Extend ActionDispatch::Request

app/controllers/turbo/frames/frame_request.rb

@@ -15,10 +15,11 @@ module Turbo::Frames::FrameRequest
   included do
     layout -> { false if turbo_frame_request? }
     etag { :frame if turbo_frame_request? }
+    before_action { request.variant.push(:turbo_frame) if turbo_frame_request? }
   end
 
   private
     def turbo_frame_request?
-      request.headers["Turbo-Frame"].present?
+      request.turbo_frame?
     end
 end

lib/turbo/engine.rb

@@ -1,5 +1,6 @@
 require "rails/engine"
 require "turbo/test_assertions"
+require "turbo/frames/request_extensions"
 
 module Turbo
   class Engine < Rails::Engine
@@ -43,6 +44,12 @@ class Engine < Rails::Engine
       Mime::Type.register "text/vnd.turbo-stream.html", :turbo_stream
     end
 
+    initializer "turbo.request_extensions" do
+      ActiveSupport.on_load(:action_dispatch_request) do
+        include Turbo::Frames::RequestExtensions
+      end
+    end
+
     initializer "turbo.renderer" do
       ActiveSupport.on_load(:action_controller) do
         ActionController::Renderers.add :turbo_stream do |turbo_streams_html, options|

lib/turbo/frames/request_extensions.rb

@@ -0,0 +1,13 @@
+module Turbo
+  module Frames
+    module RequestExtensions
+      def turbo_frame?
+        turbo_frame.present?
+      end
+
+      def turbo_frame
+        headers["Turbo-Frame"]
+      end
+    end
+  end
+end

test/dummy/app/views/trays/show.html.erb

@@ -1,3 +1,4 @@
 <turbo-frame id="tray">
   <div>This is a tray!</div>
+  <div id="variant"><%= safe_join request.variant, "," %></div>
 </turbo-frame>

test/frames/frame_request_controller_test.rb

@@ -18,4 +18,12 @@ class Turbo::FrameRequestControllerTest < ActionDispatch::IntegrationTest
 
     assert_not_equal etag_with_frame, etag_without_frame
   end
+
+  test "frame requests have a turbo_frame variant" do
+    get tray_path(id: 1)
+    assert_select "#variant", text: ""
+
+    get tray_path(id: 1), headers: { "Turbo-Frame" => "true" }
+    assert_select "#variant", text: "turbo_frame"
+  end
 end