def tasks(self):
        """Produce events until stopped"""
        self.running = True

        while self.running:
            self.update_event.clear()

            if not len(self.heap):
                self.update_event.wait()

            runtime, delay, queue = heappop(self.heap)
            now = time()
            if now < runtime:
                self.update_event.wait(runtime - now)
            else :
                try :
                    task = queue.get_nowait()
                except Queue.Empty:
                    continue # no pushback
                else :
                    yield task
                    queue.task_done()

            self.add(delay, queue)