Dependency Injection

Applications and application instances each serve a role in Ember's DI implementation.

An Application serves as a "registry" for dependency declarations. Factories (i.e. classes) are registered with an application, as well as rules about "injecting" dependencies that are applied when objects are instantiated.

An ApplicationInstance serves as the "owner" for objects that are instantiated from registered factories. Application instances provide a means to "look up" (i.e. instantiate and / or retrieve) objects.

Note: Although an Application serves as the primary registry for an app, each ApplicationInstance can also serve as a registry. Instance-level registrations are useful for providing instance-level customizations, such as A/B testing of a feature.

A factory can represent any part of your application, like a route, template, or custom class. Every factory is registered with a particular key. For example, the index template is registered with the key template:index, and the application route is registered with the key route:application.

Registration keys have two segments split by a colon (:). The first segment is the framework factory type, and the second is the name of the particular factory. Hence, the index template has the key template:index. Ember has several built-in factory types, such as service, route, template, and component.

Zoey says...

You might ask, how can I find the name of a factory?

Factories are kebab-cased and directories are followed by a forward slash. For example, a controller app/controllers/users/primary-teachers is registered as controller:users/primary-teachers.

You can create your own factory type by simply registering a factory with the new type. For example, to create a user type, you'd simply register your factory with application.register('user:user-to-register').

Factory registrations must be performed either in application or application instance initializers (with the former being much more common).

For example, an application initializer could register a Logger factory with the key logger:main:

import EmberObject from '@ember/object';

export function initialize(application) {
  let Logger = EmberObject.extend({
    log(m) {
      console.log(m);
    }
  });

  application.register('logger:main', Logger);
}

export default {
  name: 'logger',
  initialize: initialize
};

By default, Ember will attempt to instantiate a registered factory when it is looked up. When registering an already instantiated object instead of a class, use the instantiate: false option to avoid attempts to re-instantiate it during lookups.

In the following example, the logger is a plain JavaScript object that should be returned "as is" when it's looked up:

export function initialize(application) {
  let logger = {
    log(m) {
      console.log(m);
    }
  };

  application.register('logger:main', logger, { instantiate: false });
}

export default {
  name: 'logger',
  initialize: initialize
};

By default, registrations are treated as "singletons". This simply means that an instance will be created when it is first looked up, and this same instance will be cached and returned from subsequent lookups.

When you want fresh objects to be created for every lookup, register your factories as non-singletons using the singleton: false option.

In the following example, the Message class is registered as a non-singleton:

import EmberObject from '@ember/object';

export function initialize(application) {
  let Message = EmberObject.extend({
    text: ''
  });

  application.register('notification:message', Message, { singleton: false });
}

export default {
  name: 'notification',
  initialize: initialize
};

Once a factory is registered, it can be "injected" where it is needed.

Factories can be injected into whole "types" of factories with type injections. For example:

import EmberObject from '@ember/object';

export function initialize(application) {
  let Logger = EmberObject.extend({
    log(m) {
      console.log(m);
    }
  });

  application.register('logger:main', Logger);
  application.inject('route', 'logger', 'logger:main');
}

export default {
  name: 'logger',
  initialize: initialize
};

As a result of this type injection, all factories of the type route will be instantiated with the property logger injected. The value of logger will come from the factory named logger:main.

Routes in this example application can now access the injected logger:

import Route from '@ember/routing/route';

export default class IndexRoute extends Route {
  activate() {
    // The logger property is injected into all routes
    this.logger.log('Entered the index route!');
  }
}

Injections can also be made on a specific factory by using its full key:

application.inject('route:index', 'logger', 'logger:main');

In this case, the logger will only be injected on the index route.

Injections can be made into any class that requires instantiation. This includes all of Ember's major framework classes, such as components, helpers, routes, and the router.

Dependency injections can also be declared directly on Ember classes using inject. Currently, inject supports injecting controllers (via import { inject } from '@ember/controller';) and services (via import { service } from '@ember/service';).

The following code injects the shopping-cart service on the cart-contents component as the property cart:

import Component from '@glimmer/component';
import { service } from '@ember/service';

export default class CartContentComponent extends Component {
  @service('shopping-cart') cart;
}

If you'd like to inject a service with the same name as the property, simply leave off the service name (the dasherized version of the name will be used):

import Component from '@glimmer/component';
import { service } from '@ember/service';

export default class CartContentComponent extends Component {
  @service shoppingCart;
}

To fetch an instantiated factory from the running application you can call the lookup method on an application instance. This method takes a string to identify a factory and returns the appropriate object.

applicationInstance.lookup('factory-type:factory-name');

The application instance is passed to Ember's instance initializer hooks and it is added as the "owner" of each object that was instantiated by the application instance.

Instance initializers receive an application instance as an argument, providing an opportunity to look up an instance of a registered factory.

export function initialize(applicationInstance) {
  let logger = applicationInstance.lookup('logger:main');

  logger.log('Hello from the instance initializer!');
}

export default {
  name: 'logger',
  initialize: initialize
};

Ember.getOwner will retrieve the application instance that "owns" an object. This means that framework objects like components, helpers, and routes can use Ember.getOwner to perform lookups through their application instance at runtime.

For example, this component plays songs with different audio services based on a song's audioType.

import Component from '@glimmer/component';
import { getOwner } from '@ember/application';

// Usage:
//
// <PlayAudio @song=this.song />
//
export default class PlayAudioComponent extends Component {
  get audioService() {
    if (!this.args.song) {
      return null;
    }

    let applicationInstance = getOwner(this);
    let { audioType } = this.args.song;

    return applicationInstance.lookup(`service:audio-${audioType}`);
  }

  click() {
    let player = this.audioService;
    player.play(this.args.song.file);
  }
}