The Road to Ember 2.0

Intro

Today, we're announcing our plan for Ember 2.0. While the major version bump gives us the opportunity to simplify the framework in ways that require breaking changes, we are designing Ember 2.0 with migration in mind.

This is not a big-bang rewrite; we will continue development on the master branch, and roll out changes incrementally on the 1.x release train. The 2.0.0 release will simply remove features that have been deprecated between now and then. Our goal is that you can move your Ember app to 2.0 incrementally, one sprint at a time.

This RFC captures the results of the last two core team face-to-face meetings, where we discussed community feedback about the future of the project. While it explains the high-level goals and tries to paint a picture of how all the pieces fit together, this document will be updated over time with links to individual RFCs that contain additional implementation detail.

We plan to flesh out these more-detailed RFCs in the next few weeks, as the discussion here progresses, before finalizing this plan.

We are announcing Ember 2.0 through our community RFC process in advance of a release, both so our proposals can be vetted by the community and so the community can understand the goals and contribute their own ideas back.

Motivation

Stability without Stagnation

Ember is all about identifying common patterns that emerge from the web development community and rolling them into a complete front-end stack. This makes it easy to get started on new projects and jump into existing ones, knowing that you will get a best-of-breed set of tools that the community will continue to support and improve for years to come.

In the greater JavaScript community, getting the latest and greatest often means rewriting parts of your apps once a year, as the community abandons existing solutions in search of improvements. Progress is important, but so is ending the constant cycle of writing and rewriting that plagues so many applications.

The Ember community works hard to introduce new ideas with an eye towards migration. We call this "stability without stagnation", and it's one of the cornerstones of the Ember philosophy.

Below, we introduce some of the major new features coming in Ember 2.0. Each section includes a transition plan, with details on how we expect existing apps to migrate to the new API.

When breaking changes are absolutely necessary, we try to make those changes ones you can apply without too much thought. We call these "mechanical" refactors. Typically, they'll involve a change to syntax without changing semantics. These are significantly easier to adopt than those that require fundamental changes to your application architecture.

To further aid in these transitions, we are planning to add a new tab to the Ember Inspector that will list all deprecations in your application, as well as a list of the locations in the source code where the deprecated code was triggered. This should serve as a convenient "punch list" for your transitional work.

Every member of the core team works on up-to-date Ember applications, and we feel the tension between stability and progress acutely. We want to deliver cutting-edge products, but need to keep shipping, and many companies that have adopted Ember for their products tell us the same thing.

Big Bets

In 2014, we made big bets in two areas, and they've paid off.

The first bet was on open standards: JavaScript modules, promises and Web Components. We started the year off with globals-based apps, callbacks and "views", and incrementally (and compatibly) built towards standards-based solutions as those standards solidified.

The second bet was that the community was as tired as we were of hand-rolling their own build scripts for each project. We've invested heavily in Ember CLI, giving us a single tool that unifies the community and provides a venue for disseminating great ideas.

In Ember 2.0, Ember CLI and ES6 modules will become first-class parts of the Ember experience. We will update the website, guides, documentation, etc. to teach new users how to build Ember apps with the CLI tools and using JavaScript's new module syntax.

While globals-based apps will continue to work in 2.0, we may introduce new features that rely on either Ember CLI or ES6 modules. You should begin moving your app to Ember CLI as soon as possible.

All of the apps maintained by the Ember core team have been migrated to Ember CLI, and we believe that most teams should be able to make the transition incrementally.

Learning from the Community

We're well aware that we don't have a monopoly on good ideas, and we're always analyzing competing frameworks and libraries to discover great ideas that we can incorporate.

For example, AngularJS taught us the importance of making early on-ramp easy, how cool directives/components could be, and how dependency injection improves testing.

We've been analyzing and discussing React's approach to data flow and rendering for some time now, and in particular how they make use of a "virtual DOM" to improve performance.

Ember's view layer is one of the oldest parts of Ember, and was designed for a world where IE7 and IE8 were dominant. We've spent the better part of 2014 rethinking the view layer to be more DOM-aware, and the new codebase (codenamed "HTMLBars") borrows what we think are the best ideas from React. We cover the specifics below.

React's "virtual DOM" abstraction also allowed them to simplify the programming model of component-based applications. We really like these ideas, and the new HTMLBars engine, landing in the next Ember release, lays the groundwork for adopting the simplified data-flow model.

In Ember 2.0, we will be adopting a "virtual DOM" and data flow model that embraces the best ideas from React and simplifies communication between components.

Interestingly, we found that well-written Ember applications are already written with this clear and direct data flow. This change will mostly make the best patterns more explicit and easier for developers to find when starting out.

A Steady Flow of Improvement

Ember 1.0 shipped over a year ago and we have continued to improve the framework while maintaining backwards-compatibility. We are proud of the fact that Ember apps tend to track released versions.

You might expect us to do Ember 2.0 work on a separate "2.0" branch, accumulating features until we ship. We aren't going to do that.

Instead, we plan to do the vast majority of new work on master (behind feature flags), and land new features in 1.x as they become stable.

The 2.0.0 release will simply remove the cruft that naturally builds up when maintaining compatibility with old releases.

If we add features that change Ember idioms, we will add clear deprecation warnings with steps to refactor to new patterns.

Our goal is that, as much as possible, people will be able to boot up their app on the last 1.x version, update to the latest set of idioms by following the deprecation prompts, and have things working on 2.0.

Because going from the last version of Ember 1.x to Ember 2.0 will be just another six-week release, there simply won't be much time for us to make it an incredibly painful upgrade. ;)

Simplifying Ember Concepts

Ember evolved organically from a view-layer-only framework in 2011 into the route-driven, complete front-end stack it is today. Along the way, we've accumulated several concepts that are no longer widely used in idiomatic Ember apps.

These vestigial concepts make file sizes larger, code more complex, and make Ember harder to learn.

Ember 2.0 is about simplification. This lets us reduce file sizes, reduce code complexity, and generally make Ember apps easier to pick up and maintain.

The high-level set of improvements that we have planned are:

• More intuitive attribute bindings
• New HTML syntax for components
• Block parameters for components
• More consistent template scope
• One-way data binding by default, with opt-in to mutable, two-way bindings
• More explicit communication between components, which means less implicit communication via two-way bindings
• Routes drive components, instead of controller + template
• Improved actions that are invoked inside components as simple callbacks

In some sections, we provide estimates for when a feature will land. These are our best-guesses, but because of the rapid-release train model of Ember, we may be off by a version or two.

However, all features that are slated for "before 2.0" will land before we cut over to a major new version.

More Intuitive Attribute Bindings

Today's templating engine is the oldest part of Ember.js. Under the hood, it generates a string of HTML and then inserts it into the page.

One unfortunate consequence of this architecture is that it is not possible to intuitively bind values to HTML attributes.

You would expect to be able type something like:

<a href="{{url}}">Click here</a>

But instead, in today's Ember, you have to learn about and use the bind-attr helper:

<a {{bind-attr href=url}}>Click here</a>

The new HTMLBars template engine makes bind-attr a thing of the past, allowing you to type what you mean. It also makes it possible to express many attribute-related concepts simply:

<a class="{{active}} app-link" href="{{url}}.html">Click here</a>

Transition Plan

The HTMLBars templating engine is being developed on master, and parts of it have already landed in Ember 1.8. Doing the work this way means that the new engine continues to support the old syntax: your existing templates will continue to work.

The improved attribute syntax has not yet landed, but we expect it to land before Ember 1.10.

We do not plan to remove support for existing templating syntax (or no-longer-necessary helpers like bind-attr) in Ember 2.0.

More Intuitive Components

In today's Ember, components are represented in your templates as Handlebars "block helpers".

The most important problem with this approach is that Handlebars-style components do not work well with attribute bindings or the action helper. In short, a helper that is meant to be used inside an HTML tag cannot be used inside a call to a component.

Beginning in Ember 1.11, we will support an HTML-based syntax for components. The new syntax can be used to invoke existing components, and new components can be called using the old syntax.

<my-video src={{movie.url}}></my-video>

<!-- equivalent to -->

{{my-video src=movie.url}}

Transition Plan

The improved component syntax will (we hope) land in Ember 1.11. You can transition existing uses of {{component-name}} to the new syntax at that time. You will likely benefit by eliminating uses of computed properties that can now be more tersely expressed using the interpolation syntax.

We have no plans to remove support for the old component syntax in Ember 2.0.

Block Parameters

In today's templates, there are two special forms of built-in Handlebars helpers: #each post in posts and #with post as p. These allow the template inside the helper to retain the parent context, but get a piece of helper-provided information as a named value (such as post in the previous examples).

{{#with contact.person as p}}
  {{!-- this block of code is still in the parent's scope, but
        the #with helper provided a `p` name with a
        helper-provided value --}}
  
{{p.firstName}} {{p.lastName}}

  {{!-- `title` here refers to the outer scope's title --}}
  
{{title}}
{{/with}}

Today, this capability is hardcoded into the two special forms, but it can be useful for other kinds of components. For example, you may have a calendar component (ui-calendar) that displays a specified month.

The ui-calendar component may want to allow users to supply a custom template for each day in the month, but each repetition of the template will need information about the day it represents (its day of the week, date number, etc.) in order to render it.

With the new "block parameters" feature, any component will have access to the same capability as #each or #with:

<ui-calendar month={{currentMonth}} as |day|>
  <p class="title">{{day.title}}</p>
  <p class="date">{{day.date}}</p>
</ui-calendar>

In this case, the ui-calendar component iterates over all of days in currentMonth, rendering each instance of the template with information about which date it should represent.

We also think that this feature will be useful to allow container components (like tabs or forms) to supply special-case component definitions as block params. We are still working on the details, but believe that an approach along these lines could make these kinds of components simpler and more flexible.

Transition Plan

Block parameters will hopefully land in 1.12, and at that point the two special forms for {{each}} and {{with}} will be deprecated. You should refactor your templates to use the new block parameters syntax once it lands, as it is a purely mechanical refactor.

We have no plans to remove support for the {{each}} and {{with}} special forms in Ember 2.0.

More Consistent Handlebars Scope

In today's Ember, the each and with helpers come in two flavors: a "context-switching" flavor and a "named-parameter" flavor.

{{#each post in posts}}
  {{!-- the context in here is the same as the outside context,
        and `post` references the current iteration --}}
{{/each}}

{{#each posts}}
  {{!-- the context in here has shifted to the individual post.
        the outer context is no longer accessible --}}
{{/each}}

This has proven to be one of the more confusing parts of the Ember templating system. It is also not clear to beginners which to use, and when they choose the context-shifting form, they lose access to values in the outer context that may be important.

Because the helper itself offers no clue about the context-shifting behavior, it is easy (even for more experienced Ember developers) to get confused when skimming a template about which object a value refers to.

In Ember 1.10, we will deprecate the context-shifting forms of #each and #with in favor of the named-parameter forms.

Transition Plan

To transition your code to the new syntax, you can change templates that look like this:

{{#each people}}
  <p>{{firstName}} {{lastName}}</p>
  <p>{{address}}</p>
{{/each}}

with:

{{#each people as |person|}}
  <p>{{person.firstName}} {{person.lastName}}</p>
  <p>{{person.address}}</p>
{{/each}}

We plan to deprecate support for the context-shifting helpers in Ember 1.10 and remove support in Ember 2.0. This change should be entirely mechanical.

One-Way Bindings by Default

After a few years of having written Ember applications, we have observed that most of the data bindings in the templating engine do not actually require two-way bindings.

When we designed the original templating layer, we figured that making all data bindings two-way wasn't very harmful: if you don't set a two-way binding, it's a de facto one-way binding!

We have since realized (with some help from our friends at React), that components want to be able to hand out data to their children without having to be on guard for wayward mutations.

Additionally, communication between components is often most naturally expressed as events or callbacks. This is possible in Ember, but the dominance of two-way data bindings often leads people down a path of using two-way bindings as a communication channel. Experienced Ember developers don't (usually) make this mistake, but it's an easy one to make.

When you use the new component syntax, the {{}} interpolation syntax defaults to creating one-way bindings in the components.

<my-video src={{url}}></my-video>

In this example, the component's src property will be updated whenever url changes, but it will not be allowed to mutate it.

If a template wishes to allow the component to mutate a property, it can explicitly create a two-way binding using the mut helper:

<my-video paused={{mut isPaused}}></my-video>

This can help ease the transition to a more event-based style of programming.

It also eliminates the boilerplate associated with an event-based style when working with form controls. Instead of copying state out of a model, listening for callbacks, and updating the model, the input helper can be given an explicit mutable binding.

<input value={{mut firstName}}>
<input value={{mut lastName}}>

This is similar to the approach taken by React.Link, but we think that the use-case of form helpers is sufficiently common to make it ergonomic.

Transition Plan

The new one-way default is triggered by the use of new component syntax. This means that component invocations in existing templates will continue to work without changes.

When transitioning to the new HTML-based syntax, you will likely want to evaluate whether bindings are actually being mutated, and avoid using mut for values that the component never changes. This will make it easier for future readers of your template to get an understanding of what properties might be changed downstream.

To preserve the same semantics during a refactor to the new HTML-based syntax, you can simply mark all bindings as mut.

{{!-- these are semantically equivalent --}}

{{my-video src=movie.url paused=controller.isPaused}}

<my-video src={{mut movie.url}} paused={{mut controller.isPaused}}>
</my-video>

While the above example preserves the same mutability semantics, it should be clear that the video player component should never change the url of the movie model.

To make sure you get an exception should this ever happen, simply remove the mut:

<my-video src={{movie.url}} paused={{mut controller.isPaused}}>
</my-video>

We have no plans to remove the old-style component syntax in Ember 2.0, so the semantics of existing component invocations will not change.

Separated Component Parameters

In today's Ember, parameters passed to components as attributes become properties of the component itself, putting them in the same place as other internal state.

This can be somewhat confusing, because it may not be obvious to the reader of a component's JavaScript or template which values are internal, and which are passed in as part of the public API.

To remind themselves, many Ember users write their components like this:

export default Component.extend({
  /* Public API */

  src: null,
  paused: null,
  title: null,

  /* Internal */
  scrubber: null
})

It can also be unclear how to react to a change in the external properties. It is possible to use observers for this purpose in Ember, but observers feel low-level and do not coordinate very well with the rendering process.

To reduce confusion, we plan to move external attributes into a new attrs hash.

If you invoke a component like this:

<my-video src={{movie.url}}></my-video>

then the my-video component accesses the passed-in src attribute as this.attrs.src.

We also plan to provide lifecycle callbacks (modelled after React's lifecycle callbacks) for changes to attrs that will integrate with the rendering lifecycle. We plan to supplement the API with callbacks for changes in individual properties as well.

Transition Plan

In Ember 1.10, we will begin installing provided attributes in the component's attrs hash. If a provided attribute is accessed directly on the component, a deprecation warning will be issued.

In applications, you should update your component JavaScript and templates to access provided attributes via the component's attrs property.

In Ember 2.0, we will stop setting attributes as properties on the component itself.

We will also provide a transitional mixin that Ember addons can use that will make provided attributes available as attrs.*. This will allow add-ons to move to the new location, while maintaining support for older versions of Ember. We expect people to upgrade to Ember 1.10 relatively quickly, and do not expect addons to need to maintain support for Ember 1.9 indefinitely.

Routeable Components

Many people have noticed that controllers in Ember look a lot like components, but with an arbitrary division of responsibilities. We agree!

In current versions of Ember, when a route is entered, it builds a controller, associates a model with it, and hands it off to an (old-style) view for rendering. The view itself is invisible; you just write a template with the correct name.

We plan to transition to: when a route is entered, it renders a component, passing along the model as an attr. This eliminates a vestigial use of old-style views, and associates the top-level template with a regular component.

Transition Plan

Initially, we will continue to support routing to a controller+template, so nothing will break. Going forward, routes will route to a component instead.

In order to do that refactoring, several things will change:

• Instead of referring to model properties directly (or on this), you will refer to them as model.propName.
• Similarly, computed properties that move to your component will need to depend on model.propName if they are migrated from an ObjectController.
• In both cases, the short version is that you can no longer rely on the proxying behavior of ObjectController or ArrayController, but you can remedy the situation by prefixing model. to the property name.
• Unlike controllers, top-level components do not persist across navigation. Persistent state should be stored in route objects and passed as initial properties to routable components.
• In addition to the asynchronous model hook in routes, routes will also be able to define a attrs hook, which can return additional asynchronous data that should be provided to the component.
• Routeable Components should be placed in a "pod" naming convention. For example, the component for the blog-post route would be app/blog-post/component.js.

We plan to land support for routeable components in Ember 1.12, and deprecate routeable controllers at the same time. We plan to remove support for routeable controllers in Ember 2.0. This will allow you to move your codebases over to routeable components piecemeal before making the jump to 2.0.

We will also provide an optional plugin for Ember 2.0 apps that restores existing behavior. This plugin will be included in the Ember automated test suite to ensure that we do not introduce accidental regressions in future releases on the 2.x series.

We realize that this is the change has the largest transitional cost of all the planned features, and we plan to dedicate time to the precise details in the full RFC on this topic.

Improving Actions

Today's components can communicate with their parent component through actions. In particular, the sendAction method allows a child component to invoke a named action on the parent (inside of the actions hash).

Part of the reason for this API was a limitation in the original Handlebars syntax:

{{!-- we can't get too fancy with the value of key-press --}}
{{input key-press="valueChanged"}}

In this example, when the input component calls this.sendAction('key-press'), it invokes the valueChanged action on its parent component.

With the new HTML syntax for components, we have more flexibility:

<input key-press={{action "valueChanged"}}>

This will package up the parent's valueChanged action (in the actions hash) as a callback function that is available to the child component as this.attrs['key-press'].

export default Ember.Component.extend({
  keypress: function(event) {
    this.attrs['key-press'](event.target.value);
  }
});

The benefit of this approach is twofold:

• Actions are no longer treated specially in the component API. They are simply properties packaged up to be called by the child component.
• It is possible to pass an alternative function as the key-press, reducing the child component's knowledge of what the callback is doing. This has testing and abstraction benefits.

Transition Plan

We will continue to support the sendAction API for the forseeable future in today's Handlebars syntax.

When calling an existing component with new HTMLBars syntax, you do not need to change your existing actions hash. You should change syntax that looks like this:

{{video-player playing="playingBegins"}}

To this:

<video-player playing={{action "playingBegins"}}>

The video-player component's internal use of sendAction will work with both calling styles.

New components should use this.attrs.playing(), but existing components that want to continue supporting legacy callers should continue to use sendAction for now. The sendAction API will seamlessly support both calling styles, and will be supported for the forseeable future.

// instead of
this.sendAction('progress', value);

// new code can use
this.attrs.progress(value);

Onward

Version 2.0 marks the transformation of Ember from simply an MVC framework to a complete front-end stack. Between Ember's best-in-class router, revamped components with virtual DOM, easy-to-use build tools, and a growing ecosystem that makes taking advantage of additional libraries a breeze, there's no better way to get started and stay productive developing web apps today.

Hopefully, this plan demonstrates that staying on the cutting-edge can be done without rewriting your app. There are a huge number of Ember apps in production today, and we're looking forward to a time in the very near future where they can start to take advantage these new features.

Expect to see many more RFCs covering these features in depth soon (including a roadmap for Ember Data 1.0). We look forward to hearing your feedback!