Template inheritance in Ember - ember.js

Question

I have several unrelated pages that use Layout A, and another set of unrelated pages that use Layout B. It's as simple as that, but I can't figure out how to do this in Ember the DRY way.
I understand that template nesting is equal to route nesting, but I do not want to nest routes because it'd mean the URL will be also nested. I want to nest templates only because the pages are unrelated.
What I want to achieve is essentially template inheritance.
I expected this to work, but Ember throws an error.
// app/routes/samePage.js
import Ember from 'ember';
export default Ember.Route.extend({
renderTemplate(){
this.render('somePage', {
into: 'layoutA'
});
}
});
This is the error I get:
ember.debug.js:18015 Assertion Failed: You attempted to render into 'layoutA' but it was not found
I also get this warning. It tells me to read this link, but I don't think it helps me.
DEPRECATION: Rendering into a {{render}} helper that resolves to an {{outlet}} is deprecated. [deprecation id: ember-routing.top-level-render-helper] See http://emberjs.com/deprecations/v2.x/#toc_rendering-into-a-render-helper-that-resolves-to-an-outlet for more details.
Here is what layoutA.hbs would look like. I know you can't have {{outlet}} multiple times in a same template, but you probably get what I want to achieve.
<div class="header">
{{outlet}}
</div>
<div class="content">
{{outlet}}
</div>
<div class="footer">
{{outlet}}
</div>
How do I go about doing this in Ember? It sounds like such a basic task that needs to be more clear. Do I need to implement a template inheritance helper (like the one shown here) by myself? Or perhaps, there's already an Ember add-on for that?

Answer 1

Unfortunately, there is no such thing as template inheritance in Ember. But there are two features that allows to reuse html-code:
Components is well-known feature of Ember. It allows to have a re-usable template and/or js code to control such template appearance and behaviour. Component has it's own context, so all data should be passed to it via properties. I recommend to use components for custom ui elements or when you need to reuse a part of template with some logic (for example, top navigation with user menu which depends on authentication can be moved to component). There is also a trick that allows to emulate template inheritance with components using multiple {{yield}}. Maybe that's what you want.
Partials seems to be less known (they are not even mentioned in official guide for 2.x) but very useful. This helper ({{partial}}) renders any template in current context. I recommend to use it when you need to break a big template into parts.
These features are enough to reduce an amount of duplicated code. You probably can't reduce duplicated code to zero with them, but in my opinion it's not critical. Just move what you can to partials/components and your templates will be clear enough. Use that trick with component and yield if you want to emulate inheritance.
Update
Couple of words about partials
If you google "ember component vs partial", you can see a few blog posts and answers on SO, in which ppl say "don't use partials". In many cases without explaining why. The main points that I found are:
Components are more isolated, decoupled and testable. And I agree with that.
Partials may be deprecated and removed in future. However, that was first said in year 2015, but at this moment partials are still there and not deprecated.
When I suggest to use partials?
When you have a big template that is hard to maintain and you can't drop that route in parts. It happens if you use some css-framework (like bootstrap or semantic-ui) and need to implement a couple of big 3-4 step forms, or add a couple of modals or display some complex entity. Using components in this case is unneccessary (you will not use them on any other page) and their isolation adds headache (you will need to pass data that you need to display as properties and add some action(s) to get user input in case of forms).
Why I suggest to use partials in this case?
There is no need to reuse such partial, we just want to break template for easier maintenance. For example, to have each step of 3-steps form in it's own .hbs file rather than having one big template.
If partials will be deprecated, it's easy to move that pieces back to one big template.

Answer 2

It sounds to me like your solution is pretty simply just to use Ember components. Ember components have their own template which can nested into your route's template with {{name_of_component}}, however using components does not affect the routing.
Perhaps I am misunderstanding your need, but it seems to me like this is the easy fix. Read about it here. If I am misunderstanding the question, please clarify!

Answer 3

What I want to achieve is similar to using named outlets, so I can output some content into a specific place. However, it seems like Ember's recommend way to do this is to use ember-elsewhere.
The documentation says:
Before named outlets were introduced to Ember the render helper was used to declare slots for this.render in routes. This usage is not common in modern, idiomatic applications and is deprecated. In general, the pattern of named outlets or named render helpers is discouraged. Instead use of ember-elsewhere or another DOM-redirection library should better serve these use cases.
I found this add-on to be easier to understand over partials and components.
This, or using partials/components is probably the best we can get as Handlebars do not have a template inheritance feature.
Or, I should be changing the way I'm creating the app, and use routes to do template nesting. That's probably the Ember way.