app.js

angular.module('app', ['authentication'])
  .config(function ($stateProvider, $locationProvider) {
    // ...
  })
  .run(function (CurrentUser) {
    CurrentUser.session();
  })

CurrentUser.js

'use strict';

angular.module('authentication')

  .factory('CurrentUser', function ($http, $location) {
    function CurrentUser () {}

    CurrentUser.isAuthenticated = false;
    CurrentUser.requiresAuthentication = false;

    /**
     * 
     */
    
    CurrentUser.prototype.requireAuthentication = function () {
      CurrentUser.requiresAuthentication = true;
    };

    /**
     * We need to pass a function to $scope.watch
     */
    
    CurrentUser.prototype.requiresAuthentication = function () {
      return CurrentUser.requiresAuthentication;
    }

    CurrentUser.prototype.isAuthenticated = function(user) {
      if (user) {
        var key;
        for (key in user) { this[key] = user[key]; }
        CurrentUser.isAuthenticated = true;
        CurrentUser.requiresAuthentication = false;
        return CurrentUser;
      } else {
        return CurrentUser.isAuthenticated;
      }
    };


    /**
     * Signup
     */

    CurrentUser.prototype.signup = function(registration) {
      var self = this;
      return $http.post('/signup', registration).then(function (response) {
        self.isAuthenticated(response.data.user);
        $location.path('/')
      });
    };


    /**
     * Login
     */

    CurrentUser.prototype.login = function(credentials) {
      var self = this;
      return $http.post('/login', credentials).then(function (response) {
        self.isAuthenticated(response.data.user);
        $location.path('/')
      });
    };

    /**
     * Reset
     */

    CurrentUser.prototype.reset = function() {
      var key;
      for (key in this) { delete this[key]; }
      CurrentUser.isAuthenticated = false;
      CurrentUser.requiresAuthentication = false;
    };

    /**
     * Logout
     */

    CurrentUser.prototype.logout = function() {
      var self = this;
      $http.post('/logout').then(function () {
        self.reset();
        $location.path('/')
      });
    };

    /**
     * Session
     */

    CurrentUser.prototype.session = function() {
      var self = this;
      return $http.get('/session').then(function (response) {
        if (response.data.authenticated) {
          self.isAuthenticated(response.data.user);
        }
      });
    };

    /**
     * The service is an instance of CurrentUser
     */
    
    return new CurrentUser();
  })

LoginCtrl.coffee

angular.module('app').controller('LoginCtrl', ['$scope', '$location', 'CurrentUser',
  ($scope, $location, user) ->
    $scope.showLogin = false

    $scope.$watch user.requiresAuthentication, (value) ->
      $scope.showLogin = value

    $scope.reset = ->
      $scope.credentials = {}
      $scope.message = "Enter your credentials"
      $scope.messageType = "alert-warning"

    $scope.sendLogin = ->
      success = ->
        $scope.reset()
      error = (response) ->
        $scope.message = response.data.error
        $scope.messageType = "alert-error"
      user.login($scope.credentials).then success, error
      
    $scope.cancelLogin = ->
      $scope.reset()
      user.constructor.requiresAuthentication = false
      $location.path '/'

    # Initialize the form
    $scope.reset()
])

LoginFocusDirective.coffee

angular.module('app').directive('loginFocus', ['$timeout', 'CurrentUser', ($timeout, user) ->
  (scope, element, attrs) ->
    scope.$watch user.requiresAuthentication, (n,o) ->
      focus = -> element.focus()
      if n is true and o is false
        $timeout focus, 300
])

SomeCtrl

angular.module('app')
  .controller('UserMenuCtrl', function ($scope, CurrentUser) {
    $scope.currentUser = CurrentUser;

    $scope.logout = function () {
      CurrentUser.logout();
    }
  });