python 使用事件对象asyncio.Event来同步协程的操作

事件对象asyncio.Event是基于threading.Event来实现的。

事件可以一个信号触发多个协程同步工作，

例子如下：

import asyncio
import functools

def set_event(event):
  print('setting event in callback')
  event.set()

async def coro1(event):
  print('coro1 waiting for event')
  await event.wait()
  print('coro1 triggered')

async def coro2(event):
  print('coro2 waiting for event')
  await event.wait()
  print('coro2 triggered')

async def main(loop):
  # Create a shared event
  event = asyncio.Event()
  print('event start state: {}'.format(event.is_set()))

  loop.call_later(
    0.1, functools.partial(set_event, event)
  )

  await asyncio.wait([coro1(event), coro2(event)])
  print('event end state: {}'.format(event.is_set()))

event_loop = asyncio.get_event_loop()
try:
  event_loop.run_until_complete(main(event_loop))
finally:
  event_loop.close()

输出如下：

event start state: False
coro2 waiting for event
coro1 waiting for event
setting event in callback
coro2 triggered
coro1 triggered
event end state: True

补充知识： python里使用协程来创建echo客户端

在这个例子里使用asyncio.Protocol来创建一个echo客户端，先导入库asyncio和logging。

接着定义发送的消息MESSAGES。

创建连接服务器的地址SERVER_ADDRESS，接着创建EchoClient类，它是继承asyncio.Protocol。

在这个类的构造函数里，接收两个参数messages和future，

messages是指定要发送的消息数据，future是用来通知socket接收数据完成或者服务器关闭socket的事件通知，以便事件循环知道这个协程已经完成了，就可以退出整个程序。

connection_made函数是当socket连接到服务器时调用，它就立即发送数据给服务器，数据发送完成之后发送了eof标记。

服务器收到数据和标志都回复客户端，客户端data_received函数接收数据，eof_received函数接收结束标记。

connection_lost函数收到服务器断开连接。

这行代码：

client_completed = asyncio.Future()

创建一个协程完成的触发事件。

由于event_loop.create_connection函数只能接收一个参数，需要使用functools.partial来进行多个参数包装成一个参数。

后面通过事件循环来运行协程。

import asyncio
import functools
import logging
import sys

MESSAGES = [
  b'This is the message. ',
  b'It will be sent ',
  b'in parts.',
]
SERVER_ADDRESS = ('localhost', 10000)

class EchoClient(asyncio.Protocol):

  def __init__(self, messages, future):
    super().__init__()
    self.messages = messages
    self.log = logging.getLogger('EchoClient')
    self.f = future

  def connection_made(self, transport):
    self.transport = transport
    self.address = transport.get_extra_info('peername')
    self.log.debug(
      'connecting to {} port {}'.format(*self.address)
    )
    # This could be transport.writelines() except that
    # would make it harder to show each part of the message
    # being sent.
    for msg in self.messages:
      transport.write(msg)
      self.log.debug('sending {!r}'.format(msg))
    if transport.can_write_eof():
      transport.write_eof()

  def data_received(self, data):
    self.log.debug('received {!r}'.format(data))

  def eof_received(self):
    self.log.debug('received EOF')
    self.transport.close()
    if not self.f.done():
      self.f.set_result(True)

  def connection_lost(self, exc):
    self.log.debug('server closed connection')
    self.transport.close()
    if not self.f.done():
      self.f.set_result(True)
    super().connection_lost(exc)

logging.basicConfig(
  level=logging.DEBUG,
  format='%(name)s: %(message)s',
  stream=sys.stderr,
)
log = logging.getLogger('main')

event_loop = asyncio.get_event_loop()

client_completed = asyncio.Future()

client_factory = functools.partial(
  EchoClient,
  messages=MESSAGES,
  future=client_completed,
)
factory_coroutine = event_loop.create_connection(
  client_factory,
  *SERVER_ADDRESS,
)

log.debug('waiting for client to complete')
try:
  event_loop.run_until_complete(factory_coroutine)
  event_loop.run_until_complete(client_completed)
finally:
  log.debug('closing event loop')
  event_loop.close()

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