Developer Guide: DI: Understanding DI in Angular

While DI is widely used in statically typed languages such as Java or C++, it has not been widely used in JavaScript. Angular brings the benefits of DI into JavaScript apps.

In angular, DI is implemented as a subsystem that manages dependencies between services, controllers, widgets, and filters.

Services are objects that handle common tasks in web applications. Angular provides several built-in services, and you can create your own custom services.

The main job of angular's DI subsystem is to provide services to angular components that depend on them. The way the DI subsystem provides services is as follows: all services are registered with angular's service API, and all components that depend on services define those dependencies as a property ($inject). With this information, the DI subsystem manages the creation of service objects and the provision of those objects to the components that need them, at the time they need them. The following illustration steps through the sequence of events:

In the illustration above, the dependency injection sequence proceeds as follows:

  1. Module "phonecat" is created and all the service providers are registered with this module. (the "ng" module is created by Angular behind the scenes as well)
  2. ng-app triggers bootstrap sequence on given element, during which angular creates injector, loads "phonecat" and "ng" modules and compiles the template.
  3. The ng-controller directive implicitly creates a new child scope and instantiates PhoneListCtrl controller.
  4. Injector identifies the $http service as PhoneListCtrl controller's only dependency.
  5. Injector checks its instances cache whether the $http service has already been instantiated. If not uses the provider from the available modules to construct it.
  6. Injector provides the instance of $http service to the PhoneListCtrl controller constructor.

How Scope Relates to DI

The root scope of the application is just a service that is available for injection to any part of the application under the service name "$rootScope".

Inferring dependencies from the signature of the factory function or constructor

EXPERIMENTAL FEATURE: This is an experimental feature. See the important note at the end of this section for drawbacks.

We resort to $inject and our own annotation because there is no way in JavaScript to get a list of arguments. Or is there? It turns out that calling .toString() on a function returns the function declaration along with the argument names as shown below:

function myFn(a,b){}
expect(myFn.toString()).toEqual('function myFn(a,b){}');

This means that angular can infer the function names after all and use that information to generate the $inject annotation automatically. Therefore the following two function definitions are equivalent:

// given a user defined service
angular.module('module1', [], function($provide) {
  $provide.factory('serviceA', ...);
});

// inject '$window', 'serviceA', curry 'name';
function fnA($window, serviceA, name){};
fnA.$inject = ['$window', 'serviceA'];

// inject '$window', 'serviceA', curry 'name';
function fnB($window, serviceA_, name){};
// implies: fnB.$inject = ['$window', 'serviceA'];

If angular does not find a $inject annotation on the function, then it calls the .toString() method and tries to infer what should be injected by using function argument names as dependency identifiers.

IMPORTANT Minifiers/obfuscators change the names of function arguments and will therefore break the $inject inference. For this reason, either explicitly declare the $inject or do not use minifiers/obfuscators. In the future, we may provide a pre-processor which will scan the source code and insert the $inject into the source code so that it can be minified/obfuscated.

Related Topics

Related API