WIP: Add ga4-index data attrs in Ruby vs JS

We're now able to provide the index from the facet partials thus
avoiding the need for the code in
GOVUK.analyticsGa4.Ga4FinderTracker.setFilterIndexes(). However, I've
left the code to trigger the "ga4-filter-indexes-added" event for now to
reduce the scope of this change. The plan is that the whole of
setFilterIndexes() will become redundant in a subsequent commit.

Note that the index is 0-indexed vs 1-indexed since the former is how
the Rails indexing was working. This means we currently have to add 1 in
the facet partials when setting the ga4-index.index_section value. It
might make more sense to change the index in FacetsIterator to be
1-indexed and change OptionSelectFacet#closed_by_default? to work with
this.

TODO: I'm not completely convinced that the Facet#has_ga4_section?
method belongs in the model.

app/assets/javascripts/analytics-ga4/ga4-finder-tracker.js

@@ -7,20 +7,7 @@
 
   GOVUK.analyticsGa4.Ga4FinderTracker = {
 
-    // Finds the parent div containing the filters. Loops through each child div that has data-ga4-filter-parent on it . Sets an index on each of these child divs.
     setFilterIndexes: function () {
-      var filterContainer = document.querySelector('[data-ga4-filter-container]')
-
-      if (!filterContainer) {
-        return
-      }
-
-      var filterParents = filterContainer.querySelectorAll('[data-ga4-filter-parent]')
-
-      for (var i = 0; i < filterParents.length; i++) {
-        filterParents[i].setAttribute('data-ga4-index', JSON.stringify({ index_section: i + 1, index_section_count: filterParents.length }))
-      }
-
       window.GOVUK.triggerEvent(window, 'ga4-filter-indexes-added')
     },
 

app/controllers/finders_controller.rb

@@ -113,7 +113,7 @@ def all_facets
   end
 
   def facets
-    all_facets.select(&:filterable?)
+    FacetsIterator.new(all_facets.select(&:filterable?))
   end
 
   def signup_links

app/lib/facets_iterator.rb

@@ -0,0 +1,14 @@
+class FacetsIterator
+  delegate :select, :any?, to: :@facets
+
+  def initialize(facets)
+    @facets = facets
+    @facets_with_ga4_section = @facets.select(&:has_ga4_section?)
+  end
+
+  def each_with_index_and_count
+    @facets.each do |facet|
+      yield facet, @facets_with_ga4_section.index(facet), @facets_with_ga4_section.count
+    end
+  end
+end

app/models/facet.rb

@@ -59,6 +59,10 @@ def ga4_section
     nil
   end
 
+  def has_ga4_section?
+    !ga4_section.nil?
+  end
+
 private
 
   def and_word_connectors

app/views/finders/_date_facet.html.erb

@@ -7,6 +7,6 @@
     aria_controls_id: "js-search-results-info",
     date_errors_to: date_facet.error_message_to(@search_query),
     date_errors_from: date_facet.error_message_from(@search_query),
-    data_attributes:  { "ga4-filter-parent": date_facet.ga4_section }
+    data_attributes:  { "ga4-filter-parent": date_facet.ga4_section, "ga4-index": { index_section: index + 1, index_section_count: count } }
   } %>
 </span>

app/views/finders/_facet_collection.html.erb

@@ -46,8 +46,8 @@
           </div>
         </div>
         <div class="facets__content" data-module="ga4-event-tracker" data-ga4-filter-container>
-          <% facets.each.with_index do |facet, index| %>
-            <%= render partial: facet, object: facet, locals: { index: } %>
+          <% facets.each_with_index_and_count do |facet, index, count| %>
+            <%= render partial: facet, object: facet, locals: { index:, count: } %>
           <% end %>
           <div class="facets__tags-block js-mobile-facet-tag-block"  data-module="remove-filter">
             <%= render "facet_tags", facet_tags.present %>

app/views/finders/_option_select_facet.html.erb

@@ -17,7 +17,8 @@
       :closed_on_load => option_select_facet.closed_on_load?(index),
       :closed_on_load_mobile => option_select_facet.closed_on_load_mobile?,
       :show_filter => option_select_facet.show_option_select_filter,
-      :large => option_select_facet.large?
+      :large => option_select_facet.large?,
+      :data_attributes => { "ga4-change-category": "update-filter checkbox", "ga4-section": option_select_facet.ga4_section, "ga4-index": { index_section: index + 1, index_section_count: count } }
     } %>
   <% end %>
 <% end %>

app/views/finders/_radio_facet.html.erb

@@ -1,4 +1,4 @@
-<%= tag.div(class: "facet-container", data: { "ga4-change-category": "update-filter radio", "ga4-section": "", "ga4-filter-parent": radio_facet.ga4_section }) do %>
+<%= tag.div(class: "facet-container", data: { "ga4-change-category": "update-filter radio", "ga4-filter-parent": radio_facet.ga4_section, "ga4-index": { index_section: index + 1, index_section_count: count } }) do %>
   <%= render "govuk_publishing_components/components/radio", {
     name: radio_facet.key,
     small: true,

app/views/finders/_taxon_facet.html.erb

@@ -1,7 +1,7 @@
 <% unless i_am_a_topic_page_finder %>
   <%= render "components/expander", {
     title: "Topic",
-    data_attributes: { "ga4-filter-parent": taxon_facet.ga4_section }
+    data_attributes: { "ga4-filter-parent": taxon_facet.ga4_section, "ga4-index": { index_section: index + 1, index_section_count: count } }
   } do %>
     <div class="js-taxonomy-select" data-ga4-change-category="update-filter select" data-ga4-section="Topic">
       <%= render "govuk_publishing_components/components/select", {