Dart: async abstract method

Question

I'm trying to design an interface that abstracts long-running operation that should not be used from UI directly. To abstract it, I've created an abstract class with the only method to perform such an operation:

abstract class MakeSomething {

  Result make(Param param);

}

However I can't mark it as async (tried to place in before the signature, before the return type and before the semicolon). Is it possible, and, if yes - how?

Answer 1

async functions almost always must return a Future. (An uncommon exception is async functions may have a void return type to be "fire-and-forget"; in such cases, there is no automatic way for the caller to be notified when the function completes.)

If you want your make function to be asynchronous and to provide a Result to the caller, it must return Future<Result>.

Note that async is not part of the function's type signature; async is a contextual keyword that enables the use of await inside the body of the function. That means that async is not very useful when declaring an abstract interface. The important part is that the function returns a Future, and derived classes can choose whether to implement that function using async/await.