Add support for implicitly rendering renderables

In rails#36388, we added support for rendering objects that respond to
for controller actions.

This PR adds support for implicitly rendering renderables for controller actions as one can for regular templates today.

@bradgessler's post on doing this for Phlex: https://fly.io/ruby-dispatch/hacking-rails-implicit-rendering-for-view-components/
ViewComponent thread discussing the topic: ViewComponent/view_component#618

Author: joelhawksley

actionpack/lib/action_controller/metal/implicit_render.rb

@@ -35,7 +35,9 @@ module ImplicitRender
     include BasicImplicitRender
 
     def default_render
-      if template_exists?(action_name.to_s, _prefixes, variants: request.variant)
+      if renderable = find_renderable(action_name.to_s, _prefixes)
+        render(renderable.new)
+      elsif template_exists?(action_name.to_s, _prefixes, variants: request.variant)
         render
       elsif any_templates?(action_name.to_s, _prefixes)
         message = "#{self.class.name}\##{action_name} is missing a template " \
@@ -60,6 +62,10 @@ def method_for_action(action_name)
     end
 
     private
+      def find_renderable(action_name, prefixes)
+        ActionView::RenderableRegistry.get_renderables(self)[[prefixes, action_name].join("/")]
+      end
+
       def interactive_browser_request?
         request.get? && request.format == Mime[:html] && !request.xhr?
       end

actionview/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Add support for implicitly rendering renderables in controllers.
+
+    *Joel Hawksley*
+
 *   Rename `check_box*` methods into `checkbox*`.
 
     Old names are still available as aliases.

actionview/lib/action_view.rb

@@ -44,6 +44,7 @@ module ActionView
     autoload :PathRegistry
     autoload :PathSet
     autoload :RecordIdentifier
+    autoload :RenderableRegistry
     autoload :Rendering
     autoload :RoutingUrlFor
     autoload :Template

actionview/lib/action_view/renderable_registry.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module ActionView # :nodoc:
+  module RenderableRegistry # :nodoc:
+    @renderables_by_class = Hash.new({})
+
+    def self.get_renderables(klass)
+      @renderables_by_class[klass] || get_renderables(klass.superclass)
+    end
+
+    def self.set_renderable(klass, path, renderable_klass)
+      @renderables_by_class[klass][path] = renderable_klass
+    end
+  end
+end

actionview/lib/action_view/view_paths.rb

@@ -69,6 +69,10 @@ def view_paths=(paths)
         self._view_paths = _build_view_paths(paths)
       end
 
+      def register_renderable(path, renderable_klass)
+        ActionView::RenderableRegistry.set_renderable(self, path, renderable_klass)
+      end
+
       private
         # Override this method in your controller if you want to change paths prefixes for finding views.
         # Prefixes defined here will still be added to parents' <tt>._prefixes</tt>.

actionview/test/actionpack/controller/render_test.rb

@@ -39,6 +39,12 @@ class ValidatingPost < Post
   validates :title, presence: true
 end
 
+class ImplicitRenderable
+  def render_in(_)
+    "implicit renderable"
+  end
+end
+
 class TestController < ActionController::Base
   protect_from_forgery
 
@@ -287,6 +293,9 @@ def formatted_html_erb
   def formatted_xml_erb
   end
 
+  def implicit_renderable
+  end
+
   def render_to_string_test
     @foo = render_to_string inline: "this is a test"
   end
@@ -662,6 +671,7 @@ class RenderTest < ActionController::TestCase
     get :empty_partial_collection, to: "test#empty_partial_collection"
     get :formatted_html_erb, to: "test#formatted_html_erb"
     get :formatted_xml_erb, to: "test#formatted_xml_erb"
+    get :implicit_renderable, to: "test#implicit_renderable"
     get :greeting, to: "test#greeting"
     get :hello_in_a_string, to: "test#hello_in_a_string"
     get :hello_world, to: "fun/games#hello_world"
@@ -772,6 +782,9 @@ def setup
 
     @request.host = "www.nextangle.com"
 
+    # This would likely be handled at by the view object framework
+    ActionController::Base.register_renderable("test/implicit_renderable", ImplicitRenderable)
+
     @old_view_paths = ActionController::Base.view_paths
     ActionController::Base.view_paths = File.join(FIXTURE_LOAD_PATH, "actionpack")
   end
@@ -1122,6 +1135,11 @@ def test_should_render_formatted_html_erb_template_with_faulty_accepts_header
     assert_equal "<test>passed formatted HTML erb</test>", @response.body
   end
 
+  def test_should_render_implicit_renderable
+    get :implicit_renderable
+    assert_equal "implicit renderable", @response.body
+  end
+
   def test_layout_test_with_different_layout
     get :layout_test_with_different_layout
     assert_equal "<html>Hello world!</html>", @response.body