Futures and async/await in Dart

async marks the function; await pauses it, not the program.

// async marks the function; await pauses it without blocking the
// program's single thread.
Future<String> fetchForecast() async {
  await Future.delayed(const Duration(milliseconds: 50));
  return 'SW 12 kts, clearing';
}

Future<void> main() async {
  print('Requesting forecast...');
  final forecast = await fetchForecast();
  print('Forecast: $forecast');

  // Futures also chain without await.
  await fetchForecast().then((f) => print('Again: $f'));
}

How it works

1. Future<String> is a value that will arrive; async enables await.
2. await fetchForecast() reads like sync code.
3. .then chains the same future without await.

Keywords and builtins used here

• String
• async
• await
• const
• final
• return
• void

The run, in numbers

Lines

15

Characters to type

447

Tokens

87

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 75 seconds.

Type this snippet

Step 1 of 3 in Async, step 22 of 27 in Language basics.

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