Controller Injection Deprecation

text/0000-controller-injection.md

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+- Start Date: 2020-01-09
+- Relevant Team(s): Ember.js
+- RFC PR: https://github.com/emberjs/rfcs/pull/574
+- Tracking:
+
+# Deprecate Controller Injection
+
+## Summary
+
+This RFC recommends deprecating the ability to inject a controller into another.
+
+```js
+import { inject as controller } from '@ember/controller';
+
+export default SomeController extends Controller {
+    @controller other;
+
+    get otherFoo() {
+        return this.other.foo;
+    }
+}
+
+// app/controllers/other.js
+export default OtherController extends Controller {
+    foo = 1;
+}
+
+///////
+/////// In EmberObject.extend parlance:
+///////
+import { inject as controller } from '@ember/controller';
+
+export default Controller.extends({
+    other: controller(),
+
+    otherFoo: computed('other.foo', function() {
+        return this.other.foo;
+    })
+})
+
+// app/controllers/other.js
+export default Controller.extend({
+    foo: 1
+})
+```
+
+## Motivation
+
+Controller injection is not a standard paradigm in Ember. Data that needs to be shared between
+routes or controllers or components can be shared via services. Alternatively, if a controller
+needs to be accessed, the `controllerFor()` API can be used or, as a last resort, the `lookup`
+API on the ApplicationInstance (i.e. the `owner` of most Ember core class instances) can be used.
+
+Deprecating this functionality also makes it ever-so-slightly easier to make the case for removing
+controllers entirely from Ember.
+
+## Detailed design
+
+The `inject` function from the `@ember/controller` package should log a deprecation warning and
+then eventually stop being exported.
+
+## How we teach this
+
+There is not much to teach for this removal as this is not a major feature and it is not mentioned
+in the guides. There are other efforts to teach Ember developers about how to share state around
+the application.
+
+## Drawbacks
+
+It may be seen as unnecessary churn if controller injection is removed as a feature without a longer
+term plan for controllers.
+
+## Alternatives
+
+- Keep controller injection.
+- Wait for the long term plan for controllers before making any changes.
+    - I think it's good to start deprecating / removing features to narrow the scope of controllers.
+
+## Unresolved questions
+
+- What code needs to be removed once the inject function is removed?