Summary

With the goal of making significant performance improvements and of adding public API to support use cases long-served by a private API, a new API of factoryFor will be added to ApplicationInstance instances.

Motivation

Ember's dependency injection container has long supported fetching a factory that will be created with any injections present. Using the private API that provided this support allows an instance of the factory to be created with initial values passed via create. For example:

// app/logger/main.js
import Ember from 'ember';

export default Ember.Logger.extend({
  someService: Ember.inject.service()
});

import Ember from 'ember';
const { Component, getOwner } = Ember;

export default Component.extend(
  init() {
    this._super(...arguments);
    let Factory = getOwner(this)._lookupFactory('logger:main');
    this.logger = Factory.create({ level: 'low' });
  }
});

In this API, the Factory is actually a subclass the original main logger class. When _lookupFactory is called, an additional extend takes place to add any injections (such as someService above). The class/object setup looks like this:

• In the module: MyClass = Ember.Object.extend(
• In _lookupFactory: MyFactoryWithInjections = MyClass.extend(
• And when used: MyFactoryWithInjections.create(

The second call to extend implements Ember's owner/DI framework and permits someService to be resolved later. The "owner" object is merged into the new MyFactoryWithInjections class along with any registered injections.

This "double extend" (once at define time, once at _lookupFactory time) takes a toll on performance booting an app. This design flaw has motivated a desire to keep _lookupFactory private.

The MyFactoryWithInjections class also features as a cache. Because it is attached to the owner/container, it is cleared between test runs or application instances. To illustrate, this flow-chart shows how MyFactoryWithInjections diverges between tests:

               +-------------------------------+
               |                               |
               |      /app/models/foo.js       |
               |                               |
               +-------------------------------+
                               |
              first test run   |    nth test run
              +----------------+---------------+
              |                                |
              v                                v
   +---------------------+          +--------------------+
   |resolve('model:foo') |   ===    |resolve('model:foo')|
   +---------------------+          +--------------------+
              |                                |
              |                                |
              v                                v

     extend(injections)               extend(injections)

              |                                |
              |                                |
              |                                |
              v                                v
+--------------------------+     +---------------------------+
|lookupFactory('model:foo')| !== |lookupFactory('model:foo') |
+--------------------------+     +---------------------------+

Despite the design flaws in this API, it does fill a meaningful role in Ember's DI solution. Use of the private API is common. Some examples:

• ember-cart uses the functionality to create model objects without tying them to the store example a, example b
• Ember-Data's modelFactoryFor

The goal of this RFC is to create a public API for fetching factories with better performance characteristics than _lookupFactory.

Detailed design

Throughout this document I reference Ember 2.12 as it is the next LTS at writing. This proposal may ship for 2.12-LTS or be bumped to the next LTS.

This feature will be added in these steps.

1. In Ember introduce a ApplicationInstance#factoryFor based on _lookupFactory. It should be documented that certain behaviors inherent to "double extend" are not supported. In development builds and supporting browsers, wrap return values in a Proxy. The proxy should throw an error when any property besides create or class is accessed. class must return the registered factory, not the double extended factory.
2. In the same release add a deprecation message to usage of _lookupFactory. As this API is intimate it must be maintained through at least one LTS release (2.12 at this writing).
3. In 2.13 drop _lookupFactory and migrate the factoryFor implementation to avoid "double-extend" entirely.

Additionally, a polyfill will be released for this feature supporting prior versions of Ember.

Design of ApplicationInstance#factoryFor

A new API will be introduced. This API will return both the original base class registered into or resolved by the container, and will also return a function to generate a dependency-injected instance. For example:

import Ember from 'ember';
const { Component, getOwner } = Ember;

export default Component.extend(
  init() {
    this._super(...arguments);
    let factory = getOwner(this).factoryFor('logger:main');
    this.logger = factory.create({ level: 'low' });
  }
});

Unlike _lookupFactory, factoryFor will not return an extended class with DI applied. Instead it will return a factory object with two properties:

// factoryFor returns:
let {

  // a function taking an argument of initial properties passed to the object
  // and returning an instance
  create,

  // The class registered into (or resolved by) the container
  class

} = owner.factoryFor('type:name');

This API should meet two requirements of the use-cases described in "Motivation":

• Because factoryFor only returns a create method and reference to the original class, its internal implementation can diverge away from the "double extend". A side-effect of this is that the class of an object instantiated via _lookupFactory(name).create() and factoryFor(name).create() may not be the same, given the same original factory.
• The presence of class will make it easy to identify the base class of the factory at runtime.

For example today's _lookupFactory creates an inheritance structure like the following:

                    Current:
       +-------------------------------+
       |                               |
       |      /app/models/foo.js       |
       |                               |
       +-------------------------------+
                       |
                       |
                       |
                       v
            +--------------------+
            |  Class[model/Foo]  |
            +--------------------+
                       |
                       |
                       |
       first test run  |   nth test run
           +-----------+----------+
           |                      |
           |                      |
           |                      |
           v                      v
+--------------------+ +--------------------+
|     subclass of    | |     subclass of    |
|  Class[model/Foo]  | |  Class[model/Foo]  |
+--------------------+ +--------------------+

Between test runs 2 instances of model:foo will have a common shared ancestor the grandparent Class[model/Foo].

This implementation of factoryFor proposes to remove the intermediate subclass and instead have a generic factory object which holds the injections and allows for injected instances to be created. The resulting object graph would look something like this:

                  Proposed:
      +-------------------------------+
      |                               |
      |      /app/models/foo.js       |
      |                               |
      +-------------------------------+
                      |
                      |
                      |
                      v
           +--------------------+
           |  Class[model/Foo]  |
           +--------------------+
                      |
                      |
                      |
      first test run  |   nth test run
           +----------+-----------+
           |                      |
           |                      |
           |                      |
           v                      v
+--------------------+ +--------------------+
|     Factory of     | |     Factory of     |
|  Class[model/Foo]  | |  Class[model/Foo]  |
+--------------------+ +--------------------+

With factoryFor instances of model:foo will share a common constructor. Any state stored on the constructor would of course leak between the tests.

An example implementation of factoryFor can be reviewed on this GitHub comment.

Implications for owner.register

Currently, factories registered into Ember's DI system are required to provide an extend method. Removing support for extend-based DI in _lookupFactory will permit factories without extend to be registered. Instead factories must only provide a create method. For example:

let factory = {
  create(options={}) {
    /* Some implementation of `create` */
    return Object.create({});
  }
};
owner.register('my-type:a-factory', factory);
let factoryWithDI = owner.factoryFor('my-type:a-factory');

factoryWithDI.class === factory;

Development-mode Proxy

Because many developers will simply re-write _lookupFactory to factoryFor, it is important to provide some aid and ensure they actually complete the migration completely (they they avoid setting state on the factory). A proxy wrapping the return value of factoryFor and raising assertions when any property besides create or class is accessed will be added in development.

Additionally, using instanceof on the result of factoryFor should be disallowed, causing an exception to be raised.

A good rule of thumb is that, in development, using anything besides class or create on the return value of factoryFor should fail with a helpful message.

Releasing a polyfill

A polyfill addon, similar to ember-getowner-polyfill will be released for this feature. This polyfill will provide the factoryFor API going back to at least 2.8, provide the API and silence the deprecation in versions before factoryFor is available, and be a no-op in versions where factoryFor is available.

How We Teach This

This feature should be introduced along side lookup in the relevant guide. The return value of factoryFor should be taught as a POJO and not as an extended class.

Example deprecation guide: Migrating from _lookupFactory to factoryFor

Ember owner objects have long provided an intimate API used to fetch a factory with dependency injections. This API, _lookupFactory, is deprecated in Ember 2.12 and will be removed in Ember 2.13. To ease the transition to this new public API, a polyfill is provided with support back to at least Ember 2.8.

_lookupFactory returned the class of resolved factory extended with a mixin containing its injections. For example:

let factory = Ember.Object.extend();
owner.register('my-type:a-name', factory);
let klass = owner._lookupFactory('my-type:a-name');
klass.constructor.superclass === factory; // true
let instance = klass.create();

factoryFor instead returns an object with two properties: create and class. For example:

let factory = Ember.Object.extend();
owner.register('my-type:a-name', factory);
let klass = owner.factoryFor('my-type:a-name');
klass.class === factory; // true
let instance = klass.create();

A common use-case for _lookupFactory was to fetch an factory with specific needs in mind:

• The factory needs to be created with initial values (which cannot be provided at create-time via lookup.
• The instances of that factory need access to Ember's DI framework (injections, registered dependencies).

For example:

// app/widgets/slow.js
import Ember from 'ember';

export default Ember.Object.extend({
  // this instance requires access to Ember's DI framework
  store: Ember.inject.service(),

  convertToModel() {
    this.get('store').createRecord('widget', {
      widgetType: 'slow',
      name, canWobble
    });
  }

});

// app/services/widget-manager.js
import Ember from 'ember';

export default Ember.Service.extend({

  init() {
    this.set('widgets', []);
  },

  /*
   * Create a widget of a type, and add it to the widgets array.
   */
  addWidget(type, name, canWobble) {
    let owner = Ember.getOwner(this);
    // Use `_lookupFactory` so the `store` is accessible on instances.
    let WidgetFactory = owner._lookupFactory(`widget:${type}`);
    let widget = WidgetFactory.create({name, canWobble});
    this.get('widgets').pushObject(widget);
    return widget;
  }

});

For these common cases where only create is called on the factory, migration to factoryFor is mechanical. Change _lookupFactory to factoryFor in the above examples, and the migration would be complete.

Migration of static method calls

Factories may have had static methods or properties that were being accessed after resolving a factory with _lookupFactory. For example:

// app/widgets/slow.js
import Ember from 'ember';

const SlowWidget = Ember.Object.extend();
SlowWidget.reopenClass({
  SPEEDS: [
    'slow',
    'verySlow'
  ],
  hasSpeed(speed) {
    return this.SPEEDS.contains(speed);
  }
});

export default SlowWidget;

let factory = owner._lookupFactory('widget:slow');
factory.SPEEDS.length; // 2
factory.hasSpeed('slow'); // true

With factoryFor, access to these methods or properties should be done via the class property:

let factory = owner.factoryFor('widget:slow');
let klass = factory.class;
klass.SPEEDS.length; // 2
klass.hasSpeed('slow'); // true

Drawbacks

The main drawback to this solution is the removal of double extend. Double extend is a performance troll, however it also means if a single class is registered multiple times each _lookupFactory returns a unique factory. It is plausible that some use-case relying on this behavior would get trolled in the migration to factoryFor, however it is unlikely.

For example these cases where state is stored on the factory would no longer be scope to one instance of the owner (like one test). Instead, setting a value on the class would set it on the registered class.

Some real-world examples of setting state on the factory class:

• ember-model
• https://github.com/ebryn/ember-model/blob/master/packages/ember-model/lib/model.js#L404 and https://github.com/ebryn/ember-model/blob/master/packages/ember-model/lib/model.js#L457 with factoryFor will increment a shared counter across application and container instances.
• https://github.com/ebryn/ember-model/blob/master/packages/ember-model/lib/model.js#L723-L725 would also set properties on the base Ember.Model factory instead of an extension of that class.
• ember-data
• If attrs change between test runs (seems very unlikely) then https://github.com/emberjs/data/blob/387630db5e7daec6aac7ef8c6172358a3bd6394c/addon/-private/system/model/attr.js#L57 would be affected. The CP of attributes will have a value cached on the factory, and where with _lookupFactory's double-extend the cache would be on the extended class, in factoryFor that CP cache will be on the class registered as a factory.
• Any other of the following:
• lookupFactory(x).reopen / reopenClass at runtime (or test time to monkey patch code)
• lookupFactory(x).something = value

Alternatives

More aggressive timelines have been considered for this change.

However we have considered the possibility that removing _lookupFactory in 2.13 (something LTS technically permits) would be too aggressive for the community of addons. Providing a polyfill is part of the strategy to handle this change.

Unresolved questions

Are there any use-cases for the double extend not considered?