Inject sub-components from context

Hi,

Thanks so much for your work on Fractal.

I am the maintainer of the Drupal integration of Fractal: https://www.drupal.org/project/ui_patterns_fractal

According to the Fractal documentation, we need to set the sub-components inside the template file:

<div class="searchbox">
    <input type="search" name="keywords">
    {{> @button }}
</div>

https://fractal.build/guide/components/sub-components

It would be very useful to be able to keep those templates “sub-components agnostic”, by specifying a simple variable:

<div class="searchbox">
    <input type="search" name="keywords">
    {{ submit }}
</div>

And inject the sub-component from the description file context. For example:

title: Search Box
context:
  submit: @button

Other proposal, with context overriding:

title: Search Box
context:
  submit: 
    render: @button
    text: Go!

For Drupal integration, it is a must-have, because its render system works like that: https://www.drupal.org/docs/8/api/render-api/render-arrays

And I strongly believe this mechanism would be useful even for non-Drupal people.

Is it already possible with Fractal?

PS: I am using the Twig engine by the way: https://github.com/frctl/twig

[tlenex]

https://github.com/frctl/fractal/issues/278#issuecomment-316977660

[mihkeleidast]

This has been recently implemented in #573