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.
• Use the Future API.

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

Note: Although an async function might perform time-consuming operations, it doesn’t wait for those operations. Instead, the async function executes only until it encounters its first await expression. Then it returns a Future object, resuming execution only after the await expression completes.

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:

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

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>. 

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.

Note: Before using await for, be sure that it makes the code clearer and that you really do want to wait for all of the stream’s results. For example, you usually should not use await for for UI event listeners, because UI frameworks send endless streams of events.

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:

Future main() async { 
   // ... 
   await for (var request in requestServer) { 
      handleRequest(request); 
   } 
   // ... 
}