CODEWITHSUNDEEP
dart
Chris G.
Chris G.

Reputation: 25974

Why is await not waiting

In the following, I would expect:

This output:

Start sleep1
End sleep1
Start sleep2
End sleep2

But I get output:

Start sleep1
Start sleep2
End sleep1
End sleep2

Code:

Future printSleep1() async{
  print('Start sleep1');
  return await new Future.delayed(const Duration(seconds: 1), () => print('End sleep1'));
}

Future printSleep2() async{
  print('Start sleep2');
  return new Future.delayed(const Duration(seconds: 1), () => print('End sleep2'));
}

main() {
    printSleep1();
  printSleep2();
}

Also when not returning a Future:

printSleep1() async{
  print('Start sleep1');
  await new Future.delayed(const Duration(seconds: 1), () => print('End sleep1'));
}

printSleep2() async{
  print('Start sleep2');
  new Future.delayed(const Duration(seconds: 1), () => print('End sleep2'));
}

main() {
    printSleep1();
  printSleep2();
}

Upvotes: 0

Views: 118

Answers (1)

Richard Heap
Richard Heap

Reputation: 51732

Both printSleep1 and printSleep2 return Futures, so they don't necessarily start execution when you call them. (Dart 2 changes this subtly, in that they execute up to the first await, but that's not relevant here.)

Change you main to:

main() async {
  await printSleep1();
  await printSleep2();
}

so that printSleep2 won't get called until after printSleep1's Future completes.

Upvotes: 2

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