How to call and wait async function done inside sync function in flutter

Question

I want to call and wait async function done before return from a sync function

// async function
Future<User> getUser(String username) async {
   ...
}

In dart, i could use https://api.dart.dev/stable/2.9.2/dart-cli/waitFor.html to wait a async function before go to next statement.

bool checkUser(String username, String encodedPwd) {
    var user = waitFor<User>(getUser(username));
    if (user.pwd == encodedPwd) 
       return true;
    else
       return false;
}

Because the require of the framework, the checkUser function will be call by framework, and must be a sync function.

In flutter, I could not use dart:cli, I implement by pattern .then() .whenComplete(), but when call checkUser the statement print(1) will be call and end the function without wait for getUser finish.

bool checkUser(String username, String pwd) {
  getUser(username).then((user) { 
     if (user.pwd == encodePwd(pwd)) {
        return true;
     } else {
        return false;
     }
 );
 print(1);
}

How to call async function inside sync function and wait the async function done before return?

The function getUser() is a Future. So obviously the sync statement print(1) will be executed first. — Prasanna Kumar
– Prasanna Kumar, Commented Nov 29, 2020 at 16:33
if you need to wait for a async function to complete you need to use await , and you can only use the await inside an async function, i guess you are thinking of some over engineering here — Yadhu
– Yadhu, Commented Nov 30, 2020 at 6:56

MickaelHrndz · Accepted Answer · 2020-11-30 06:21:09Z

1

Being able to do what you ask would basically render the distinction between sync and async functions useless (and block the main thread I think). The function you linked "should be considered a last resort".

I think what you want is :

Future<bool> checkUser(String username, String pwd) async { 
  var user = await getUser(username); 
  return user.pwd == encodePwd(pwd) ? true : false;
}

answered Nov 30, 2020 at 6:21

MickaelHrndz

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