Add GA4 attribute to start now buttons

- add a custom GA4 attribute to be picked up by the link tracker for the 'start now' variant of the button component

app/views/govuk_publishing_components/components/_button.html.erb

@@ -1,5 +1,6 @@
 <%
   add_gem_component_stylesheet("button")
+  disable_ga4 ||= false
 
   # button_helper.css_classes generates "gem-c-button"
   shared_helper = GovukPublishingComponents::Presenters::SharedHelper.new(local_assigns)

app/views/govuk_publishing_components/components/docs/button.yml

@@ -44,6 +44,14 @@ examples:
       href: '#'
       start: true
       rel: external
+  start_now_button_without_ga4_attributes:
+    description: By default the start now button version of this component includes a `data-ga4-attributes` attribute that is used by the specialist (external) link tracker to uniquely identify start now buttons. This attribute can be removed using the `disable_ga4` option.
+    data:
+      text: Start now
+      disable_ga4: true
+      href: '#'
+      start: true
+      rel: external
   secondary_button:
     data:
       text: Secondary button

lib/govuk_publishing_components/presenters/button_helper.rb

@@ -28,6 +28,7 @@ class ButtonHelper
                   :aria_describedby
 
       def initialize(local_assigns)
+        @disable_ga4 = local_assigns[:disable_ga4]
         @href = local_assigns[:href]
         @text = local_assigns[:text]
         @title = local_assigns[:title]
@@ -45,6 +46,7 @@ def initialize(local_assigns)
         @target = local_assigns[:target]
         @type = local_assigns[:type]
         @start = local_assigns[:start]
+        @data_attributes[:ga4_attributes] = ga4_attribute if start
         @secondary = local_assigns[:secondary]
         @secondary_quiet = local_assigns[:secondary_quiet]
         @secondary_solid = local_assigns[:secondary_solid]
@@ -129,6 +131,10 @@ def get_margin_bottom(margin, info_text)
 
         [*0..9].include?(margin) ? "govuk-!-margin-bottom-#{margin}" : legacy_class
       end
+
+      def ga4_attribute
+        { type: "start button" }.to_json unless @disable_ga4
+      end
     end
   end
 end

spec/components/button_spec.rb

@@ -15,6 +15,7 @@ def component_name
     assert_select ".govuk-button[type=submit]", text: "Submit"
     assert_select ".govuk-button--start", false
     assert_select ".gem-c-button__info-text", false
+    assert_select "[data-ga4-attributes]", false
   end
 
   it "renders text correctly" do
@@ -46,6 +47,14 @@ def component_name
     render_component(text: "Start now", href: "#", start: true)
     assert_select ".govuk-button[href='#'] span", text: /Start now/
     assert_select ".govuk-button--start"
+    assert_select ".govuk-button--start[data-ga4-attributes='{\"type\":\"start button\"}']"
+  end
+
+  it "renders start now button with GA4 attributes disabled" do
+    render_component(text: "Start now", href: "#", start: true, disable_ga4: true)
+    assert_select ".govuk-button[href='#'] span", text: /Start now/
+    assert_select ".govuk-button--start"
+    assert_select ".govuk-button--start[data-ga4-attributes='{\"type\":\"start button\"}']", false
   end
 
   it "renders secondary button" do