CODEWITHSUNDEEP
flutterdart
shawnblais
shawnblais

Reputation: 1248

Is there a SynchronousFuture equivalent of `async`

For some reason the compiler is complaining about this:

  SynchronousFuture<void> setNewRoutePath(AppLink newLink) {
    _currentLink = newLink;
    // return null; // compiler really wants to see this null return...why?
  }

But is happy with this:

  Future<void> setNewRoutePath(AppLink newLink) async {
    _currentLink = newLink;
  }

Seems like async keyword is handling the implicit return here. Is there some equivalent for SynchronousFuture?

Upvotes: 1

Views: 954

Answers (2)

jamesdlin
jamesdlin

Reputation: 89936

  SynchronousFuture<void> setNewRoutePath(AppLink newLink) {
  _currentLink = newLink;
  // return null; // compiler really wants to see this null return...why?
}

You need a return value because your function is declared to return a non-void value, so it must return something. You could argue that all functions could implicitly return null if there's no explicit return statement, but that would be error-prone:

int f(String someValue) {
  if (someCondition) {
    return 42;
  }
} // f didn't return anything if someCondition is false. Accidental or intentional?

Futures themselves aren't really special. If you have a non-async function that returns a Future, you must still have an explicit return statement:

Future<void> f() {
  print('Hello world!');
} // Error: f doesn't end with a return statement.

However, the async keyword does a few things:

  • Primarily it enables the use of the await keyword.
  • It automatically wraps returned values in a Future. This includes implicit return values from void functions.

SynchronousFuture is just an ordinary class provided by Flutter that implements the Future interface. It is not special. It's not part of the Dart language, so there is not going to be any keyword that does automatic return value wrapping like what the async keyword does.

Also note that in your function that returns a SynchronousFuture, you should not just sprinkle return null statements. With an async function, a return; statement (or exiting the function without an explicit return statement) ultimately returns a Future<void>() to the caller. Callers expect to be able to call methods (e.g. .then()) on the returned Future to add completion callbacks. If you return null for a SynchronousFuture, calling .then() on it will result in a null pointer exception at runtime. You instead would need to use return SynchronousFuture<void>(null);.

I also should point out that the SynchronousFuture documentation states:

In general use of this class should be avoided as it is very difficult to debug such bimodal behavior.

so unless you have some clear need to use a SynchronousFuture, you're better off avoiding it.

Upvotes: 4

Rohan Thacker
Rohan Thacker

Reputation: 6337

Unless you have created a class called SynchronousFuture the compiler will complain as there is no class called SynchronousFuture in the dart standard library, so either you or a library you import must define this class.

In dart all async functions must have a return value of type Future<T>.

// return null; // compiler really wants to see this null return...why?

Because Future is not the same as void, when you return a value in an async function, that value is subsequently used as value for the future, as future is a generic class.

Upvotes: 4

Related Questions