static Future<bool> isDirectory(String path) =>
      _getType(_toUtf8Array(path), true)
          .then((type) => (type == FileSystemEntityType.directory));

  /// Synchronously checks whether [path] refers to a link.
  ///
  /// Checks whether `typeSync(path, followLinks: false)` returns
  /// [FileSystemEntityType.link].
  static bool isLinkSync(String path) => _isLinkRawSync(_toUtf8Array(path));

  static bool _isLinkRawSync(rawPath) =>
      (_getTypeSync(rawPath, false) == FileSystemEntityType.link);

  /// Synchronously checks whether [path] refers to a file.
  ///
  /// Checks whether `typeSync(path)` returns
  /// [FileSystemEntityType.file].
  static bool isFileSync(String path) =>
      (_getTypeSync(_toUtf8Array(path), true) == FileSystemEntityType.file);

  /// Synchronously checks whether [path] refers to a directory.
  ///
  /// Checks whether `typeSync(path)` returns
  /// [FileSystemEntityType.directory].
  static bool isDirectorySync(String path) =>
      (_getTypeSync(_toUtf8Array(path), true) ==
          FileSystemEntityType.directory);

  external static _getTypeNative(
      _Namespace namespace, Uint8List rawPath, bool followLinks);
  external static _identicalNative(
      _Namespace namespace, String path1, String path2);
  external static _resolveSymbolicLinks(_Namespace namespace, Uint8List path);

  // Finds the next-to-last component when dividing at path separators.
  static final RegExp _parentRegExp = Platform.isWindows
      ? new RegExp(r'[^/\\][/\\]+[^/\\]')
      : new RegExp(r'[^/]/+[^/]');

  /// The parent path of a path.
  ///
  /// Finds the final path component of a path, using the platform's
  /// path separator to split the path, and returns the prefix up to
  /// that part.
  ///
  /// Will not remove the root component of a Windows path, like "C:\\" or
  /// "\\\\server_name\\". Includes a trailing path separator in the last
  /// part of [path], and leaves no trailing path separator.
  static String parentOf(String path) {
    int rootEnd = -1;
    if (Platform.isWindows) {
      if (path.startsWith(_absoluteWindowsPathPattern)) {
        // Root ends at first / or \ after the first two characters.
        rootEnd = path.indexOf(new RegExp(r'[/\\]'), 2);
        if (rootEnd == -1) return path;
      } else if (path.startsWith('\\') || path.startsWith('/')) {
        rootEnd = 0;
      }
    } else if (path.startsWith('/')) {
      rootEnd = 0;
    }
    // Ignore trailing slashes.
    // All non-trivial cases have separators between two non-separators.
    int pos = path.lastIndexOf(_parentRegExp);
    if (pos > rootEnd) {
      return path.substring(0, pos + 1);
    } else if (rootEnd > -1) {
      return path.substring(0, rootEnd + 1);
    } else {
      return '.';
    }
  }

  /// The parent directory of this entity.
  Directory get parent => new Directory(parentOf(path));

  static FileSystemEntityType _getTypeSyncHelper(
      Uint8List rawPath, bool followLinks) {
    var result = _getTypeNative(_Namespace._namespace, rawPath, followLinks);
    _throwIfError(result, 'Error getting type of FileSystemEntity');
    return FileSystemEntityType._lookup(result);
  }

  static FileSystemEntityType _getTypeSync(
      Uint8List rawPath, bool followLinks) {
    IOOverrides? overrides = IOOverrides.current;
    if (overrides == null) {
      return _getTypeSyncHelper(rawPath, followLinks);
    }
    return overrides.fseGetTypeSync(
        _toStringFromUtf8Array(rawPath), followLinks);
  }

  static Future<FileSystemEntityType> _getTypeRequest(
      Uint8List rawPath, bool followLinks) {
    return _File._dispatchWithNamespace(
        _IOService.fileType, [null, rawPath, followLinks]).then((response) {
      _checkForErrorResponse(response, "Error getting type",
          utf8.decode(rawPath, allowMalformed: true));
      return FileSystemEntityType._lookup(response as int);
    });
  }

  static Future<FileSystemEntityType> _getType(
      Uint8List rawPath, bool followLinks) {
    IOOverrides? overrides = IOOverrides.current;
    if (overrides == null) {
      return _getTypeRequest(rawPath, followLinks);
    }
    return overrides.fseGetType(_toStringFromUtf8Array(rawPath), followLinks);
  }

  static _throwIfError(Object result, String msg, [String? path]) {
    if (result is OSError) {
      throw new FileSystemException(msg, path, result);
    } else if (result is ArgumentError) {
      throw result;
    }
  }

  // TODO(bkonyi): find a way to do this with raw paths.
  static String _trimTrailingPathSeparators(String path) {
    // TODO(40614): Remove once non-nullability is sound.
    ArgumentError.checkNotNull(path, "path");
    if (Platform.isWindows) {
      // "C:" and "C:\" are semantically different on Windows and only "C:\" is an
      // absolute path.
      // See https://learn.microsoft.com/en-us/dotnet/standard/io/file-path-formats
      final minimumCharToNotTrim = _isAbsolute(path) ? 3 : 1;
      while (path.length > minimumCharToNotTrim &&
          (path.endsWith(Platform.pathSeparator) || path.endsWith('/'))) {
        path = path.substring(0, path.length - 1);
      }
    } else {
      while (path.length > 1 && path.endsWith(Platform.pathSeparator)) {
        path = path.substring(0, path.length - 1);
      }
    }
    return path;
  }

  // TODO(bkonyi): find a way to do this with raw paths.
  static String _ensureTrailingPathSeparators(String path) {
    if (path.isEmpty) path = '.';
    if (Platform.isWindows) {
      while (!path.endsWith(Platform.pathSeparator) && !path.endsWith('/')) {
        path = "$path${Platform.pathSeparator}";
      }
    } else {
      while (!path.endsWith(Platform.pathSeparator)) {
        path = "$path${Platform.pathSeparator}";
      }
    }
    return path;
  }
}

/// Base event class emitted by [FileSystemEntity.watch].
sealed class FileSystemEvent {
  /// Bitfield for [FileSystemEntity.watch], to enable [FileSystemCreateEvent]s.
  static const int create = 1 << 0;

  /// Bitfield for [FileSystemEntity.watch], to enable [FileSystemModifyEvent]s.
  static const int modify = 1 << 1;

  /// Bitfield for [FileSystemEntity.watch], to enable [FileSystemDeleteEvent]s.
  static const int delete = 1 << 2;

  /// Bitfield for [FileSystemEntity.watch], to enable [FileSystemMoveEvent]s.
  static const int move = 1 << 3;

  /// Bitfield for [FileSystemEntity.watch], for enabling all of [create],
  /// [modify], [delete] and [move].
  static const int all = create | modify | delete | move;

  static const int _modifyAttributes = 1 << 4;
  static const int _deleteSelf = 1 << 5;
  static const int _isDir = 1 << 6;

  /// The type of event. See [FileSystemEvent] for a list of events.
  final int type;

  /// The path that triggered the event.
  ///
  /// Depending on the platform and the [FileSystemEntity], the path may be
  /// relative.
  final String path;

  /// Is `true` if the event target was a directory.
  ///
  /// Note that if the file has been deleted by the time the event has arrived,
  /// this will always be `false` on Windows. In particular, it will always be
  /// `false` for `delete` events.
  final bool isDirectory;

  FileSystemEvent._(this.type, this.path, this.isDirectory);
}

/// File system event for newly created file system objects.
final class FileSystemCreateEvent extends FileSystemEvent {
  /// Constructs a new [FileSystemCreateEvent].
  FileSystemCreateEvent(String path, bool isDirectory)
      : super._(FileSystemEvent.create, path, isDirectory);

  String toString() =>
      "FileSystemCreateEvent('$path', isDirectory=$isDirectory)";
}

/// File system event for modifications of file system objects.
final class FileSystemModifyEvent extends FileSystemEvent {
  /// If the content was changed and not only the attributes, [contentChanged]
  /// is `true`.
  final bool contentChanged;

  /// Constructs a new [FileSystemModifyEvent].
  FileSystemModifyEvent(String path, bool isDirectory, this.contentChanged)
      : super._(FileSystemEvent.modify, path, isDirectory);

  String toString() =>
      "FileSystemModifyEvent('$path', isDirectory=$isDirectory, "
      "contentChanged=$contentChanged)";
}

/// File system event for deletion of file system objects.
final class FileSystemDeleteEvent extends FileSystemEvent {
  /// Constructs a new [FileSystemDeleteEvent].
  FileSystemDeleteEvent(String path, bool isDirectory)
      : super._(FileSystemEvent.delete, path, isDirectory);

  String toString() =>
      "FileSystemDeleteEvent('$path', isDirectory=$isDirectory)";
}

/// File system event for moving of file system objects.
final class FileSystemMoveEvent extends FileSystemEvent {
  /// The destination path of the file being moved.
  ///
  /// The destination is `null` if the underlying implementation
  /// is unable to identify the destination of the moved file.
  ///
  /// The source path is available as [path].
  final String? destination;

  /// Constructs a new [FileSystemMoveEvent].
  FileSystemMoveEvent(String path, bool isDirectory, this.destination)
      : super._(FileSystemEvent.move, path, isDirectory);

  String toString() => "FileSystemMoveEvent('$path', "
      "isDirectory=$isDirectory, destination=$destination)";
}

abstract class _FileSystemWatcher {
  external static Stream<FileSystemEvent> _watch(
      String path, int events, bool recursive);
  external static bool get isSupported;
}