Why use services in Angular?

Question

I am just starting out with Angular. Reading the example of a service in the Google documentation, I just wonder why you would choose to use a service rather keeping the variables and function right in the controller?

angular.
 module('MyServiceModuleDI', []).
 factory('notify', function($window) {
    var msgs = [];
    return function(msg) {
      msgs.push(msg);
      if (msgs.length == 3) {
        $window.alert(msgs.join("\n"));
        msgs = [];
      }
    };
  });

function myController($scope, notify) {
  $scope.callNotify = function(msg) {
    notify(msg);
  };
}

When would you choose to use a service in this case?

Answer 1

In my opinions the main reasons are:

• Persist and share data between Controllers.
  I.E: You create a service that fetchs data form a database, if you store it inside a controller, once you change to another Controller the data will be discarded (unless you store it in the $rootScope but this is not the best way to do it) , but if you keep it inside a service (services are singletons), the data will persist when you change controllers.

• Abstract data access logic by creating an API that will be used by your controllers/directives/services.
  Keep business logic inside Controllers and data logic inside services.

• DRY (Don't repeat yourself).
  I.E: You have a series of functions that you need in different controllers, without the service you would have to repeat your code in each controller, with a service you can create a single API for this functions and inject it in every Controller/Directive/Service you need.

Here is an example:

var myApp = angular.module('myApp',[]);


//Here is the service Users with its functions and attributes
//You can inject it in any controller, service is a singleton and its data persist between controllers
myApp.factory('Users', function () {

    //data logic
    //fetch data from db and populate...
    var name = "John";
    var surname = "Doe" 

    function getName() { return name; }
    function getFullName() { return name + ' ' + surname; }
    function setName(newName) { name = newName; }

    //API
    return {
        getName: getName,
        getFullName: getFullName,
        setName: setName
    }
});

//An Util service with methods I will use in different controllers   
myApp.factory('Util', function () {

    //a bunch of useful functions I will need everywhere
    function daysInMonth (month,year) {
        return new Date(year, month+1,0).getDate();
    }

    return {
        daysInMonth: daysInMonth    
    };
});   

//Here I am injecting the User and Util services in the controllers   
myApp.controller('MyCtrl1', ['$scope', 'Users', 'Util', function ($scope, Users, Util) {

    $scope.user = Users.getFullName(); //"John Doe";
    Users.setName('Bittle');

    //Using Util service
    $scope.days = Util.daysInMonth(05,2013);
}]);

myApp.controller('MyCtrl2', ['$scope', 'Users', 'Util', function ($scope, Users, Util) {

    $scope.user = Users.getFullName(); //"Bittle Doe"; //The change that took place in MyCtrl1 hhas persisted.

}]);