Why does this Flutter example not have a return keyword?

Question

I am reading over the documentation for Flutter.

On this page, I have observed the following curious method. In the method, the declared return type is Future. The method though, does not have the return keyword present anywhere. Why is this?

Future<void> _incrementCounter() async {
  setState(() {
    _counter++;
  });
  Directory directory = await getApplicationDocumentsDirectory();
  final String dirName = directory.path;
  await File('$dir/counter.txt').writeAsString('$_counter');
}

I have been able to ascertain that all flutter functions return a value, and the default return value is null. But if this method always returns null, then why declare a return type of Future<void>?

Rémi Rousselet Rémi Rousselet · Accepted Answer · 2019-06-20T16:08:07

That is because the function is marked with the async modifier:

Future foo() async {
  print('hello world');
}

is equivalent to

Future foo() {
  try {
    print('hello world');
    return Future.value(null);
  } catch (err) {
    return Future.error(err);
  }
}

Why does this Flutter example not have a return keyword?

2 Answers