Question

让我们说我有一辆从某处接收消息的总线。每条消息都有target和msg，我想实现订阅mecahnism，因此其他协同程序可以subscribe到特定target

subscriptions = {}
async def subscribe(target):
    subscriptions[target]= wait_for_messages()

async def proc_msg(target,msg);
    subscriptions[target].send(msg)

async def wait_for_messages():
     while True:
        asyncio.sleep(1)

async def subscribe(target)
    async for msg in subscribe(target):
      print(msg)

我的问题是关于wait_for_messages，即使睡眠类型有效（代码仅用于演示），如何在不睡觉和不断唤醒的情况下等待发送命令。

Answer 1

订阅机制来自回调世界和asyncio.Future - 是从基于回调的世界到基于协程的世界的某种桥梁：有了它，你可以等待发生的事情。在msg即将到来时使用set_result，在后面使用await来获取coroutine中的msg。

例如：

import asyncio
from random import randint
from collections import defaultdict
from contextlib import suppress


# MSG SUBSCRIBE/SEND API:
_futures = defaultdict(asyncio.Future)


async def msg_for_target(target):
    """Await for this coroutine to recieve msg."""
    return await _futures[target]


def send_msg(target, msg):
    _futures[target].set_result(msg)
    del _futures[target]  # We would need new Future for future subscribers


# TEST:
async def random_sender():
    """Send random messages."""
    i = 0
    while True:
        i += 1
        await asyncio.sleep(0.5)

        target = randint(0, 3)
        msg = f'msg {i}'        
        print(f'Sending msg "{msg}" for target "{target}" ...')
        send_msg(target, msg)


async def main():
    task = asyncio.ensure_future(random_sender())

    for target in (2, 0, 3, 1):
        print(f'> Subscribed for target "{target}"')
        msg = await msg_for_target(target)
        print(f'> Recieved "{msg}"')
        print()

    # Cleanup, see https://stackoverflow.com/a/43810272/1113207
    task.cancel()
    with suppress(asyncio.CancelledError):
        await task


if __name__ ==  '__main__':
    loop = asyncio.get_event_loop()
    try:
        loop.run_until_complete(main())
    finally:
        loop.run_until_complete(loop.shutdown_asyncgens())
        loop.close()

<强>输出：

> Subscribed for target "2"
Sending msg "msg 1" for target "2" ...
> Recieved "msg 1"

> Subscribed for target "0"
Sending msg "msg 2" for target "3" ...
Sending msg "msg 3" for target "0" ...
> Recieved "msg 3"

> Subscribed for target "3"
Sending msg "msg 4" for target "2" ...
Sending msg "msg 5" for target "2" ...
Sending msg "msg 6" for target "2" ...
Sending msg "msg 7" for target "1" ...
Sending msg "msg 8" for target "3" ...
> Recieved "msg 8"

> Subscribed for target "1"
Sending msg "msg 9" for target "0" ...
Sending msg "msg 10" for target "0" ...
Sending msg "msg 11" for target "2" ...
Sending msg "msg 12" for target "2" ...
Sending msg "msg 13" for target "1" ...
> Recieved "msg 13"

asyncio如何在调用send之前暂停coroutine

1 个答案: