how to invoke async function in sync function in flutter

Question

Is it possible to invoke async function in flutter sync function without add the async keyword to parent function? I want to invoke the async function like this:

 void getEventsForDay(DateTime day) {
    var result = await TaskProvider.getTasksByRangeDate(monthStartMilliseconds, monthEndMilliseconds); 
 }

I could not add async keywords in the function getEventsForDay because the outer component did not allow. But I still need to wait the getTasksByRangeDate return to load some initial data. what should I do to invoke an async function without add async keywords in parent function? BTW, this is the full code:

  List<TodoTask> getEventsForDay(DateTime day) {
    int monthStartMilliseconds = DateTimeUtils.startOfMonthMilliseconds(DateTime.now());
    int monthEndMilliseconds = DateTimeUtils.endOfMonthMilliseconds(DateTime.now());
    var result = await TaskProvider.getTasksByRangeDate(monthStartMilliseconds, monthEndMilliseconds);
    tasks.addAll(result);
    buildHashMap(result);
    final DateFormat formatter = DateFormat('yyyy-MM-dd');
    String dateString = formatter.format(day);
    List<TodoTask> dayTodoTask = taskMap[dateString] ?? [];
    return dayTodoTask;
  }

I have already tried using then like this:

 List<TodoTask> getEventsForDay(DateTime day) {
    int monthStartMilliseconds = DateTimeUtils.startOfMonthMilliseconds(DateTime.now());
    int monthEndMilliseconds = DateTimeUtils.endOfMonthMilliseconds(DateTime.now());
    TaskProvider.getTasksByRangeDate(monthStartMilliseconds, monthEndMilliseconds).then((value) => {getTasks(value, day)});
  }

  List<TodoTask> getTasks(List<TodoTask> tasks, DateTime day) {
    tasks.addAll(tasks);
    buildHashMap(tasks);
    final DateFormat formatter = DateFormat('yyyy-MM-dd');
    String dateString = formatter.format(day);
    List<TodoTask> dayTodoTask = taskMap[dateString] ?? [];
    return dayTodoTask;
  }

seems could not return the task list to outer layer funciton. I have tried like this:

List<TodoTask> getEventsForDay(DateTime day) {
    var taskResult;
    int monthStartMilliseconds = DateTimeUtils.startOfMonthMilliseconds(DateTime.now());
    int monthEndMilliseconds = DateTimeUtils.endOfMonthMilliseconds(DateTime.now());
    TaskProvider.getTasksByRangeDate(monthStartMilliseconds, monthEndMilliseconds)
        .then((value) => {() => taskResult = getTasks(value, day)});
    return taskResult ?? [];
  }

  List<TodoTask> getTasks(List<TodoTask> tasks, DateTime day) {
    tasks.addAll(tasks);
    buildHashMap(tasks);
    final DateFormat formatter = DateFormat('yyyy-MM-dd');
    String dateString = formatter.format(day);
    List<TodoTask> dayTodoTask = taskMap[dateString] ?? [];
    return dayTodoTask;
  }

but the taskResult result always be Null.

Answer 1

Yes, it is possible to call async code from a synchronous function !

It will just be executed on another thread.

Method 1 (Clean)

What we usually do is create an async function and call it from the synchronous one :

import 'dart:async';

void main() {
  
  doSomething();
  
  print('Hello');
  
}

Future doSomething() async {
  await Future.delayed(const Duration(seconds: 2))
        .then((_) => print("Oops I am late"));
}

Method 2 (Fast)

import 'dart:async';

void main() {
  
  unawaited(() async {
    await Future.delayed(const Duration(seconds: 2))
        .then((_) => print("Oops I am late"));
  }.call());
  
  print('Hello');
  
}

PS : If you don't want to call unawaited(), you can just remove it, but it is not recommended. After you remove it, it would just look like this :

() async {
      // Do something asyncrhronous
}.call();

Conclusion

Try the code on the DartPad !

You can see that we don't use async in the method main() for both cases. I will let you choose which one you prefer. I personally use the Method 2 for debugging purposes.