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 simples t 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);
  }
}