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require 'fiber'
module ActionView
# Consider the following layout:
#
# <%= yield :header %>
# 2
# <%= yield %>
# 5
# <%= yield :footer %>
#
# And template:
#
# <%= provide :header, "1" %>
# 3
# 4
# <%= provide :footer, "6" %>
#
# It will stream:
#
# "1\n", "2\n", "3\n4\n", "5\n", "6\n"
#
# Notice that once you <%= yield %>, it will render the whole template
# before streaming again. In the future, we can also support streaming
# from the template and not only the layout.
#
# Also, notice we use +provide+ instead of +content_for+, as +provide+
# gives the control back to the layout as soon as it is called.
# With +content_for+, it would render all the template to find all
# +content_for+ calls. For instance, consider this layout:
#
# <%= yield :header %>
#
# With this template:
#
# <%= content_for :header, "1" %>
# <%= provide :header, "2" %>
# <%= provide :header, "3" %>
#
# It will return "12\n" because +content_for+ continues rendering the
# template but it is returns back to the layout as soon as it sees the
# first +provide+.
#
# == TODO
#
# * Add streaming support in the controllers with no-cache settings
# * What should happen when an error happens?
# * Support streaming from child templates, partials and so on.
# * Support on sprockets async JS load?
#
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)
@start.call(block)
self
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)
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, everytime 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
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