Improve rendering outside request

[seanpdoyle]

Follow-up to #529
Closes #613

Remove the & operator from the Turbo::Frames::FrameRequest concern,
since it isn't solving the original issues in the way that it intended.

Instead, add documentation to the README.md that highlights
turbo-rails's compatibility with ActionController::Renderer.

[seanpdoyle]

@jwoodrow thank you for sharing more clarifying details to #613.

I've opened this PR to add stronger guarding conditionals instead of using the & safe access operator.

Having fixed the issue, I want to acknowledge that I dislike adding these kinds of conditionals to concerns that are mixed into a controller context, since the presence of a request is crucial to that context and feels like a safe assumption to have been making.

In #613 you've shared some sample code:

ApplicationController.new.render_to_string(
  formats:  [:xlsx],
  handlers: [:axlsx],
  filename:,
  template: 'path/to/view',
  locals:
)

If this call were split up into two successive calls, you'd have an opportunity to assign a request instance:

controller = ApplicationController.new
controller.request = ActionDispatch::Request.new({})

controller.render_to_string(
  formats:  [:xlsx],
  handlers: [:axlsx],
  filename:,
  template: 'path/to/view',
  locals:
)

If you made that change to your tests, would they pass?

Could you share more context about your particular use case? I wonder if there's a different solution to the underlying problem.

[eugeneius]

Rails provides ActionController:Renderer for rendering templates outside of a request:
https://api.rubyonrails.org/v7.1.3.2/classes/ActionController/Renderer.html

Internally it builds a dummy request object:
https://github.com/rails/rails/blob/v7.1.3.2/actionpack/lib/action_controller/renderer.rb#L124

The sample code from #613 should be rewritten as:

ApplicationController.renderer.render_to_string(
  formats:  [:xlsx],
  handlers: [:axlsx],
  filename:,
  template: 'path/to/view',
  locals:
)

I think instead of this change, #532 should be reverted, since it didn't fix anything and was attempting to support usage that isn't Rails public API.

[seanpdoyle]

@eugeneius great point. I've updated this PR to be more focused on documentation instead.

[brunoprietog]

Thanks @seanpdoyle!

/cc @jorgemanrubia