CODEWITHSUNDEEP
flutterdartasynchronousasync-await
TheKnax
TheKnax

Reputation: 43

How do you correctly await multiple asynchronous methods in dart?

I recently starting developing in flutter. When starting my app I first want to load Data from different sources. I am having trouble with this and it seems like I haven't understood async / await yet.

What I want to do

When starting my App I want to load Data from multiple Sources. This should be done parallel but should only continue once all data sources are fully loaded.

What I tried

import 'package:flutter/material.dart';

class AsyncTester extends StatefulWidget {
  @override
  _AsyncTesterState createState() => _AsyncTesterState();
}

class _AsyncTesterState extends State<AsyncTester> {
  @override
  void initState() {
    super.initState();
    startApplication();
  }

  @override
  Widget build(BuildContext context) {
    return Container();
  }

  void startApplication() async {
    await loadData();
    print("starting Application!");
  }

  loadData() {
    loadDataSource1();
    loadDataSource2();
  }

  void loadDataSource1() async {
    await Future.delayed(Duration(seconds: 3));
    print("Data Source 1 loaded.");
  }

  void loadDataSource2() async {
    await Future.delayed(Duration(seconds: 2));
    print("Data Source 2 loaded.");
  }
}

The output is:

I/flutter (23100): starting Application!
I/flutter (23100): Data Source 2 loaded.
I/flutter (23100): Data Source 1 loaded.

I don't understand why startApplication() does not wait for loadData() to finish. I thought that is exactly what await does?

BTW I am nesting loadDataSource1() and loadDataSource2() in loadData() because doing this

void startApplication() async {
   await loadDataSource1();
   await loadDataSource2();
   print("starting Application!");
}

would load the Data after one another. Is this correct or is there a better way to do this?

Upvotes: 3

Views: 2700

Answers (1)

Guillaume Roux
Guillaume Roux

Reputation: 7308

You should use Future.wait, this method will execute both of your asynchronous operation in parallel and will complete once they are all finished:

void startApplication() async {
  await Future.wait([
    loadDataSource1(),
    loadDataSource2(),
  ]);
  print("starting Application!");
}

Upvotes: 11

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