Document view components support (#38656)

actionpack/lib/action_controller/renderer.rb

@@ -82,8 +82,12 @@ def initialize(controller, env, defaults)
     #   need to call <tt>.to_json</tt> on the object you want to render.
     # * <tt>:body</tt> - Renders provided text and sets content type of <tt>text/plain</tt>.
     #
-    # If no <tt>options</tt> hash is passed or if <tt>:update</tt> is specified, the default is
-    # to render a partial and use the second parameter as the locals hash.
+    # If no <tt>options</tt> hash is passed or if <tt>:update</tt> is specified, then:
+    #
+    # If an object responding to `render_in` is passed, `render_in` is called on the object,
+    # passing in the current view context.
+    #
+    # Otherwise, a partial is rendered using the second parameter as the locals hash.
     def render(*args)
       raise "missing controller" unless controller
 

actionpack/test/controller/renderer_test.rb

@@ -70,8 +70,8 @@ def test_render_component
     renderer = ApplicationController.renderer
 
     assert_equal(
-      %(<span title="my title">(Inline render)</span>),
-      renderer.render(TestComponent.new(title: "my title")).strip
+      %(Hello, World!),
+      renderer.render(TestComponent.new)
     )
   end
 

actionpack/test/lib/test_component.rb

 # frozen_string_literal: true
 
-class TestComponent < ActionView::Base
-  delegate :render, to: :view_context
-
-  def initialize(title:)
-    @title = title
-  end
-
-  def render_in(view_context)
-    self.class.compile
-    @view_context = view_context
-    rendered_template
+class TestComponent
+  def render_in(_view_context)
+    "Hello, World!"
   end
 
   def format
     :html
   end
-
-  def self.template
-    <<~'erb'
-    <span title="<%= title %>">(<%= render(plain: "Inline render") %>)</span>
-    erb
-  end
-
-  def self.compile
-    @compiled ||= nil
-    return if @compiled
-
-    class_eval(
-      "def rendered_template; @output_buffer = ActionView::OutputBuffer.new; " +
-      ActionView::Template::Handlers::ERB.erb_implementation.new(template, trim: true).src +
-      "; end"
-    )
-
-    @compiled = true
-  end
-
-private
-  attr_reader :title, :view_context
 end

actionview/lib/action_view/helpers/rendering_helper.rb

@@ -22,8 +22,12 @@ module RenderingHelper
       #   type of <tt>text/plain</tt> from <tt>ActionDispatch::Response</tt>
       #   object.
       #
-      # If no options hash is passed or :update specified, the default is to render a partial and use the second parameter
-      # as the locals hash.
+      # If no <tt>options</tt> hash is passed or if <tt>:update</tt> is specified, then:
+      #
+      # If an object responding to `render_in` is passed, `render_in` is called on the object,
+      # passing in the current view context.
+      #
+      # Otherwise, a partial is rendered using the second parameter as the locals hash.
       def render(options = {}, locals = {}, &block)
         case options
         when Hash

actionview/test/lib/test_component.rb

 # frozen_string_literal: true
 
-class TestComponent < ActionView::Base
-  delegate :render, to: :view_context
-
-  def initialize(title:)
-    @title = title
-  end
-
-  # Entrypoint for rendering. Called by ActionView::RenderingHelper#render.
-  #
-  # Returns ActionView::OutputBuffer.
-  def render_in(view_context, &block)
-    self.class.compile
-    @view_context = view_context
-    @content = view_context.capture(&block) if block_given?
-    rendered_template
-  end
-
-  def self.template
-    <<~'erb'
-    <span title="<%= title %>"><%= content %> (<%= render(plain: "Inline render") %>)</span>
-    erb
+class TestComponent
+  def render_in(_view_context)
+    "Hello, World!"
   end
-
-  def self.compile
-    @compiled ||= nil
-    return if @compiled
-
-    class_eval(
-      "def rendered_template; @output_buffer = ActionView::OutputBuffer.new; " +
-      ActionView::Template::Handlers::ERB.erb_implementation.new(template, trim: true).src +
-      "; end"
-    )
-
-    @compiled = true
-  end
-
-private
-  attr_reader :content, :title, :view_context
 end

actionview/test/template/render_test.rb

@@ -681,8 +681,8 @@ def test_render_throws_exception_when_no_extensions_passed_to_register_template_
 
   def test_render_component
     assert_equal(
-      %(<span title="my title">Hello, World! (Inline render)</span>),
-      @view.render(TestComponent.new(title: "my title")) { "Hello, World!" }.strip
+      %(Hello, World!),
+      @view.render(TestComponent.new)
     )
   end
 end

guides/source/layouts_and_rendering.md

@@ -277,6 +277,16 @@ since an attacker could use this action to access security sensitive files in yo
 
 TIP: `send_file` is often a faster and better option if a layout isn't required.
 
+#### Rendering objects
+
+Rails can render objects responding to `:render_in`.
+
+```ruby
+render MyComponent.new
+```
+
+This calls `render_in` on the provided object with the current view context.
+
 #### Options for `render`
 
 Calls to the `render` method generally accept six options: