# 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".dup
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
fiber = Fiber.new do
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
