AnotherController.js

(function() {
    'use strict';
    
    function AnotherController($scope, socket) {
        // use the socket factory through your app, but you must use socket.then
        // so that the actions occur once the socket is established
        socket.then(function(socket) {
            socket.emit('some_socket_event', {});
        });
        
        $scope.buttonClicked = function() {
            socket.then(function(socket) {
                socket.emit('button_clicked_event', {});
            });
        };
        
    }
    
    AnotherController.$inject = [
        '$rootScope',
        'socket' // this is the SocketFactory which returns a promise
    ];
    
    angular.module('controllers')
        .controller('AnotherController', AnotherController);
})();

LoginController.js

(function() {
    'use strict';
    
    function LoginController(LoginService) {
        LoginService.login({
            username: $username,
            password: $password
        }, function(user) {
            if(user) {
                // broadcast that there was a successful authentication
                // to cause the socket to resolve.
                $rootScope.$broadcast('authenticated');
                
            } else {
                // handle everything else
            }
        });
    }
    
    LoginController.$inject = [
        '$rootScope',
        'socket' // this is the SocketFactory which returns a promise
    ];

    angular.module('controllers')
        .controller('LoginController', LoginController);
    
})();

SocketFactory.js

(function() {

    'use strict';

    function SocketFactory($q, $rootScope, socketFactory, $timeout, config) {
        // create a promise instance
        var socket = $q.defer();
        
        // listen for the authenticated event emitted on the rootScope of 
        // the Angular app. Once the event is fired, create the socket and resolve
        // the promise.
        $rootScope.$on('authenticated', function() {
            
            // resolve in another digest cycle
            $timeout(function() {
                // create the socket
                var newSocket = (function() {
                    return socketFactory({
                        ioSocket: io.connect(config.API.SOCKET, {
                            query: config.API_HEADER + "=" + config.API_KEY
                        })
                    });
                })();
                
                // resolve the promise
                socket.resolve(newSocket);
            });
        });
        
        // return the promise
        return socket.promise;
    }

    SocketFactory.$inject = [
        '$q',
        '$rootScope',
        'socketFactory',
        '$timeout',
        'CONFIG'
    ];

    angular.module('factories')
        .factory('socket', SocketFactory);

})();