module ActionView
# = Action View Cache Helper
module Helpers
module CacheHelper
# This helper exposes a method for caching fragments of a view
# rather than an entire action or page. This technique is useful
# caching pieces like menus, lists of new topics, static HTML
# fragments, and so on. This method takes a block that contains
# the content you wish to cache.
#
# The best way to use this is by doing key-based cache expiration
# on top of a cache store like Memcached that'll automatically
# kick out old entries. For more on key-based expiration, see:
# http://signalvnoise.com/posts/3113-how-key-based-cache-expiration-works
#
# When using this method, you list the cache dependency as the name of the cache, like so:
#
# <% cache project do %>
# All the topics on this project
# <%= render project.topics %>
# <% end %>
#
# This approach will assume that when a new topic is added, you'll touch
# the project. The cache key generated from this call will be something like:
#
# views/projects/123-20120806214154/7a1156131a6928cb0026877f8b749ac9
# ^class ^id ^updated_at ^template tree digest
#
# The cache is thus automatically bumped whenever the project updated_at is touched.
#
# If your template cache depends on multiple sources (try to avoid this to keep things simple),
# you can name all these dependencies as part of an array:
#
# <% cache [ project, current_user ] do %>
# All the topics on this project
# <%= render project.topics %>
# <% end %>
#
# This will include both records as part of the cache key and updating either of them will
# expire the cache.
#
# ==== Template digest
#
# The template digest that's added to the cache key is computed by taking an md5 of the
# contents of the entire template file. This ensures that your caches will automatically
# expire when you change the template file.
#
# Note that the md5 is taken of the entire template file, not just what's within the
# cache do/end call. So it's possible that changing something outside of that call will
# still expire the cache.
#
# Additionally, the digestor will automatically look through your template file for
# explicit and implicit dependencies, and include those as part of the digest.
#
# The digestor can be bypassed by passing skip_digest: true as an option to the cache call:
#
# <% cache project, skip_digest: true do %>
# All the topics on this project
# <%= render project.topics %>
# <% end %>
#
# ==== Implicit dependencies
#
# Most template dependencies can be derived from calls to render in the template itself.
# Here are some examples of render calls that Cache Digests knows how to decode:
#
# render partial: "comments/comment", collection: commentable.comments
# render "comments/comments"
# render 'comments/comments'
# render('comments/comments')
#
# render "header" => render("comments/header")
#
# render(@topic) => render("topics/topic")
# render(topics) => render("topics/topic")
# render(message.topics) => render("topics/topic")
#
# It's not possible to derive all render calls like that, though. Here are a few examples of things that can't be derived:
#
# render group_of_attachments
# render @project.documents.where(published: true).order('created_at')
#
# You will have to rewrite those to the explicit form:
#
# render partial: 'attachments/attachment', collection: group_of_attachments
# render partial: 'documents/document', collection: @project.documents.where(published: true).order('created_at')
#
# === Explicit dependencies
#
# Some times you'll have template dependencies that can't be derived at all. This is typically
# the case when you have template rendering that happens in helpers. Here's an example:
#
# <%= render_sortable_todolists @project.todolists %>
#
# You'll need to use a special comment format to call those out:
#
# <%# Template Dependency: todolists/todolist %>
# <%= render_sortable_todolists @project.todolists %>
#
# The pattern used to match these is /# Template Dependency: ([^ ]+)/, so it's important that you type it out just so.
# You can only declare one template dependency per line.
#
# === External dependencies
#
# If you use a helper method, for example, inside of a cached block and you then update that helper,
# you'll have to bump the cache as well. It doesn't really matter how you do it, but the md5 of the template file
# must change. One recommendation is to simply be explicit in a comment, like:
#
# <%# Helper Dependency Updated: May 6, 2012 at 6pm %>
# <%= some_helper_method(person) %>
#
# Now all you'll have to do is change that timestamp when the helper method changes.
def cache(name = {}, options = nil, &block)
if controller.perform_caching
safe_concat(fragment_for(cache_fragment_name(name, options), options, &block))
else
yield
end
nil
end
# Cache fragments of a view if +condition+ is true
#
# <%= cache_if admin?, project do %>
# All the topics on this project
# <%= render project.topics %>
# <% end %>
def cache_if(condition, name = {}, options = nil, &block)
if condition
cache(name, options, &block)
else
yield
end
nil
end
# Cache fragments of a view unless +condition+ is true
#
# <%= cache_unless admin?, project do %>
# All the topics on this project
# <%= render project.topics %>
# <% end %>
def cache_unless(condition, name = {}, options = nil, &block)
cache_if !condition, name, options, &block
end
# This helper returns the name of a cache key for a given fragment cache
# call. By supplying skip_digest: true to cache, the digestion of cache
# fragments can be manually bypassed. This is useful when cache fragments
# cannot be manually expired unless you know the exact key which is the
# case when using memcached.
def cache_fragment_name(name = {}, options = nil)
skip_digest = options && options[:skip_digest]
if skip_digest
name
else
fragment_name_with_digest(name)
end
end
private
def fragment_name_with_digest(name) #:nodoc:
if @virtual_path
names = Array(name.is_a?(Hash) ? controller.url_for(name).split("://").last : name)
digest = Digestor.digest name: @virtual_path, finder: lookup_context, dependencies: view_cache_dependencies
[ *names, digest ]
else
name
end
end
# TODO: Create an object that has caching read/write on it
def fragment_for(name = {}, options = nil, &block) #:nodoc:
read_fragment_for(name, options) || write_fragment_for(name, options, &block)
end
def read_fragment_for(name, options) #:nodoc:
controller.read_fragment(name, options)
end
def write_fragment_for(name, options) #:nodoc:
# VIEW TODO: Make #capture usable outside of ERB
# This dance is needed because Builder can't use capture
pos = output_buffer.length
yield
output_safe = output_buffer.html_safe?
fragment = output_buffer.slice!(pos..-1)
if output_safe
self.output_buffer = output_buffer.class.new(output_buffer)
end
controller.write_fragment(name, fragment, options)
end
end
end
end