CODEWITHSUNDEEP
node.jswebsocketsocket.ioyeoman
Zhehao Victor Zhou
Zhehao Victor Zhou

Reputation: 203

Socket.io Callback Function Parameter Confused

I am not sure I understand the server side configuration of the socket. 

var app = express();
var server = require('http').createServer(app);
var socketio = require('socket.io')(server, {
    serveClient: config.env !== 'production',
    path: '/socket.io-client'
});

Here, the code creates a socket server "attached" with the http server according to the api reference. What is attach?

 socketio.on('connection', function (socket) {
    socket.on('create', function(room) {
        console.log('joining a room');
        socket.join(room);
        console.log('socket joined room: ', room);
    });

   socket.address = socket.handshake.address !== null ?
        socket.handshake.address.address + ':' + socket.handshake.address.port :
        process.env.DOMAIN;

   socket.connectedAt = new Date();

  // Call onDisconnect.
  socket.on('disconnect', function () {
      onDisconnect(socket);
      console.info('[%s] DISCONNECTED', socket.address);
  });

   // Call onConnect.
   onConnect(socket);
   console.info('[%s] CONNECTED', socket.address);
  });
};

Question: Here, the 'socket' variable is server's or client's ? if it is server's then why the socket.join(room) works ? (client's been added to a room) If it is client's, then why it has to listen to 'create' event.(client emits an event called create to change the room.)

to conclude, I confused by the three 'socket' in the following code. 

 socketio.on('connection', function (socket) {
    socket.on('create', function(room) {
       console.log('joining a room');
       socket.join(room);
       console.log('socket joined room: ', room);
    });
 });

Upvotes: 0

Views: 1417

Answers (1)

jfriend00
jfriend00

Reputation: 707846

Here, the code creates a socket server "attached" with the http server according to the api reference. What is attach?

webSocket connections (which socket.io is built on top of) all get initiated from the client by first making an HTTP connection. Thus, there must be an HTTP server that can be used for socket.io connections. That's why the initialization of socket.io needs an HTTP connection. Often, that web server is also acting as a normal web server too and thus can be used for both purposes. This simplifies cross-origin issues since all browsers all clients to connect to the same origin from which their web page was served. If, you don't already have another web server, socket.io can create it's own.

Question: Here, the 'socket' variable is server's or client's ?

It is the server-side object that represents the connection to a particular client. Socket.io has a socket object on the client that represents the client-side connection to the server and it has a socket object on the server (for each separate client connection) that represents the connection to a particular client.

So, if you want to send data to a particular client, you use the socket object that represents that client and you do:

socket.emit(msg, data);

if it is server's then why the socket.join(room) works ?

This works because this socket object represents the connection to a particular client. It is a server-side object, but it is specific to a particular client (e.g. there's a different one for each client connection).

to conclude, I confused by the three 'socket' in the following code.

 socketio.on('connection', function (socket) {
    socket.on('create', function(room) {
       console.log('joining a room');
       socket.join(room);
       console.log('socket joined room: ', room);
    });
 });

socketio represents the overall socket.io module and is how you execute global commands. In this case, you are executing a global command to listen for any newly connection clients.

When you get a connection event, the argument to that event is the newly created socket object (a server-side object that represents a connection to a particular client).

The socket.on('create', ...) is a listener for the create message sent from the client to the server. So, this line of code says to listen for a create message sent from this particular (newly connected) client and when that message arrives, call a callback and pass it the data sent with the message (in this case a room name).

The socket.join(room); line uses the same socket as above and joins it to a specific room on the server.

Upvotes: 4

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