A pitch to have the ability to go between several form builders in a form, focused on Bullet Train's themed field partials.

themed_form_with.rb

# Provide a BulletTrain::Theme::FormBuilder, with extensions to
# go between Rails standard form helpers and BulletTrain's themed versions seamlessly.

# With standard Rails:
# form_with model: Post.new do |form|
#   form.themed.text_field # Use BulletTrain theming on just one field
#
#   form.themed.fields do # Need a nested section of fields but themed?
#   end
# end

# themed_form_with model: Post.new do |form|
#   # Use BulletTrain theming on every field, but:
#   form.unthemed.text_field # Allow opting out on a field-by-field basis.
#   form.original.text_field # An alternative call name in case we're using multiple different form builders.
# end

class BulletTrain::Theme::FormBuilder < ActionView::Helpers::FormBuilder
  #   themed_form_with model: Post.new do |form|
  #     form.text_field :text_field_value, autofocus: true
  #   end
  #
  # versus:
  #
  #   form_with model: Post.new do |form|
  #     render "shared/fields/text_field", form: form, method: :text_field_value, options: { autofocus: true }
  #   end
  def text_field(method, other_options: nil, **options)
    @template.render "shared/fields/text_field", form: self, other_options: other_options, options: options
  end

  # Then define the other fields…
  def date_field(...)
  end

  # Provide an out to the old form builder. E.g.:
  #   form.unthemed.text_field
  alias_method :unthemed, :original
end

module BulletTrain::Theme::FormExtensions
  ActionView::Helpers::FormBuilder.extend self

  def register_form_builder(accessor_name, klass)
    define_method(accessor_name) { klass.new(@object_name, @object, @template, @options) }
  end

  register_form_builder :original, self
  register_form_builder :themed, BulletTrain::Theme::FormBuilder
  register_from_builder :shoelaced, Shoelace::FormBuilder # Potential builder that'll output `sl-input` etc.
end

# Could also have apps set default_form_builder to BulletTrain::Theme::FormBuilder via an option,
# then form_with would automatically use the theme aware methods.
def themed_form_with(**options, &block)
  form_with(**options, builder: BulletTrain::Theme::FormBuilder, &block)
end