:meth:`.WPollingThreadTask._polling_iteration` implementation

def _polling_iteration(self):
		""" :meth:`.WPollingThreadTask._polling_iteration` implementation
		"""
		if len(self.__task_chain) > 0:
			if self.__current_task is None:
				self.__current_task = 0

			task = self.__task_chain[self.__current_task]
			if task.thread() is None:
				task.start()
			elif task.ready_event().is_set() is True:
				task.stop()
				if self.__current_task < (len(self.__task_chain) - 1):
					self.__current_task += 1
				else:
					self.ready_event().set()
			elif task.exception_event().is_set() is True:
				raise RuntimeError('Child thread failed')
		else:
			self.ready_event().set()