Native Class Roadmap

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+- Start Date: 2018-06-14
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# Native Class Roadmap
+
+## Summary
+
+Now that we’ve (almost) achieved feature parity for using native classes to
+extend from EmberObject, the next step for Ember is to move toward using and
+recommending native classes by default, with the long term goal of deprecating
+EmberObject and removing it from the framework. This RFC seeks to outline where
+we want to go (e.g. what Ember looks like without EmberObject), constraints we
+face, and potential paths forward.
+
+## Design Constraints
+
+In order to figure out the path forward to native classes, we need to first
+develop an idea of what the future of Ember looks like once we have fully
+adopted them and removed EmberObject entirely. Below are several design
+considerations for the new object model:
+
+- **It should leverage the platform.** We should use and encourage standard
+Javascript classes and idioms wherever possible. This means leveraging classes,
+class fields, and decorators for most features that the object model provides.
+- **Ember should remain as neutral as possible about the implementation details.**
+Part of the reason we are going to have migration pains as a community moving
+forward is that Ember is tied to the behavior of EmberObject in a myriad of
+small ways. In an ideal world, these details would be opaque to Ember, allowing
+users to implement and change-out object models at their own convenience. This
+would have made the transition much easier, and will provide flexibility in the
+community for pursuing new language features and libraries (e.g. mixins)
+- **It should be possible to adopt incrementally.** This will be one of the largest
+changes the community has ever gone through, on the same level if not larger than
+dropping Views and adopting Components. EmberObject is pervasive - it's been
+used for every construct imaginable in Ember apps and addons due to its
+versatility and simplicity. Ideally users will be able to make incremental
+changes toward pure native classes by first rewriting their existing classes
+using native class syntax (as is already possible) and then dropping EmberObject
+altogether.
+- **We should consider the long tail of adoption, particularly for addons.** There
+will be a long period where the community will need to support older versions of
+Ember at the same time as newer versions. Ideally it should be possible to
+support both the new and old model at once without separate code paths, or the
+overlap period should be long enough that addons can drop EmberObject without
+breaking Ember versions that are still part of their support matrix.
+- **If possible, conversion should be automate-able.** This is not a hard
+constraint, since it limits us to having very similar behavior to the existing
+object model. However, if it is possible to create scripts that speed up
+adoption it will help the community tremendously, so if we are at an impasse on
+particular behaviors or implementations, we should consider whether one would be
+easier to automate towards.
+
+## Detailed design
+
+As mentioned above, one of the main reasons this conversion is going to be a
+difficult and long process is the fact that both user code and Ember code are
+very intertwined with the existing object model. Specifically:
+
+- They rely on the behavior of static methods such as `create`, `extend`, and
+  `destroy`.
+- They rely on the behavior of EmberObject's constructor
+- They rely on a non-trivial amount of functionality provided via inheritance
+- They rely on features of EmberObject's inheritance model that do not exist in
+native Javascript (Mixins, concatenated/merged properties, nuances of observers
+and listeners, etc.)
+
+The fact that these are so intertwined makes it very difficult (if not
+impossible) for us to rewrite the core classes that exist in Ember without
+breaking user land code.
+
+One strategy we can implement to prevent this from happening again in the future
+is to make the details of classes opaque to Ember. Rather than providing base
+classes that can be extended by users to add functionality and implement their
+applications, we can rewrite Ember to treat user classes as delegates. In other
+words, Ember will make no assumptions about the behavior of a class, other than
+that it fulfills a certain interface.
+
+For instance, a Component can be any class which receives an `args` hash and
+optionally implements any of the component lifecycle hooks:
+
+```js
+export default class FooComponent {
+  didInsertElement() {
+    if (this.args.isShowing) {
+      // do something
+    }
+  }
+}
+```
+
+This method allows users to provide base-less classes, classes which do not
+extend anything, to Ember as well. Ember could continue to provide empty base
+classes to extend from, but it wouldn't have to.
+
+Following a strategy of delegates and interfaces would disentangle Ember from
+the details of the object model as much as possible. Ember would not care about
+the implementation of a class, it would simply assume that it operates by the
+same rules as standard Javascript classes do.
+
+This method would also allow us to extract CoreObject from Ember and provide it
+as a separate package for users who want to continue using the legacy object
+model. So long as their objects match the new interfaces provided by Ember, it
+shouldn't matter what class system they are using.
+
+> Note: Just because we could continue to provide CoreObject as a separate
+package doesn't mean we _should_. The point is that this strategy makes us
+flexible enough to support it, if we choose to.
+
+### Implementation and Adoption
+
+The largest concerns for adopting the delegate pattern are:
+
+1. Implementing the delegate pattern before removing EmberObject, so app
+developers can switch seamlessly.
+2. Giving a large enough buffer to addon authors so that they can switch to the
+delegate pattern without dropping support for Ember versions that are still in
+common use.
+
+We can solve #1 during the Ember v3 lifecycle by changing Ember's code
+internally to treat the existing base classes and native classes as delegates.
+However, we cannot count on being able to remove EmberObject for v4, since
+it's likely that there will still be users of Ember versions that do not support
+delegates. So, the overall roadmap would look like:
+
+- Ember v3: Implement delegates in Ember, allowing users to adopt the new world.
+- Ember v4: Deprecate EmberObject, and prepare the framework for removal.
+- Ember v5: Remove EmberObject.
+
+## How we teach this
+
+Once implemented, we can begin providing examples of creating base-less classes
+for all of Ember's standard constructs. We should update the guides to recommend
+this method, make announcments, and move toward officially deprecating
+EmberObject.
+
+We should also write migration guides that help users through the process of
+converting to native classes. In particular, non-Ember constructs that use
+EmberObject will likely have to change a fair amount. Demonstrating various
+conversions in blog posts and guides will help to show people the path forward,
+and spread knowledge of best practices when using native classes.
+
+## Drawbacks
+
+This approach limits what we can do significantly in framework code (no
+inheritance, can't provide defaults, must provide all behavior external to user
+code). There are many established software patterns other than inheritance for
+providing most behavior, so in practice this shouldn't be an issue.
+
+## Alternatives
+
+- **Gradually rewrite current base classes.** We could attempt to gradually rewrite
+Ember constructs such as Services, Routes, and Components in place. This would
+give us more flexibility for a lot of things, like constructor behavior and
+default method implementations, etc. It would probably take about as long to do
+this as the delegate pattern, and could get very messy along the way, especially
+for removing certain constructs (like Mixins).
+- **Provide new base classes at the same time as the old ones.** We could provide
+new base classes, exposed at different import paths, that users could gradually
+switch to over time. This would give us more flexibility as well, but it would
+also be about as much work and the same adoption timeline as the delegate
+pattern, since Ember would have to support both the old Ember object model and
+the new base classes at the same time until the switch. It would also likely be
+fairly confusing to users, since they would have to make sure they were
+importing the correct classes.
+
+## Unresolved questions
+
+If we go this direction, it means we cannot specify any base class behavior. One
+issue that arises is the fact that dependency injections will not be available
+to the class _during_ object construction. They can be assigned _after_ only.
+
+There are a few options for how we could resolve this:
+
+1. **Provide injected properties as named params in the constructor.** This would
+allow us to mirror the injection definitions, which is fairly intuitive.
+However, accessors and methods which depend on services will not work unless
+the user assigns the service, which may be counterintuitive:
+
+  ```js
+  class Foo {
+    @service time;
+
+    getCurrentTime() {
+      return this.time.now();
+    }
+
+    constructor({ time }) {
+      this.currentTime = time.now(); // this works
+      this.currentTime = this.getCurrentTime(); // this does not
+    }
+  }
+  ```
+
+2. **Provide injected properties as positional params to the constructor, using a
+separate decorator.** This would completely separate named injections from
+constructor injections entirely. This would likely be confusing to newcomers,
+but would be similar to how other DI systems work:
+
+  ```js
+  @inject('time', 'date')
+  class Foo {
+    @service time;
+
+    constructor(time, date) {
+      this.currentTime = time.now();
+      this.currentDate = date.today();
+    }
+  }
+  ```
+
+
+3. **Utilize private global state to provide the container to injections during
+construction.** If done correctly, this would allow users to use injected
+properties during all phases of construction. This would be the most intuitive
+for users since it mirrors the existing behavior, but would be the most
+difficult to maintain:
+
+  ```js
+  const constructionContainers = [];
+
+  function service(target, key, desc) {
+    desc.get = function() {
+      let owner = getOwner(this)
+        || constructionContainers[constructionContainers.length - 1);
+
+      return owner.lookup(`service:${key}`);
+    }
+  }
+
+  class Container {
+    lookup(className) {
+      let Class = this.registry.lookup(className);
+
+      // We push the current container onto a stack, and pop it off when done
+      // constructing, because other lookups may occur during the construction
+      // of this object
+      constructionContainers.push(this);
+      let instance;
+
+      try {
+        instance = new Class();
+      } finally {
+        // remove the current container, even in the case of an error
+        constructionContainers.pop();
+      }
+
+      return instance;
+    }
+  }
+
+  class Foo {
+    @service time;
+
+    constructor() {
+      this.currentTime = this.time.now();
+    }
+  }
+  ```
+
+4. **We could not support injections during object construction.** Instead, we could
+formalize the `init` hook as a separate hook from the constructor, which
+semantically means “initialized by the container”. This may be a reasonable hook
+for the framework to have and could be very useful, even if it seems a bit
+confusing (e.g. what’s the difference between `init` and `constructor`?)