Async and Practical

async/await

Pause and Resume

async marks a function that returns a Future; await pauses the surrounding async function until the future is ready, then resumes with the value. Future.value(x) produces an already-completed future, so the example is deterministic and free of timing.

Program

Play the program to await two futures in sequence and watch each resume bind its result.

async_await.dart
Replay: real traced execution (multi-file project)
Future<int> getCount() {
  return Future.value(7);
}

Future<int> twice(int n) async {
  return n * 2;
}

Future<void> main() async {
  var a = await getCount();
  var b = await twice(a);
  print('$a $b');
}

  1. awaiting ← getCount()

    9Future<void> main() async {10  var a = await getCount();11  var b = await twice(a);
    values this stepgetCount()awaiting

  2. return value ← Future.value(7)

    1Future<int> getCount() {2  return Future.value(7);3}
    values this stepFuture.value(7)return value

  3. resumed ← a = 7

    9Future<void> main() async {10  var a = await getCount();11  var b = await twice(a);
    values this stepa = 7resumed

  4. awaiting ← twice(7)

    10var a = await getCount();11var b = await twice(a);12print('$a $b');
    values this steptwice(7)awaiting7a

  5. return value ← 14

    5Future<int> twice(int n) async {6  return n * 2;7}
    values this step14return value7n

  6. resumed ← b = 14

    10var a = await getCount();11var b = await twice(a);12print('$a $b');
    values this stepb = 14resumed

  7. print('$a $b');

    11  var b = await twice(a);12  print('$a $b');13}
    output7 14
    values this step7a14b
async function An `async` function (like `twice`) returns a `Future<T>` automatically; `return n * 2` becomes `Future.value(n * 2)`.
Future.value `Future.value(7)` is an already-completed future, so the example needs no timers, files, network, or external events.
pause and resume Each `await fn()` suspends `main` until the future is ready, then resumes with the value bound into a local.