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While Angular offers several useful services, for any nontrivial application you'll find it useful to write your own custom services. To do this you begin by registering a service factory function with a module either via the `Module#factory api` or directly via the `$provide` api inside of module config function.

All Angular services participate in dependency injection (DI) by registering themselves with Angular's DI system (injector) under a `name` (id) as well as by declaring dependencies which need to be provided for the factory function of the registered service. The ability to swap dependencies for mocks/stubs/dummies in tests allows for services to be highly testable.

## Registering Services

To register a service, you must have a module that this service will be part of. Afterwards, you can register the service with the module either via the `Module api` or by using the `$provide` service in the module configuration function.The following pseudo-code shows both approaches:

Using the angular.Module api:

    var myModule = angular.module('myModule', []);
    myModule.factory('serviceId', function() {
      var shinyNewServiceInstance;
      //factory function body that constructs shinyNewServiceInstance
      return shinyNewServiceInstance;
    });

Using the $provide service:

    angular.module('myModule', [], function($provide) {
      $provide.factory('serviceId', function() {
        var shinyNewServiceInstance;
        //factory function body that constructs shinyNewServiceInstance
        return shinyNewServiceInstance;
      });
    });

Note that you are not registering a service instance, but rather a factory function that will create this instance when called.

## Dependencies

Services can not only be depended upon, but can also have their own dependencies. These can be specified as arguments of the factory function. Read more about dependency injection (DI) in Angular and the use of array notation and the $inject property to make DI annotation minification-proof.

Following is an example of a very simple service. This service depends on the `$window` service (which is passed as a parameter to the factory function) and is just a function. The service simply stores all notifications; after the third one, the service displays all of the notifications by window alert.

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

## Instantiating Angular Services

All services in Angular are instantiated lazily. This means that a service will be created only when it is needed for instantiation of a service or an application component that depends on it. In other words, Angular won't instantiate services unless they are requested directly or indirectly by the application.

## Services as singletons

Lastly, it is important to realize that all Angular services are application singletons. This means that there is only one instance of a given service per injector. Since Angular is lethally allergic to global state, it is possible to create multiple injectors, each with its own instance of a given service, but that is rarely needed, except in tests where this property is crucially important.

### Related Topics

- Understanding Angular Services
- Managing Service Dependencies
- Injecting Services Into Controllers
- Testing Angular Services

### Related API

- Angular Service API