我如何在龙卷风中的相同URL上提供HTTP页面和Websocket

Question

I have an API and, depending on the request protocol, I want to either serve the client an HTTP response, or I want to connect a websocket and send the response followed by incremental updates. However in Tornado I can only specify a single handler for any URL.

Answer 1

The difference between a request for an HTTP page and a websocket is the presence of a header. Unfortunately afaik there is no way to tell the tornado router to choose one or the other handler based on a header (other than host).

I can however make a handler that inherits from both my already quite elaborate MyBaseRequestHandler and WebSocketHandler , with some magic. The following code works in python 3.5 and tornado 4.3, your mileage may vary on other versions:

class WebSocketHandlerMixin(tornado.websocket.WebSocketHandler):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        # since my parent doesn't keep calling the super() constructor,
        # I need to do it myself
        bases = type(self).__bases__
        assert WebSocketHandlerMixin in bases
        meindex = bases.index(WebSocketHandlerMixin)
        try:
            nextparent = bases[meindex + 1]
        except IndexError:
            raise Exception("WebSocketHandlerMixin should be followed "
                            "by another parent to make sense")

        # undisallow methods --- t.ws.WebSocketHandler disallows methods,
        # we need to re-enable these methods
        def wrapper(method):
            def undisallow(*args2, **kwargs2):
                getattr(nextparent, method)(self, *args2, **kwargs2)
            return undisallow

        for method in ["write", "redirect", "set_header", "set_cookie",
                       "set_status", "flush", "finish"]:
            setattr(self, method, wrapper(method))
        nextparent.__init__(self, *args, **kwargs)

    async def get(self, *args, **kwargs):
        if self.request.headers.get("Upgrade", "").lower() != 'websocket':
            return await self.http_get(*args, **kwargs)
        # super get is not async
        super().get(*args, **kwargs)


class MyDualUseHandler(WebSocketHandlerMixin, MyBaseHandler):
    def http_get():
        self.write("My HTTP page")

    def open(self, *args, **kwargs):
        self.write_message("Hello WS buddy")

    def on_message(self, message):
        self.write_message("I hear you: %s" % message)