尝试用 aiohttp 写爬虫，但这么写不知道该怎么停止循环？

使用 aiohttp 试着写了一个爬虫，但是发现可能会出现 一级页面还在抓取的时候，由于队列为空，直接退出的情况。不知该如何去做这个判断？另外不知以下代码这么写是否有其他的问题？？

# coding:utf-8

import asyncio

import aiohttp


class Task(object):
    def __init__(self, info, priority):
        self.priority = priority
        self.info = info

    def __lt__(self, other):
        return self.priority < other.priority


class Spider(object):

    def __init__(self, loop=None):
        self.loop = loop
        conn = aiohttp.TCPConnector(limit=3)
        self.session = aiohttp.ClientSession(loop=loop, connector=conn)
        self.queue = asyncio.PriorityQueue()

    def start(self, page):
        task_info = {'callback': self.parse_1, 'page': page}
        return task_info

    async def set_item(self, task):
        pass

    async def fetch(self, task):
        await asyncio.sleep(2)
        task['callback'](task['page'])

    async def worker(self):
        while True:
            next_task = await self.queue.get()
            if next_task.info.get('type') == 'item':
                asyncio.ensure_future(self.set_item(next_task.info))
            else:
                asyncio.ensure_future(self.fetch(next_task.info))
            self.queue.task_done()

            # if self.queue.empty():
            #     await asyncio.sleep(1)
            #     if self.queue.empty():
            #         break

    def run(self):
        for page in range(1, 10):
            self.queue.put_nowait(Task(self.start(page), 0))

        self.loop.run_until_complete(self.worker())

    def close(self):
        if not self.session.closed:
            if self.session._connector_owner:
                self.session._connector.close()
            self.session._connector = None

    def parse_1(self, meta):
        print('parse_1-----', meta)
        for page in range(20, 30):
            task = {'callback': self.parse_2, 'page': page}
            self.queue.put_nowait(Task(task, 1))

    def parse_2(self, meta):
        print('parse2----', meta)
        for page in range(30, 40):
            task = {'callback': self.parse_3, 'page': page}
            self.queue.put_nowait(Task(task, 0))

    def parse_3(self, meta):
        print('parse3----', meta)


loop = asyncio.get_event_loop()
sp = Spider(loop=loop)
sp.run()
sp.close()