Websocket subprotocol support

[hballard]

The websockets library that Sanic utilizes supports passing a list of acceptable websocket subprotocols into their server or client implementations, to be used during the opening handshake ("Sec-WebSocket-Protocol"). Sanic does not support this currently and does not do anything w/ subprotocols in the websocket_handshake method. The eventual websocket object that gets passed to the Sanic handler has an "subprotocol" attribute, which would normally be set during the handshake. Currently it is empty. Is there any plan to support subprotocols in the future? Thanks for the excellent framework!

[yunstanford]

hmm.. seems like Sanic is using WebSocketCommonProtocol, not WebSocketServerProtocol. But WebSocketCommonProtocol doesn't support to pass in subprotocols directly (we can hook it up if really needs). @r0fls is there any reason to use WebSocketCommonProtocol, instead of WebSocketServerProtocol ?

Also , https://github.com/channelcat/sanic/blob/master/sanic/websocket.py#L48 this piece of code is mainly from the implementation of WebSocketServerProtocol.

[messense]

cc @miguelgrinberg

[miguelgrinberg]

Yes, the support for negotiating a subprotocol is in the WebSocketServerProtocol class, which I think would be harder to integrate with the sanic request object than WebSocketCommonProtocol, which only has the low-level WebSocket support.

What I think makes the most sense here is to implement the negotiation of the subprotocol in websocket_handshake. We can add a subprotocols argument to the @websocket decorator so that the application can specify what subprotocols it accepts. Example:

@app.websocket('/feed', subprotocols=['foo', 'bar'])
async def feed(request, ws):
    if ws.subprotocol:
        print('The subprotocol is: ', ws.subprotocol)
    while True:
        data = 'hello!'
        print('Sending: ' + data)
        await ws.send(data)
        data = await ws.recv()
        print('Received: ' + data)