let shouldStart = _lock.calculateLocked { () -> Bool in
            self._queue.enqueue(event)

        switch self._state {
        case .stopped:
            self._state = .running
            return true
        case .running:
            return false
        }
    }

let (nextEvent, observer) = self._lock.calculateLocked { () -> (Event<E>?, O) in
        if self._queue.count > 0 {
            return (self._queue.dequeue(), self._observer)
        }
        else {
            self._state = .stopped
            return (nil, self._observer)
        }
    }