Understanding Angular Services

Understanding Angular Services

What are Angular Services?

Angular services are singletons objects or functions
that carry out specific tasks common to web apps. Angular has a number of built in services, such as the

$http
service

, which provides access to the browser's

XMLHttpRequest

object
for making requests to a server. Like other core Angular variables and identifiers, the built-in services always start with

$

(such
as

$http

mentioned above). You can also create your own custom services.

Using a Service

To use an Angular service, you identify it as a dependency for the component
(controller, service, filter or directive) that depends on the service. Angular's dependency injection subsystem takes care of the rest. The Angular injector subsystem is in charge of service instantiation, resolution
of dependencies, and provision of dependencies to components as requested.

Angular injects dependencies using "constructor" injection. The dependency is passed to the component's
factory/constructor function. Because JavaScript is a dynamically typed language, Angular's dependency injection subsystem cannot use static types to identify service dependencies. For this reason a component must, explicitly, define its dependencies by using
one of the injection annotation methods. For example, by providing a

$inject

property:

var MyController = function($location) { ... };
MyController.$inject = ['$location'];
myModule.controller('MyController', MyController);

Or by providing an "inline" injection annotation:

var myService = function($http) { ... };
myModule.factory('myService', ['$http', myService]);

Defining a Service

Application developers are free to define their own services by registering their name, and service factory function, in Angular modules.

The purpose of the service factory function is to generate the single object, or function, that represents the service to the rest of the application. That object, or function, will then be injected into any component (controller, service,
filter or directive) that specifies a dependency on the service.

Angular factory functions are executed lazily. That is, they are only executed when needed to satisfy a dependency, and are then executed exactly once for each service. Everything that is dependent on this service gets a reference to the single instance generated
by the service factory.

Related Topics

About Angular Dependency Injection
Creating Angular Services
Managing Service Dependencies
Testing Angular Services

Related API

Angular Service API

Injector
API