Handling Futures

Handling Futures

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

• Use async and await.
• Use the Future API, as describedin 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 awaitto wait for the result of an asynchronous function:

await lookUpVersion();

To use await, code must be in an async function—afunction 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 (details). 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 usesawait:

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 timesfor 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()}');
}