CODEWITHSUNDEEP
flutterdartasynchronousasync-awaitfuture
SGhaleb
SGhaleb

Reputation: 1007

Wait until function returns true

Does Dart have a method to wait for a function to return a true/false then call the next function after it?

I have seen similar questions here, but I cannot seem to get them to work in a way to achieve my goal. Some of the answers use explicit delayed time (.e.g wait 3 seconds). But this doesn't work for my method.

E.g.

print('START');
await fetchResults(); 
print('COMPLETE'); // This is not called until the fetchResults returns true/false

NOTE: I have tried something like:

Future<bool> fetchResults() { 
   await Future.doWhile(() => return true);
}

But this puts my function in infinite loop.

Upvotes: 2

Views: 8388

Answers (2)

Farhan Syah
Farhan Syah

Reputation: 903

You need to use two async function for that


Future<void> execute()async {
  print('START');
  final result = await fetchResults(); 
  print('COMPLETE'); // This is not called until the fetchResults returns true/false
}



Future<bool> fetchResults() async { 

   // Your fetchResult logic here
   // e.g: 

   final int result = 1+1;
   print('In between'); 
   if(result == 2) return true;
   else return false;
}

Output

START
In between
COMPLETE

Upvotes: 1

Peter Koltai
Peter Koltai

Reputation: 9744

In an async function you can use await to wait for a function call to be completed and get its result. This will wait 3 seconds before printing COMPLETE:

void waitForIt async {
  print('START');
  if (await (fetchResults())) {
    print('COMPLETE');
  }
}

Future<bool> fetchResults() async {
  return Future.delayed(const Duration(milliseconds: 3000), () {
    return true;
  });
}

But it is also possible that your fetchResults is not async, but sync. The following produces the same 3 seconds delay:

import 'dart:io';

bool fetchResults() {
  sleep(Duration(seconds: 3));
  return true;
}

Upvotes: 2

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