1 files changed, 105 insertions, 0 deletions
diff --git a/actionview/lib/action_view/renderer/streaming_template_renderer.rb b/actionview/lib/action_view/renderer/streaming_template_renderer.rb
new file mode 100644
index 0000000000..bb9db21e32
--- /dev/null
+++ b/actionview/lib/action_view/renderer/streaming_template_renderer.rb
@@ -0,0 +1,105 @@
+# frozen_string_literal: true
+
+require "fiber"
+
+module ActionView
+  # == TODO
+  #
+  # * Support streaming from child templates, partials and so on.
+  # * Rack::Cache needs to support streaming bodies
+  class StreamingTemplateRenderer < TemplateRenderer #:nodoc:
+    # A valid Rack::Body (i.e. it responds to each).
+    # It is initialized with a block that, when called, starts
+    # rendering the template.
+    class Body #:nodoc:
+      def initialize(&start)
+        @start = start
+      end
+
+      def each(&block)
+        begin
+          @start.call(block)
+        rescue Exception => exception
+          log_error(exception)
+          block.call ActionView::Base.streaming_completion_on_exception
+        end
+        self
+      end
+
+      private
+
+        # This is the same logging logic as in ShowExceptions middleware.
+        def log_error(exception)
+          logger = ActionView::Base.logger
+          return unless logger
+
+          message = +"\n#{exception.class} (#{exception.message}):\n"
+          message << exception.annoted_source_code.to_s if exception.respond_to?(:annoted_source_code)
+          message << "  " << exception.backtrace.join("\n  ")
+          logger.fatal("#{message}\n\n")
+        end
+    end
+
+    # For streaming, instead of rendering a given a template, we return a Body
+    # object that responds to each. This object is initialized with a block
+    # that knows how to render the template.
+    def render_template(template, layout_name = nil, locals = {}) #:nodoc:
+      return [super] unless layout_name && template.supports_streaming?
+
+      locals ||= {}
+      layout   = layout_name && find_layout(layout_name, locals.keys, [formats.first])
+
+      Body.new do |buffer|
+        delayed_render(buffer, template, layout, @view, locals)
+      end
+    end
+
+    private
+
+      def delayed_render(buffer, template, layout, view, locals)
+        # Wrap the given buffer in the StreamingBuffer and pass it to the
+        # underlying template handler. Now, every time something is concatenated
+        # to the buffer, it is not appended to an array, but streamed straight
+        # to the client.
+        output  = ActionView::StreamingBuffer.new(buffer)
+        yielder = lambda { |*name| view._layout_for(*name) }
+
+        instrument(:template, identifier: template.identifier, layout: layout.try(:virtual_path)) do
+          outer_config = I18n.config
+          fiber = Fiber.new do
+            I18n.config = outer_config
+            if layout
+              layout.render(view, locals, output, &yielder)
+            else
+              # If you don't have a layout, just render the thing
+              # and concatenate the final result. This is the same
+              # as a layout with just <%= yield %>
+              output.safe_concat view._layout_for
+            end
+          end
+
+          # Set the view flow to support streaming. It will be aware
+          # when to stop rendering the layout because it needs to search
+          # something in the template and vice-versa.
+          view.view_flow = StreamingFlow.new(view, fiber)
+
+          # Yo! Start the fiber!
+          fiber.resume
+
+          # If the fiber is still alive, it means we need something
+          # from the template, so start rendering it. If not, it means
+          # the layout exited without requiring anything from the template.
+          if fiber.alive?
+            content = template.render(view, locals, &yielder)
+
+            # Once rendering the template is done, sets its content in the :layout key.
+            view.view_flow.set(:layout, content)
+
+            # In case the layout continues yielding, we need to resume
+            # the fiber until all yields are handled.
+            fiber.resume while fiber.alive?
+          end
+        end
+      end
+  end
+end