Dart - Distinguish between sync and async void callbacks

Question

Suppose we have the following method:

Future<String> foo(void callback()) async{
    ...
    callback();
    ...
}

Then the callback is allowed to be either sync or async when defined. Is there a clean way to await it if it is async? Now if I await it here I get this warning from IDE:

'await' applied to 'void', which is not a 'Future'

Answer 1

You can't. The whole point of making an async function return void instead of Future<void> is so that the function is "fire-and-forget" and so that the caller can't wait for it to complete.

If you want to allow your foo function to take asynchronous functions as arguments, then its parameter should be a Future<void> Function() instead. That way callers can easily adapt a synchronous function with an asynchronous wrapper. (The reverse is not possible.)

Alternatively you could make the callback parameter return a FutureOr<void>:

Future<String> foo(FutureOr<void> Function callback) async {
    ...
    await callback();
    ...
}

or if you don't want an unnecessary await if callback is synchronous:

Future<String> foo(FutureOr<void> Function callback) async {
    ...
    if (callback is Future<void> Function) {
      await callback();
    } else {
      callback();
    }
    ...
}