Include minimal layout for frame responses

Previously, responses to frame requests were rendered without any layout. Since Turbo does not need the content outside of the frame, reducing the amount that is rendered can be a useful optimisation. However, this optimisation prevents responses from specifying `head` content as well. There are cases where it would be useful for Turbo to have access to items specified in head, like the meta tags used for specifying visit and cache control. To get around this, we change the optimisation to use a minimal layout for frame responses, rather than no layout. With this, applications can render content into the `head` if they want, and the Turbo Rails helpers like `turbo_page_requires_reload` will also work.
hotwired · Feb 7, 2023 · 5614797 · 5614797
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diff --git a/app/controllers/turbo/frames/frame_request.rb b/app/controllers/turbo/frames/frame_request.rb
@@ -1,19 +1,27 @@
 # Turbo frame requests are requests made from within a turbo frame with the intention of replacing the content of just
 # that frame, not the whole page. They are automatically tagged as such by the Turbo Frame JavaScript, which adds a
-# <tt>Turbo-Frame</tt> header to the request. When that header is detected by the controller, we ensure that any
-# template layout is skipped (since we're only working on an in-page frame, thus can skip the weight of the layout), and
-# that the etag for the page is changed (such that a cache for a layout-less request isn't served on a normal request
-# and vice versa).
+# <tt>Turbo-Frame</tt> header to the request.
 #
-# This is merely a rendering optimization. Everything would still work just fine if we rendered everything including the layout.
-# Turbo Frames knows how to fish out the relevant frame regardless.
+# When that header is detected by the controller, we substitute our own minimal layout in place of the
+# application-supplied layout (since we're only working on an in-page frame, thus can skip the weight of the layout). We
+# use a minimal layout, rather than avoid the layout entirely, so that it's still possible to render content into the
+# <tt>head<tt>.
+#
+# Accordingly, we ensure that the etag for the page is changed, such that a cache for a minimal-layout request isn't
+# served on a normal request and vice versa.
+#
+# This is merely a rendering optimization. Everything would still work just fine if we rendered everything including the
+# full layout. Turbo Frames knows how to fish out the relevant frame regardless.
+#
+# The layout used is <tt>turbo_rails/frame.html.erb</tt>. If there's a need to customize this layout, an application can
+# supply its own (such as <tt>app/views/layouts/turbo_rails/frame.html.erb</tt>) which will be used instead.
 #
 # This module is automatically included in <tt>ActionController::Base</tt>.
 module Turbo::Frames::FrameRequest
   extend ActiveSupport::Concern
 
   included do
-    layout -> { false if turbo_frame_request? }
+    layout -> { "turbo_rails/frame" if turbo_frame_request? }
     etag { :frame if turbo_frame_request? }
   end
 

diff --git a/app/views/layouts/turbo_rails/frame.html.erb b/app/views/layouts/turbo_rails/frame.html.erb
@@ -0,0 +1,8 @@
+<html>
+  <head>
+    <%= yield :head %>
+  </head>
+  <body>
+    <%= yield %>
+  </body>
+</html>
diff --git a/test/dummy/app/views/trays/show.html.erb b/test/dummy/app/views/trays/show.html.erb
@@ -1,3 +1,7 @@
+<% content_for :head do %>
+  <meta name="test" content="present" />
+<% end %>
+
 <turbo-frame id="tray">
   <div>This is a tray!</div>
   <div><%= @frame_id %></div>

diff --git a/test/frames/frame_request_controller_test.rb b/test/frames/frame_request_controller_test.rb
@@ -1,14 +1,30 @@
 require "test_helper"
 
 class Turbo::FrameRequestControllerTest < ActionDispatch::IntegrationTest
-  test "frame requests are rendered without a layout" do
+  test "frame requests are rendered with a minimal layout" do
     get tray_path(id: 1)
     assert_select "title", count: 1
 
     get tray_path(id: 1), headers: { "Turbo-Frame" => "true" }
     assert_select "title", count: 0
   end
 
+  test "frame request layout includes `head` content" do
+    get tray_path(id: 1), headers: { "Turbo-Frame" => "true" }
+
+    assert_select "head", count: 1
+    assert_select "meta[name=test][content=present]"
+  end
+
+  test "frame request layout can be overridden" do
+    with_prepended_view_path "test/frames/views" do
+      get tray_path(id: 1), headers: { "Turbo-Frame" => "true" }
+    end
+
+    assert_select "meta[name=test][content=present]"
+    assert_select "meta[name=alternative][content=present]"
+  end
+
   test "frame requests get a unique etag" do
     get tray_path(id: 1)
     etag_without_frame = @response.headers["ETag"]
@@ -28,4 +44,13 @@ class Turbo::FrameRequestControllerTest < ActionDispatch::IntegrationTest
     get tray_path(id: 1), headers: { "Turbo-Frame" => turbo_frame_request_id }
     assert_match /#{turbo_frame_request_id}/, @response.body
   end
+
+  private
+    def with_prepended_view_path(path, &block)
+      previous_view_paths = ApplicationController.view_paths
+      ApplicationController.prepend_view_path path
+      yield
+    ensure
+      ApplicationController.view_paths = previous_view_paths
+    end
 end
diff --git a/test/frames/views/layouts/turbo_rails/frame.html.erb b/test/frames/views/layouts/turbo_rails/frame.html.erb
@@ -0,0 +1,9 @@
+<html>
+  <head>
+    <meta name="alternative" content="present" />
+    <%= yield :head %>
+  </head>
+  <body>
+    <%= yield %>
+  </body>
+</html>