asynchronous - What's the difference between returning void vs returning Future<void>?

Question

Is there a difference between an async method that returns void, and one that returns Future<void>? It seems that both are valid in Dart:

void main() async {
    await myVoid();
    await myFutureVoid();
}


void myVoid() async {
    // Do something
}


Future<void> myFutureVoid() async {
    // Do something
}

Are they identical?

If so, why is void allowed when for example int is not? The compiler says "Functions marked 'async' must have a return type assignable to 'Future'".

Answer 1

void f() and Future<void> f() are not identical. (The presence of the async keyword doesn't actually matter. The async keyword primarily enables the use of the await keyword in the function body.)

void f() declares a function that returns nothing. If it does asynchronous work, then that work will be "fire-and-forget": there is no opportunity for the caller of f to wait for it to finish.

In contrast, Future<void> f() declares a function that returns a Future that the caller can wait for (either by using await or by registering a Future.then() callback). There's no value returned by the asynchronous work, but callers can determine when it is finished.

Functions marked with async usually should return a Future. If you have a function that does asynchronous work that produces an actual value (such as an int), then the caller must wait for that value to be computed before it can be used. That function therefore must return a Future.

As a special case, an async function can return void instead of Future<void> to indicate that it is fire-and-forget.