fs_shim

A portable file system API for Dart and Flutter: File, Directory and Link (an asynchronous subset of dart:io) behind a FileSystem object. The same code runs on io (Dart VM, Flutter mobile and desktop), in the browser (IndexedDB or OPFS) and in memory (tests).

Getting started

Add the dependency:

dart pub add fs_shim

Write your code against a FileSystem, then pick one:

import 'package:fs_shim/fs_shim.dart';

Future<void> main() async {
  // io on the Dart VM and Flutter native, IndexedDB in the browser.
  final fs = fileSystemDefault;

  final dir = fs.directory(fs.path.join(fs.currentDirectory.path, 'demo'));
  await dir.create(recursive: true);
  final file = dir.file('hello.txt');
  await file.writeAsString('Hello world!');
  print(await file.readAsString()); // Hello world!
  await dir.delete(recursive: true);
}

Every call is asynchronous (there is no ...Sync method): the browser storage cannot be read synchronously.

Platform	File system	Needs
Dart VM, Flutter mobile and desktop	fileSystemIo (the default there)	nothing
Browser, Flutter web (dart2js, wasm)	fileSystemWeb on IndexedDB (the default)	IndexedDB, size limited by the browser quota
Browser, Flutter web (dart2js, wasm)	fileSystemOpfsWeb on OPFS	navigator.storage.getDirectory()
Everywhere, for tests	fileSystemMemory, newFileSystemMemory()	nothing (content lost at exit)

Every import is safe on every platform. Reading fileSystemIo on the web, or fileSystemWeb / fileSystemOpfsWeb off the web, throws UnimplementedError.

Which file system to use

File system	Import	Links	Random access
fileSystemDefault	package:fs_shim/fs_shim.dart	✅¹	✅
fileSystemIo	package:fs_shim/fs_shim.dart	✅¹	✅
fileSystemMemory, newFileSystemMemory()	package:fs_shim/fs_memory.dart	✅	✅
fileSystemWeb, getFileSystemWeb(), newFileSystemWeb()	package:fs_shim/fs_browser.dart	✅	✅ (paging for speed, see below)
fileSystemOpfsWeb	package:fs_shim/fs_opfs_web.dart	❌	❌

¹ No file links on Windows (fs.supportsFileLink is false).

• fileSystemDefault is fileSystemWeb on the web and fileSystemIo elsewhere. The File(path), Directory(path) and Link(path) constructors use it. It is a variable: a test can assign newFileSystemMemory() to it.
• fileSystemMemory is shared by the whole program; newFileSystemMemory() creates an empty one (one per test).
• Prefer passing a FileSystem to your code over using fileSystemDefault: the code then runs unchanged on any implementation.
• Check capabilities at runtime with fs.supportsLink, fs.supportsFileLink and fs.supportsRandomAccess.

Common pitfalls

• Build paths with the file system's own context, fs.path.join(...), or dir.file('name'). Memory, IndexedDB and OPFS paths are posix (root /) on every host; the top-level join of package:path uses \ on Windows.
• writeAsString, writeAsBytes and openWrite do not create the parent directory: call file.create(recursive: true) (or file.parent.create(recursive: true)) first, or use writeString from package:fs_shim/utils/read_write.dart.
• writeAsBytes takes a Uint8List, not a List<int>.
• Do not import dart:io next to fs_shim.dart: File, Directory... clash. For io only code, import package:fs_shim/fs_io.dart instead (see below).
• Catch the FileSystemException of fs_shim and test its status (FileSystemException.statusNotFound...), not the dart:io one.
• await sink.close() after file.openWrite(); the content is written once close() (and done) complete.
• CopyOptions() of package:fs_shim/utils/copy.dart is not recursive: pass recursive: true or use defaultCopyOptions.
• entity.exists() checks the type of the entity on io only. When the kind matters, use fs.isFile(path), fs.isDirectory(path) or fs.type(path).

Usage

In memory

import 'package:fs_shim/fs_shim.dart';

Future<void> main() async {
  final fs = fileSystemMemory;

  // A file in a top level directory
  final dir = fs.directory('/dir');
  final file = dir.file('file');

  await file.create(recursive: true);
  await file.writeAsString('Hello world!');
  print('file: ${await file.readAsString()}');

  // Use a file link if supported
  if (fs.supportsFileLink) {
    final link = dir.link('link');
    await link.create(file.path);
    print('link: ${await fs.file(link.path).readAsString()}');
  }

  // List the directory content
  print(await dir.list(recursive: true, followLinks: true).toList());
}

The same code runs unchanged on another implementation: only the fs instance changes.

io

Use fileSystemIo (or fileSystemDefault) from fs_shim.dart.

To port existing io only code, replace

import 'dart:io';

with

import 'package:fs_shim/fs_io.dart';

It exports dart:io with File, Directory, Link, FileSystemEntity, FileSystemEntityType, FileMode, FileStat, FileSystemException and OSError replaced by the fs_shim ones, plus fileSystemIo. Only the asynchronous methods remain. wrapIoFile/unwrapIoFile (and the same for directories, links...) convert between dart:io and fs_shim objects.

import 'package:fs_shim/fs_io.dart';
import 'package:path/path.dart';

Future<void> main() async {
  final dir = Directory(join(Directory.current.path, '.dart_tool', 'demo'));
  await dir.create(recursive: true);
  final file = File(join(dir.path, 'file'));
  await file.writeAsString('Hello world!');
  print('file: ${await file.readAsString()}');
  print('os: ${Platform.operatingSystem}'); // still dart:io
}

Browser: IndexedDB

import 'package:fs_shim/fs_browser.dart';

final fs = fileSystemWeb;

fileSystemWeb (the default on the web) stores the files in the IndexedDB database lfs.db through idb_shim, with links and random access. It stores each file in one record: good for whole file reads and writes. For random access or partial reads of large files, use 16 KiB pages:

import 'package:fs_shim/fs_browser.dart';

// Same database, 16 KiB pages.
final fs = getFileSystemWeb();
// Another database, custom page size.
final cacheFs = newFileSystemWeb(
  name: 'cache.db',
  options: const FileSystemIdbOptions(pageSize: 64 * 1024),
);

Files stay readable when the page size changes: a file is converted when it is next written.

Browser: OPFS (Origin Private File System)

fs_shim also provides an implementation on top of the browser Origin Private File System, a thin layer over the native navigator.storage.getDirectory() API (using dart:js_interop, so it is wasm compatible).

import 'package:fs_shim/fs_opfs_web.dart';

final fs = fileSystemOpfsWeb;

There is a single OPFS per origin, fileSystemOpfsWeb is a shared instance.

When to use OPFS instead of fileSystemWeb:

• OPFS uses the browser's native file storage, well suited for large files and binary content.
• fileSystemWeb (IndexedDB) supports links and random access, OPFS does not (fs.supportsLink, fs.supportsFileLink and fs.supportsRandomAccess are false, file.open() throws UnsupportedError).

A file system can also be rooted at any JS FileSystemDirectoryHandle (File System Access API), for example a directory picked by the user:

import 'package:fs_shim/fs_opfs_web.dart';

/// Call it from a click handler (pickers are Chromium only).
Future<FileSystem> pickDirectory() async {
  final handle = await FileSystemOpfsWeb.showDirectoryPicker(
    const FileSystemOpfsWebShowDirectoryPickerOptions(
      mode: FileSystemOpfsWebShowDirectoryPickerOptions.modeReadWrite,
    ),
  );
  return FileSystemOpfsWeb.withRootHandle(handle);
}

From a JS FileSystemDirectoryHandle of any interop binding, use FileSystemOpfsWeb.withRootHandle(FileSystemOpfsWebDirectoryHandle.fromJsObject(jsHandle)). Write operations need the readwrite permission. Handles do not survive a page reload unless the application saves them (handle.jsObject in IndexedDB) and asks for the permission again (FileSystemOpfsWeb.queryPermission, requestPermission).

Random access

file.open(mode: ...) returns a RandomAccessFile (setPosition, read, writeFrom, writeString, truncate, length...) on io, memory and IndexedDB. As in dart:io, FileMode.write empties an existing file; use FileMode.append to keep the content and move with setPosition.

import 'package:fs_shim/fs_shim.dart';

Future<void> patchHeader(File file) async {
  final raf = await file.open(mode: FileMode.append);
  try {
    await raf.setPosition(0);
    await raf.writeString('v2');
  } finally {
    await raf.close();
  }
}

On the web, prefer a paged file system (getFileSystemWeb()) for random access: without pages each write reads and rewrites the whole file.

Links

link.create(target) takes an absolute target or one relative to the link's directory. fs.type(path) follows links (pass followLinks: false to see the link itself). As on io, a link loop is reported as not found (or as a FileSystemException), and a recursive list(followLinks: true) reports a link to a directory it is already listing as a Link instead of entering it again.

Sandbox

fs.sandbox(path: dirPath) (or dir.sandbox()) returns a FileSystem whose root / is that directory: every path, .. included, stays inside it. file.unsandbox() gives the entity in the original file system.

Utilities

• package:fs_shim/utils/copy.dart: copy a file or a directory tree, even between two file systems (copyDirectory, copyFile, CopyOptions with include/exclude globs).
• package:fs_shim/utils/read_write.dart: writeString, writeLines, readString, readLines (creating the parent directory), dir.emptyOrCreate().
• package:fs_shim/utils/glob.dart: lightweight glob (**, * and ? in a posix style path).
• package:fs_shim/utils/path.dart: convert between posix and windows paths.

Testing

Write the code under test against a FileSystem and run it on a new memory file system, then on the real one:

import 'package:fs_shim/fs_memory.dart';
import 'package:test/test.dart';

Future<int> countWords(File file) async =>
    (await file.readAsString()).split(' ').length;

void main() {
  late FileSystem fs;
  setUp(() => fs = newFileSystemMemory());
  test('countWords', () async {
    final file = fs.file('/doc.txt');
    await file.writeAsString('one two three');
    expect(await countWords(file), 3);
  });
}

To use the latest code from git:

dependencies:
  fs_shim:
    git:
      url: https://github.com/tekartik/fs_shim.dart
      path: fs

Limitations

• Asynchronous API only, a subset of dart:io (no Process, no watch, no permissions other than the executable bit on io).
• On Windows, file links are not supported (fs.supportsFileLink is false) and directory link targets are absolute.
• On the web, IndexedDB and OPFS storage are limited by the browser quota.
• OPFS: no links and no random access.

Source code: github.com/tekartik/fs_shim.dart