Confused about Service vs Factory

manpreet Best Answer 2 years ago

As I understand it, when inside a factory I return an object that gets injected into a controller. When inside a service I am dealing with the object using this and not returning anything.

I was under the assumption that a service was always a singleton, and that a new factory objectgets injected in every controller. However, as it turns out, a factory object is a singleton too?

Example code to demonstrate:

var factories = angular.module('app.factories', []);
var app = angular.module('app',  ['ngResource', 'app.factories']);

factories.factory('User', function () {
  return {
    first: 'John',
    last: 'Doe'
  };
});

app.controller('ACtrl', function($scope, User) {
  $scope.user = User;
});

app.controller('BCtrl', function($scope, User) {
  $scope.user = User;
});

When changing user.first in ACtrl it turns out that user.first in BCtrl is also changed, e.g. User is a singleton?

My assumption was that a new instance was injected in a controller with a factory?

manpreet 2 years ago

For me the revelation came when I realise that they all work the same way: by running something once, storing the value they get, and then cough up that same stored value when referenced through Dependency Injection.

Say we have:

app.factory('a', fn);
app.service('b', fn);
app.provider('c', fn);

The difference between the three is that:

1. a's stored value comes from running fn , in other words: fn()
2. b’s stored value comes from newing fn, in other words: new fn()
3. c’s stored value comes from first getting an instance by newing fn, and then running a $getmethod of the instance

which means, there’s something like a cache object inside angular, whose value of each injection is only assigned once, when they've been injected the first time, and where:

cache.a = fn()
cache.b = new fn()
cache.c = (new fn()).$get()

This is why we use this in services, and define a this.$get in providers.

Hope this helps.