What happens when I return a Future from a function marked as async in Dart?

Question

Consider this example:

Future<int> doAsyncThing() => new Future.value(42);

usingAsync() async => doAsyncThing();

main() {
  var thing = usingAsync();
  // what is the runtimeType of thing ?
}

What is the runtimeType of the object returned by usingAsync() ? Would it be Future<Future<int>> or Future<int> or something else ?

Answer 1

The return type of usingAsync() is technically dynamic because there is no return type annotation given for usingAsync(). Omitting a return type annotation is the same as using dynamic for the return type annotation.

The runtimeType of the object returned by usingAsync() is Future<dynamic>. Functions marked with async simply always return a Future<dynamic> object.

When the Future from usingAsync() completes, it "flattens" its contained Future, and completes with an int.

import 'dart:async';

Future<int> doAsyncThing() => new Future.value(42);

usingAsync() async => doAsyncThing();

main() {
  var thing = usingAsync();

  // what is the runtimeType of thing ?
  print(thing.runtimeType); // Future

  thing.then((value) {
    print(value);  // 42
    print(value.runtimeType);  // int
  });
}

The author of the code sample in the original question probably wants to write:

Future<int> usingAsync() async => await doAsyncThing();

Or, even cleaner:

Future<int> usingAsync() => doAsyncThing();

That is, if your function returns a Future, you might not need to mark your function as async. Just return the Future.

See it in action: https://dartpad.dartlang.org/73fed0857efab196e3f9