Declare ActionView::Helpers::FormBuilder#id and #field_id

[seanpdoyle]

Summary

ActionView::Helpers::FormBuilder#id

Generate an HTML id attribute value.

Return the <form> element's id attribute.

<%= form_for @post do |f| %>
  <%# ... %>

  <% content_for :sticky_footer do %>
    <%= form.button(form: f.id) %>
  <% end %>
<% end %>

In the example above, the :sticky_footer content area will exist
outside of the <form> element. By declaring the form HTML
attribute, we hint to the browser that the
generated <button> element should be treated as the <form> element's
submit button, regardless of where it exists in the DOM.

A similar pattern could be used for <input>
elements (or other form controls) that do not
descend from the <form> element.

ActionView::Helpers::FormBuilder#field_id

Generate an HTML id attribute value for the given field

Return the value generated by the FormBuilder for the given
attribute name.

<%= form_for @post do |f| %>
  <%= f.label :title %>
  <%= f.text_field :title, aria: { describedby: f.field_id(:title, :error) } %>
  <span id="<%= f.field_id(:title, :error) %>">is blank</span>
<% end %>

In the example above, the element built by
the call to FormBuilder#text_field declares an
aria-describedby attribute referencing the
element, sharing a common id root (post_title, in this
case).

This method is powered by the field_id helper declared in
action_view/helpers/form_tag_helper, which is made available for
general template calls, separate from a FormBuilder instance.

[seanpdoyle]

Copying #40127 (comment) here, since the line in question has changed.

This set of changes was lifted directly from ActionView::Helpers::Tags::Base.

rails/actionview/lib/action_view/helpers/tags/base.rb

Lines 119 to 137 in 7101489

	def tag_id(index = nil)
	# a little duplication to construct fewer strings
	case
	when @object_name.empty?
	sanitized_method_name.dup
	when index
	"#{sanitized_object_name}_#{index}_#{sanitized_method_name}"
	else
	"#{sanitized_object_name}_#{sanitized_method_name}"
	end
	end
	def sanitized_object_name
	@sanitized_object_name ||= @object_name.gsub(/\]\[|[^-a-zA-Z0-9:.]/, "_").delete_suffix("_")
	end
	def sanitized_method_name
	@sanitized_method_name ||= @method_name.delete_suffix("?")
	end

Given the "a little duplication to construct fewer strings", it seemed like that code was finely tuned for performance, which made me hesitant to abstract out a shared and re-usable chunk of code.

My gut tells me that FormBuilder#field_id will be invoked less frequently than the Tags::Base code. That being said, if there is an abstraction that feels right, I'm all for extracting it.

[seanpdoyle]

My first attempt is to extract out a shared constant (as introduced in 419e179).

[rafaelfranca]

AFAIK ActionView::Helpers::Tags::Base has acces to the template so we should probably just move tag_id implementation to field_id and call field_id from tag_id.

[seanpdoyle]

@rafaelfranca I'll try that out. The # a little duplication to construct fewer strings comments make me a little nervous about moving that code around.

To move that definition elsewhere, we'd likely lose some of the memoization that's going on. Is there a way to gauge the impact of these changes?

[seanpdoyle]

@rafaelfranca I've pushed up c5869ce to try that out.

[rafaelfranca]

Right, that is why I suggested moving the entire code, so we keep the # a little duplication to construct fewer strings comment and the duplication to construct fewer strings.

[p8]

This could come in handy 😄
I like the field_id method better than the earlier id(:title) implementation.
field_id looks somewhat similar to dom_id https://api.rubyonrails.org/classes/ActionView/RecordIdentifier.html#method-i-dom_id
As field_id would also be used for other form tags like textarea, checkbox and select, maybe we can doform.dom_id(:title, :error) instead?

[seanpdoyle]

maybe we can do form.dom_id(:title, :error) instead?

The field_id helper is directly inspired by dom_id!

However, there are some differences that could be confusing and surprising if they shared the same name. In our examples, the :error value is a suffix added after the post_title portion, whereas dom_id's second argument is a prefix added before.

[rails-bot]

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[rafaelfranca]

Just a small comment, otherwise looks good to me. Can you squash all commits when you are done with the last fix?

[seanpdoyle]

Squashed and pushed. Thank you @rafaelfranca.