How to use socket event from another function? - node.js

Question

In socket.io we create a instance and use socket like this
var server = require('http').createServer();
var io = require('socket.io')(server);
io.on('connection', function(socket){
socket.on('event', function(data){});
socket.on('disconnect', function(){});
});
server.listen(3000);
How we can create a socket event in other function like this. Is there any way or trick to this properly.
var server = require('http').createServer();
var io = require('socket.io')(server);
io.on('connection', function(socket){
socket.on('event', function(data){});
socket.on('disconnect', function(){});
});
var f1 = function (socketID) {
if(socketID){
io.to(socketID).emit('event1', 'event1 data'); //working
socket.on('call2', function (data) { // not working
});
}
}
server.listen(3000);

Answer 1

When you are sending a request and trying to wait for a specific response to that request, it is bit tricky to use a message-based protocol like socket.io for that situation. HTTP is perfectly designed for request/response since a single connection is used only for that one request/response and therefore the response is the response that belongs to your request.
This is not true with socket.io. It's a message based scheme. When you send a message and then wait for a message back from the other end (in a multi-user environment), you have to be very careful that you don't:
Install too many event listeners and end up with duplicates.
That the response you're getting is specifically for your original request and not for some other request that might be happening in a similar time frame.
One way to solve this issue is to add a request identifier to all your request/response messages. When you send the request, you add a unique request ID. When the response is returned, it includes that request ID and then your code can tell which response belongs with which request.
Here's an idea for how to implement the request ID concept (this needs the other end to cooperate and return the same request ID that it was sent):
var idCntr = 0;
function f1(socketID) {
if(socketID){
var id = idCntr++;
io.to(socketID).emit('event1', {id: id, data: 'event data'});
function listener(data) {
// if this is our message
if (data.id === id) {
// once our response is received, remove event handler
socket.removeListener('responseToEvent1', listener);
// process response here
// code here...
}
}
// wait for our response
socket.on('responseToEvent1', listener);
}
}
Or, here's a more object oriented scheme that adds a method to the socket object called .emitRequest():
var idCntr = 0;
socket.emitRequest = function(msg, data, responseMsg, responseFn) {
var id = idCntr++;
function listener(response) {
// if this is our message
if (response.id === id) {
// once our response is received, remove event handler
this.removeListener(responseMsg, listener);
// call response handler
responseFn(response.data);
}
}
this.emit(msg, {id: id, data: data});
this.on(responseMsg, listener);
}
Note: With both of these pieces of code, the other end of your connection needs to cooperate and return the id property of the original request so that the receiving listener can tell which response goes with which request.