Async and Practical

Futures

async and await

An async function returns a Future. await suspends until that future completes.

Program

Play the program to await an asynchronous name and greet.

futures.dart
Replay: real traced execution (multi-file project)
Future<String> fetchName() async {
  await Future.delayed(Duration.zero);
  return 'Ada';
}

Future<void> main() async {
  var name = await fetchName();
  print('Hello, $name');
}

  1. call ← await fetchName()

    6Future<void> main() async {7  var name = await fetchName();8  print('Hello, $name');
    values this stepawait fetchName()call

  2. awaiting ← Future.delayed(zero)

    1Future<String> fetchName() async {2  await Future.delayed(Duration.zero);3  return 'Ada';
    values this stepFuture.delayed(zero)awaiting

  3. return value ← Future<String> -> Ada

    2  await Future.delayed(Duration.zero);3  return 'Ada';4}
    values this stepFuture<String> -> Adareturn value

  4. name ← Ada

    6Future<void> main() async {7  var name = await fetchName();8  print('Hello, $name');
    values this stepAdaname

  5. print('Hello, $name');

    7  var name = await fetchName();8  print('Hello, $name');9}
    outputHello, Ada
    values this stepAdaname
async An `async` function returns a `Future` automatically.
await `await` pauses the function until the future resolves.
event loop `Future.delayed(Duration.zero)` yields to the event loop.