Asynchronous iterable mapping in Dart

Question

Can I map some Iterable using async mapping function? Maybe it is a bug, that this code prints list of _Future imidiately, not ints after 1 or 5 seconds?

import 'dart:async';

Future<int> foo(int i) {
  var c = new Completer();
  new Timer(new Duration(seconds: 1), () => c.complete(i));
  return c.future;
}

main() {
  var list = [1,2,3,4,5];
  var mappedList = list.map((i) async => await foo(i));
  print(mappedList);
}

Answer 1

The expression (i) async => await foo(i) still returns a future. You can use Future.wait(mappedList) to wait till all created futures are completed.

Answer 2

Your misunderstanding is that async functions return a Future, not a value. await does not convert async to sync.

var mappedList = list.map(
  (i) async => await foo(i) // Returns a Future, not an int
);

You are printing are the Futures returned by (i) async => await foo(i).

Those Futures complete when the chain of Futures within them complete. When the Timer fires: foo() completes, then await foo(i), then your mapping function.

Compare with:

main() async {
  List<int> list = [1,2,3,4,5];
  Iterable<Future<int>> mapped;

  // Prints ints 1 second apart
  mapped = list.map((i) => foo(i));
  for(Future<int> f in mapped) {
    print(await f);
  }

  // Prints ints all at once, after 1 second wait
  mapped = list.map((i) => foo(i));
  for(Future<int> f in mapped) {
    f.then(print);
  }
}

On Dartpad: https://dartpad.dartlang.org/151949be67c0cdc0c54742113c98b291

Some things to note:

List.map() returns a lazy Iterable (not a List) which means the mapping function isn't called until the Iterable is iterated through.

The first loop waits for each Future to complete before printing and moving on to the next item in the Iterable, the mapping function for the next item (and hence foo()) is called after printing each value, so values are printed at 1 second intervals.

The second loop iterates through the Iterable immediately, setting up a print function to execute after each Future completes. 5 instances of function foo() are called at once, which all return approximately 1 second later, then all 5 values are printed.