how do I return a value from my async function?

Question

So I have an async funtion:

Future decodeToken() async {
  SharedPreferences prefs = await SharedPreferences.getInstance();
  var token = await prefs.getString('token');
  final Map<String, dynamic> payload = json.decode(
    ascii.decode(
      base64.decode(base64.normalize(token.split(".")[1])),
    ),
  );
  return payload;
}

and when I try do do:

var payload = await decodeToken();

it throws an error saying "The await expression can only be used in an async function."

I tried taking out the "await" and print the payload variable but obviously it prints "Instance of Future"

if I do:

decodeToken().then((value)=>print(value))

it prints the value correctly, but I want to use it in a variable, how do I return the vvalue of an async function?

Please do not edit answers into the question. If you have an answer, post an answer to your own question. Since, your question is closed as a duplicate, add an answer to the duplicate if you believe it will help future users. — Christopher Moore, Mar 04 '21 at 15:44

score 2 · Answer 1 · answered Mar 03 '21 at 20:00

The error message is saying you can only use await in a method that is marked as async, so you are calling an asynchronous method from a synchronous one and Dart doesn't like that.

You have several options:

Use the Future...then pattern:

Map<String, dynamic> payload;
decodeToken().then((data) {
  payload = data;
});

Make the surrounding method async:

void foo() async {
  var payload = await decodeToken();
}

Use a FutureBuilder (if you need your future's data in the build method):

Widget build(BuildContext context) {
  return FutureBuilder(
    future: decodeToken(),
    builder: (context, snapshot) {
      if (!snapshot.hasData) {
        // Future hasn't completed yet, display something in the meantime
        return CircularProgressIndicator();
      }
      // Future has completed, get data
      var payload = snapshot.data;
      ...
    },
  );
}

score 0 · Answer 2 · answered Mar 03 '21 at 17:51

Define the return type as `Future<Map<String, dynamic>>`

Future<Map<String, dynamic>> decodeToken() async {
  SharedPreferences prefs = await SharedPreferences.getInstance();
  var token = await prefs.getString('token');
  final Map<String, dynamic> payload = json.decode(
    ascii.decode(
      base64.decode(base64.normalize(token.split(".")[1])),
    ),
  );
  return payload;
}

how do I return a value from my async function?

2 Answers2

Define the return type as Future<Map<String, dynamic>>

Define the return type as `Future<Map<String, dynamic>>`