socket.io如何从客户端获取参数？

Question

I have the following code on the server side:

var io = socket(server);

io.on('connection', function(socket){
    console.log('made socket connection with ID:', socket.id);
});

Here is what I understand.

We make a socket on the server side and pass that (instance?) to io. Then, we say io.on('connection') which is saying listen to the connection event. We also make a callback function to which we are passing socket.

I don't understand the callback function(socket) part. Where are we passing it socket? on the client side I have this

var socket = io.connect('http://localhost:4000');

I get that a connection will be made and it will start listening but how and where are we calling the function form the client side?

Answer 1

We are not calling the callback, and so we are not passing it anything. You can't call a serverside function from the clientside.

The callback is called (and given the argument) by the JavaScript's event loop, as triggered by socket.io library's internals - more specifically, as far as I can see, in Engine.io :

Server.prototype.handshake = function (transportName, req) {
  ...
  self.emit('connection', socket);
  ...
};

This is done through Node's native EventEmitter , which allows you to register a callback for an event ( io.on('connection', function(socket) { ... }) ), and emit the event with data for the callback ( self.emit('connection', socket) ). The data passed with the event emission is the data received by any callback registered for the event on the emitter object.

Thus, the flow is roughly as follows:

• Serverside, you register a callback handler for the connection event on the Engine.io object (passed to you by Socket.io's socket function).
• Clientside, you invoke the connect function. It starts a Websocket request.
• Serverside, Engine.io receives the Websocket request. It creates a socket object to represent the connection with that particular client, then emits the connection event, with the socket object as accompanying data.
• The connection event causes all handlers registered for that event to be invoked, and passed the accompanying data (the socket object), your function being among them.