Completer<T> - интерфейс Completer<T>
Completer is used to produce
Futures and supply their value when it
A service that provides values to callers, and wants to return
Completer as follows:
Completer completer = new Completer(); // send future object back to client... return completer.future; ...
// later when value is available, call: completer.complete(value);
// alternatively, if the service cannot produce the value, it // can provide an exception: completer.completeException(exception);