flutter async to sync programming

Question

It's probably just me but asynchronous programming in flutter seems to be an absolute nightmare. Something really simple in javascript/typescript just seems to become unnecessarily hard. for example, I'm trying to make a simple function that checks if a user is authenticated:

Future<bool> isLoggedIn() async {
  var user = await FirebaseAuth.instance.currentUser();
  return user == null ? false : true;
}

and then use this function in a scenario like this:

Widget _buildChild() async {
    var auth = await user.isLoggedIn();
    if (auth) {
      return new Navigation();
    } else {
      return new LoginUI();
    }
}

But then the second function also needs to return a future?

Functions marked 'async' must have a return type assignable to 'Future'.

using then() instead of await doesn't work either. I've run into this issue several times before when using asynchronous programming in a synchronous context. maybe it's just the that it looks and feels very similar to Promises that I'm totally missing it's functionality from the docs.

I feel your pain.

Chris Nadovich
– Chris Nadovich

2023-02-25 00:49:25 +00:00
Commented Feb 25, 2023 at 0:49 — Chris Nadovich
– Chris Nadovich, Commented Feb 25, 2023 at 0:49

Andrii Turkovskyi · Accepted Answer · 2019-10-16 14:36:44Z

6

You can change your code this way:

Widget _buildChild() {
  return FutureBuilder(builder: (context, AsyncSnapshot<bool> snapshot) {
    if (snapshot.hasData)
      return snapshot.data ? Navigation() : LoginUI();
    else
      return Container();
  },
  future: user.isLoggedIn(),);
}

It returns widget synchronously. If there is no data yet - it returns empty Container, and when isLoggedIn() returns value - this method will return needed widget

answered Oct 16, 2019 at 14:36

Andrii Turkovskyi

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3 Comments

cas Over a year ago

it works! awesome quick response. but how does this work, or is there any good documentation out there that explains futurebuilders in more detail?

Lukas Over a year ago

look at this explanation: medium.com/nonstopio/…

Andrii Turkovskyi Over a year ago

For handling some async operations I'd recommend to look FutureBuilder and StreamBuilder on the official site api.flutter.dev/flutter/widgets/widgets-library.html

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