Step 1 - Read and write with await

import 'dart:io';

Future<void> main() async {
  final f = File('async_demo.txt');
  await f.writeAsString('one\n');
  final text = await f.readAsString();
  print(text.trim());
}

Step 2 - Create directory asynchronously

import 'dart:io';

Future<void> ensureDataDir() async {
  final dir = Directory('data');
  if (!await dir.exists()) {
    await dir.create(recursive: true);
  }
}

Future<void> main() async {
  await ensureDataDir();
  final f = File('data/note.txt');
  await f.writeAsString('ok');
}

Step 3 - Error handling

import 'dart:io';

Future<String?> readOptional(File f) async {
  try {
    if (!await f.exists()) return null;
    return await f.readAsString();
  } on FileSystemException catch (e) {
    print(e.message);
    return null;
  }
}

Good habit

• Propagate Future from the entrypoint (main async) or use then; avoid mixing sync and async for the same file without a clear reason.

Practice tasks

• Rewrite ensureFileWithDefault from the sync guide using exists() and writeAsString.
• Use readAsString with try/catch and return Result<String> from Level 6.