Asynchrony support

Dart libraries are full of functions that return Future or Stream objects. These functions are asynchronous: they return after setting up a possibly time-consuming operation (such as I/O), without waiting for that operation to complete.

The async and await keywords support asynchronous programming, letting you write asynchronous code that looks similar to synchronous code.

Handling Futures

When you need the result of a completed Future, you have two options:

• Use async and await, as described here and in the asynchronous programming codelab.
• Use the Future API, as described in the library tour.

Code that uses async and await is asynchronous, but it looks a lot like synchronous code. For example, here’s some code that uses await to wait for the result of an asynchronous function:

await lookUpVersion();

To use await, code must be in an async function—a function marked as async:

Future<void> checkVersion() async {
  var version = await lookUpVersion();
  // Do something with version
}

Use try, catch, and finally to handle errors and cleanup in code that uses await:

try {
  version = await lookUpVersion();
} catch (e) {
  // React to inability to look up the version
}

You can use await multiple times in an async function. For example, the following code waits three times for the results of functions:

var entrypoint = await findEntryPoint();
var exitCode = await runExecutable(entrypoint, args);
await flushThenExit(exitCode);

In await expression, the value of expression is usually a Future; if it isn’t, then the value is automatically wrapped in a Future. This Future object indicates a promise to return an object. The value of await expression is that returned object. The await expression makes execution pause until that object is available.

If you get a compile-time error when using await, make sure await is in an async function. For example, to use await in your app’s main() function, the body of main() must be marked as async:

void main() async {
  checkVersion();
  print('In main: version is ${await lookUpVersion()}');
}

For an interactive introduction to using futures, async, and await, see the asynchronous programming codelab.

Declaring async functions

An async function is a function whose body is marked with the async modifier.

Adding the async keyword to a function makes it return a Future. For example, consider this synchronous function, which returns a String:

String lookUpVersion() => '1.0.0';

If you change it to be an async function—for example, because a future implementation will be time consuming—the returned value is a Future:

Future<String> lookUpVersion() async => '1.0.0';

Note that the function’s body doesn’t need to use the Future API. Dart creates the Future object if necessary. If your function doesn’t return a useful value, make its return type Future<void>.

For an interactive introduction to using futures, async, and await, see the asynchronous programming codelab.

Handling Streams

When you need to get values from a Stream, you have two options:

• Use async and an asynchronous for loop (await for).
• Use the Stream API, as described in the library tour.

An asynchronous for loop has the following form:

await for (varOrType identifier in expression) {
  // Executes each time the stream emits a value.
}

The value of expression must have type Stream. Execution proceeds as follows:

1. Wait until the stream emits a value.
2. Execute the body of the for loop, with the variable set to that emitted value.
3. Repeat 1 and 2 until the stream is closed.

To stop listening to the stream, you can use a break or return statement, which breaks out of the for loop and unsubscribes from the stream.

If you get a compile-time error when implementing an asynchronous for loop, make sure the await for is in an async function. For example, to use an asynchronous for loop in your app’s main() function, the body of main() must be marked as async:

void main() async {
  // ...
  await for (final request in requestServer) {
    handleRequest(request);
  }
  // ...
}

For more information about asynchronous programming, in general, see the dart:async section of the library tour.