# frozen_string_literal: true
# :markup: markdown
module ActionController # :nodoc:
# # Action Controller Streaming
#
# Allows views to be streamed back to the client as they are rendered.
#
# By default, Rails renders views by first rendering the template and then the
# layout. The response is sent to the client after the whole template is
# rendered, all queries are made, and the layout is processed.
#
# Streaming inverts the rendering flow by rendering the layout first and
# subsequently each part of the layout as they are processed. This allows the
# header of the HTML (which is usually in the layout) to be streamed back to
# client very quickly, enabling JavaScripts and stylesheets to be loaded earlier
# than usual.
#
# Several Rack middlewares may not work and you need to be careful when
# streaming. This is covered in more detail below, see the Streaming@Middlewares
# section.
#
# Streaming can be added to a given template easily, all you need to do is to
# pass the `:stream` option to `render`.
#
# class PostsController
# def index
# @posts = Post.all
# render stream: true
# end
# end
#
# ## When to use streaming
#
# Streaming may be considered to be overkill for lightweight actions like `new`
# or `edit`. The real benefit of streaming is on expensive actions that, for
# example, do a lot of queries on the database.
#
# In such actions, you want to delay queries execution as much as you can. For
# example, imagine the following `dashboard` action:
#
# def dashboard
# @posts = Post.all
# @pages = Page.all
# @articles = Article.all
# end
#
# Most of the queries here are happening in the controller. In order to benefit
# from streaming you would want to rewrite it as:
#
# def dashboard
# # Allow lazy execution of the queries
# @posts = Post.all
# @pages = Page.all
# @articles = Article.all
# render stream: true
# end
#
# Notice that `:stream` only works with templates. Rendering `:json` or `:xml`
# with `:stream` won't work.
#
# ## Communication between layout and template
#
# When streaming, rendering happens top-down instead of inside-out. Rails starts
# with the layout, and the template is rendered later, when its `yield` is
# reached.
#
# This means that, if your application currently relies on instance variables
# set in the template to be used in the layout, they won't work once you move to
# streaming. The proper way to communicate between layout and template,
# regardless of whether you use streaming or not, is by using `content_for`,
# `provide`, and `yield`.
#
# Take a simple example where the layout expects the template to tell which
# title to use:
#
#
# <%= yield :title %>
# <%= yield %>
#
#
# You would use `content_for` in your template to specify the title:
#
# <%= content_for :title, "Main" %>
# Hello
#
# And the final result would be:
#
#
# Main
# Hello
#
#
# However, if `content_for` is called several times, the final result would have
# all calls concatenated. For instance, if we have the following template:
#
# <%= content_for :title, "Main" %>
# Hello
# <%= content_for :title, " page" %>
#
# The final result would be:
#
#
# Main page
# Hello
#
#
# This means that, if you have `yield :title` in your layout and you want to use
# streaming, you would have to render the whole template (and eventually trigger
# all queries) before streaming the title and all assets, which defeats the
# purpose of streaming. Alternatively, you can use a helper called `provide`
# that does the same as `content_for` but tells the layout to stop searching for
# other entries and continue rendering.
#
# For instance, the template above using `provide` would be:
#
# <%= provide :title, "Main" %>
# Hello
# <%= content_for :title, " page" %>
#
# Resulting in:
#
#
# Main
# Hello
#
#
# That said, when streaming, you need to properly check your templates and
# choose when to use `provide` and `content_for`.
#
# See also ActionView::Helpers::CaptureHelper for more information.
#
# ## Headers, cookies, session, and flash
#
# When streaming, the HTTP headers are sent to the client right before it
# renders the first line. This means that, modifying headers, cookies, session
# or flash after the template starts rendering will not propagate to the client.
#
# ## Middlewares
#
# Middlewares that need to manipulate the body won't work with streaming. You
# should disable those middlewares whenever streaming in development or
# production. For instance, `Rack::Bug` won't work when streaming as it needs to
# inject contents in the HTML body.
#
# Also `Rack::Cache` won't work with streaming as it does not support streaming
# bodies yet. Whenever streaming `Cache-Control` is automatically set to
# "no-cache".
#
# ## Errors
#
# When it comes to streaming, exceptions get a bit more complicated. This
# happens because part of the template was already rendered and streamed to the
# client, making it impossible to render a whole exception page.
#
# Currently, when an exception happens in development or production, Rails will
# automatically stream to the client:
#
# ">