The only possible way to do it is to render a controller in custom tag template. There should be a more simple and flexible solution.

We should allow Developers to configure for each Custom Tag (or Style for that matter) either a controller or renderer which will take over the responsibility of rendering template. Depending on overhead it might be something lighter than fully-featured Symfony controller, but that needs to be discovered during development unfortunately.

Before:

ezrichtext:     custom_tags:         my_tag:             template: '@MyBundle/my_tag.html.twig'
# ...

After:

ezrichtext:     custom_tags:         my_tag:             template: '@MyBundle/my_tag.html.twig'
            controller: 'MyBundle:renderTag'
# ...

Or simpler:

ezrichtext:     custom_tags:         my_tag:             controller: 'MyBundle:renderTag'
# ...

Alternative solution for renderer - TBD.