简单理解 Python EventLoop 事件循环

简介

在 python 3中，加入了 asyncio 模块，来实现协程，其中一个很重要的概念是事件循环，整个异步流程都是事件循环推动的。下面自己实现一个相对简单的EventLoop，了解一下事件循环是如何进行运转的。

事件循环

下面看一下整个流程的实现过程

将以下代码写入 spider_event_loop.py 文件：

# spider_event_loop.py

import time
import os
import socket
from urllib.parse import urlparse
from collections import deque
from selectors import DefaultSelector, EVENT_WRITE, EVENT_READ

# selector = DefaultSelector()
urls = ['https://p9-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/9bc51bc53f634bf79b5de5c8b9810817~tplv-k3u1fbpfcp-watermark.image',
        'https://p1-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/2b95ac8571ba403180743495ed56e492~tplv-k3u1fbpfcp-watermark.image',
        'https://p6-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/459c6c73887a4206a33b13ef23988809~tplv-k3u1fbpfcp-watermark.image',
        'https://p6-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/80701ade82d345cd8aa0b08fef008fe1~tplv-k3u1fbpfcp-watermark.image',
        'https://p6-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/99855ed1f79c456f9fdcd83ce5cff4f2~tplv-k3u1fbpfcp-watermark.image',
        'https://p1-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/0c4bd38c2fe3434587025748d051362e~tplv-k3u1fbpfcp-watermark.image',
        'https://p1-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/80cd2a1d165b43beb6234d893ce391e2~tplv-k3u1fbpfcp-watermark.image',
        'https://p3-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/ea18df95f26f4314a0a36d11d4a067d0~tplv-k3u1fbpfcp-watermark.image',
        'https://p1-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/095c342a796c411cad1800396746bdaf~tplv-k3u1fbpfcp-watermark.image',
        'https://p6-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/d3769e4d468a4f64a1e2879f94ad742b~tplv-k3u1fbpfcp-watermark.image'
]

# 事件循环实现
class EventLoop:

    def __init__(self):
     	# 整个流程是否
        self._stopped = False
        self._ready = deque()
        self._selector = DefaultSelector()

    # 向 _ready 队列中加入回调方法
    def call_soon(self, callback, *args):
        # 将事件添加到队列里
        handle = Handle(callback, self, *args)
        self._ready.append(handle)

    # 套接字注册读事件
    def add_writer(self, fd, callback, *args):
        # 将回调方法封装成Handle
        handle = Handle(callback, self, *args)
        
        try:
            # 检查有没有注册过
            key = self._selector.get_key(fd)
        except KeyError:
            # 没有注册过，进行写事件注册
            self._selector.register(fd, EVENT_WRITE, handle)
        else:
            # 注册过，添加写事件
            mask = key.events
            self._selector.modify(fd, mask | EVENT_WRITE, handle)
    
    # 套接字注销读事件
    def remove_writer(self, fd):
        try:
            # 检查有没有注册过
            self._selector.get_key(fd)
        except KeyError:
            # 没有，直接返回
            return
        # 注销事件
        self._selector.unregister(fd)

    # 套接字注册写事件
    def add_reader(self, fd, callback, *args):
        # 将回调方法，封装成Handle
        handle = Handle(callback, self, *args)
        try:
            # 检查是否注册过
            key = self._selector.get_key(fd)
        except KeyError:
            # 没有注册过，进行读事件注册
            self._selector.register(fd, EVENT_READ, handle)
        else:
            # 注册过，添加读事件
            mask = key.events
            self._selector.modify(fd, mask | EVENT_READ, handle)

    # 套接字注销写事件
    def remove_reader(self, fd):
        try:
            # 检查有没有注册过
            self._selector.get_key(fd)
        except KeyError:
            # 没有，直接返回
            return
        # 注