Completer<T> class abstract interface

A way to produce Future objects and to complete them later with a value or error.

Most of the time, the simplest way to create a future is to just use one of the Future constructors to capture the result of a single asynchronous computation:

Future(() { doSomething(); return result; }); 

or, if the future represents the result of a sequence of asynchronous computations, they can be chained using Future.then or similar functions on Future:

Future doStuff(){ return someAsyncOperation().then((result) { return someOtherAsyncOperation(result); }); } 

If you do need to create a Future from scratch — for example, when you're converting a callback-based API into a Future-based one — you can use a Completer as follows:

class AsyncOperation { final Completer _completer = new Completer(); Future<T> doOperation() { _startOperation(); return _completer.future; // Send future object back to client. } // Something calls this when the value is ready. void _finishOperation(T result) { _completer.complete(result); } // If something goes wrong, call this. void _errorHappened(error) { _completer.completeError(error); } }