require 'cgi'
require 'action_view/helpers/url_helper'
require 'action_view/helpers/tag_helper'
module ActionView
module Helpers #:nodoc:
# This module provides methods for generating HTML that links views to assets such
# as images, javascripts, stylesheets, and feeds. These methods do not verify
# the assets exist before linking to them.
#
# === Using asset hosts
# By default, Rails links to these assets on the current host in the public
# folder, but you can direct Rails to link to assets from a dedicated assets server by
# setting ActionController::Base.asset_host in your config/environment.rb. For example,
# let's say your asset host is assets.example.com.
#
# ActionController::Base.asset_host = "assets.example.com"
# image_tag("rails.png")
# =>
# stylesheet_link_tag("application")
# =>
#
# This is useful since browsers typically open at most two connections to a single host,
# which means your assets often wait in single file for their turn to load. You can
# alleviate this by using a %d wildcard in asset_host (for example, "assets%d.example.com")
# to automatically distribute asset requests among four hosts (e.g., "assets0.example.com" through "assets3.example.com")
# so browsers will open eight connections rather than two.
#
# image_tag("rails.png")
# =>
# stylesheet_link_tag("application")
# =>
#
# To do this, you can either setup 4 actual hosts, or you can use wildcard DNS to CNAME
# the wildcard to a single asset host. You can read more about setting up your DNS CNAME records from
# your ISP.
#
# Note: This is purely a browser performance optimization and is not meant
# for server load balancing. See http://www.die.net/musings/page_load_time/
# for background.
#
# Alternatively, you can exert more control over the asset host by setting asset_host to a proc
# that takes a single source argument. This is useful if you are unable to setup 4 actual hosts or have
# fewer/more than 4 hosts. The example proc below generates http://assets1.example.com and
# http://assets2.example.com randomly.
#
# ActionController::Base.asset_host = Proc.new { |source| "http://assets#{rand(2) + 1}.example.com" }
# image_tag("rails.png")
# =>
# stylesheet_link_tag("application")
# =>
#
# The proc takes a source parameter (which is the path of the source asset) and an optional
# request parameter (which is an entire instance of an ActionController::AbstractRequest
# subclass). This can be used to generate a particular asset host depending on the asset path and the particular
# request.
#
# ActionController::Base.asset_host = Proc.new { |source|
# if source.starts_with?('/images')
# "http://images.example.com"
# else
# "http://assets.example.com"
# end
# }
# image_tag("rails.png")
# =>
# stylesheet_link_tag("application")
# =>
#
# The optional request parameter to the proc is useful in particular for serving assets from an
# SSL-protected page. The example proc below disables asset hosting for HTTPS connections, while still sending
# assets for plain HTTP requests from asset hosts. This is useful for avoiding mixed media warnings when serving
# non-HTTP assets from HTTPS web pages when you don't have an SSL certificate for each of the asset hosts.
#
# ActionController::Base.asset_host = Proc.new { |source, request|
# if request.ssl?
# "#{request.protocol}#{request.host_with_port}"
# else
# "#{request.protocol}assets.example.com"
# end
# }
#
# === Using asset timestamps
#
# By default, Rails will append all asset paths with that asset's timestamp. This allows you to set a cache-expiration date for the
# asset far into the future, but still be able to instantly invalidate it by simply updating the file (and hence updating the timestamp,
# which then updates the URL as the timestamp is part of that, which in turn busts the cache).
#
# It's the responsibility of the web server you use to set the far-future expiration date on cache assets that you need to take
# advantage of this feature. Here's an example for Apache:
#
# # Asset Expiration
# ExpiresActive On
#
# ExpiresDefault "access plus 1 year"
#
#
# Also note that in order for this to work, all your application servers must return the same timestamps. This means that they must
# have their clocks synchronized. If one of them drift out of sync, you'll see different timestamps at random and the cache won't
# work. Which means that the browser will request the same assets over and over again even thought they didn't change. You can use
# something like Live HTTP Headers for Firefox to verify that the cache is indeed working (and that the assets are not being
# requested over and over).
module AssetTagHelper
ASSETS_DIR = defined?(Rails.public_path) ? Rails.public_path : "public"
JAVASCRIPTS_DIR = "#{ASSETS_DIR}/javascripts"
STYLESHEETS_DIR = "#{ASSETS_DIR}/stylesheets"
JAVASCRIPT_DEFAULT_SOURCES = ['prototype', 'effects', 'dragdrop', 'controls'].freeze unless const_defined?(:JAVASCRIPT_DEFAULT_SOURCES)
# Returns a link tag that browsers and news readers can use to auto-detect
# an RSS or ATOM feed. The +type+ can either be :rss (default) or
# :atom. Control the link options in url_for format using the
# +url_options+. You can modify the LINK tag itself in +tag_options+.
#
# ==== Options:
# * :rel - Specify the relation of this link, defaults to "alternate"
# * :type - Override the auto-generated mime type
# * :title - Specify the title of the link, defaults to the +type+
#
# ==== Examples
# auto_discovery_link_tag # =>
#
# auto_discovery_link_tag(:atom) # =>
#
# auto_discovery_link_tag(:rss, {:action => "feed"}) # =>
#
# auto_discovery_link_tag(:rss, {:action => "feed"}, {:title => "My RSS"}) # =>
#
# auto_discovery_link_tag(:rss, {:controller => "news", :action => "feed"}) # =>
#
# auto_discovery_link_tag(:rss, "http://www.example.com/feed.rss", {:title => "Example RSS"}) # =>
#
def auto_discovery_link_tag(type = :rss, url_options = {}, tag_options = {})
tag(
"link",
"rel" => tag_options[:rel] || "alternate",
"type" => tag_options[:type] || Mime::Type.lookup_by_extension(type.to_s).to_s,
"title" => tag_options[:title] || type.to_s.upcase,
"href" => url_options.is_a?(Hash) ? url_for(url_options.merge(:only_path => false)) : url_options
)
end
# Computes the path to a javascript asset in the public javascripts directory.
# If the +source+ filename has no extension, .js will be appended.
# Full paths from the document root will be passed through.
# Used internally by javascript_include_tag to build the script path.
#
# ==== Examples
# javascript_path "xmlhr" # => /javascripts/xmlhr.js
# javascript_path "dir/xmlhr.js" # => /javascripts/dir/xmlhr.js
# javascript_path "/dir/xmlhr" # => /dir/xmlhr.js
# javascript_path "http://www.railsapplication.com/js/xmlhr" # => http://www.railsapplication.com/js/xmlhr.js
# javascript_path "http://www.railsapplication.com/js/xmlhr.js" # => http://www.railsapplication.com/js/xmlhr.js
def javascript_path(source)
JavaScriptTag.create(self, @controller, source).public_path
end
alias_method :path_to_javascript, :javascript_path # aliased to avoid conflicts with a javascript_path named route
# Returns an html script tag for each of the +sources+ provided. You
# can pass in the filename (.js extension is optional) of javascript files
# that exist in your public/javascripts directory for inclusion into the
# current page or you can pass the full path relative to your document
# root. To include the Prototype and Scriptaculous javascript libraries in
# your application, pass :defaults as the source. When using
# :defaults, if an application.js file exists in your public
# javascripts directory, it will be included as well. You can modify the
# html attributes of the script tag by passing a hash as the last argument.
#
# ==== Examples
# javascript_include_tag "xmlhr" # =>
#
#
# javascript_include_tag "xmlhr.js" # =>
#
#
# javascript_include_tag "common.javascript", "/elsewhere/cools" # =>
#
#
#
# javascript_include_tag "http://www.railsapplication.com/xmlhr" # =>
#
#
# javascript_include_tag "http://www.railsapplication.com/xmlhr.js" # =>
#
#
# javascript_include_tag :defaults # =>
#
#
# ...
#
#
# * = The application.js file is only referenced if it exists
#
# Though it's not really recommended practice, if you need to extend the default JavaScript set for any reason
# (e.g., you're going to be using a certain .js file in every action), then take a look at the register_javascript_include_default method.
#
# You can also include all javascripts in the javascripts directory using :all as the source:
#
# javascript_include_tag :all # =>
#
#
# ...
#
#
#
#
# Note that the default javascript files will be included first. So Prototype and Scriptaculous are available to
# all subsequently included files.
#
# If you want Rails to search in all the subdirectories under javascripts, you should explicitly set :recursive:
#
# javascript_include_tag :all, :recursive => true
#
# == Caching multiple javascripts into one
#
# You can also cache multiple javascripts into one file, which requires less HTTP connections to download and can better be
# compressed by gzip (leading to faster transfers). Caching will only happen if ActionController::Base.perform_caching
# is set to true (which is the case by default for the Rails production environment, but not for the development
# environment).
#
# ==== Examples
# javascript_include_tag :all, :cache => true # when ActionController::Base.perform_caching is false =>
#
#
# ...
#
#
#
#
# javascript_include_tag :all, :cache => true # when ActionController::Base.perform_caching is true =>
#
#
# javascript_include_tag "prototype", "cart", "checkout", :cache => "shop" # when ActionController::Base.perform_caching is false =>
#
#
#
#
# javascript_include_tag "prototype", "cart", "checkout", :cache => "shop" # when ActionController::Base.perform_caching is true =>
#
#
# The :recursive option is also available for caching:
#
# javascript_include_tag :all, :cache => true, :recursive => true
def javascript_include_tag(*sources)
options = sources.extract_options!.stringify_keys
cache = options.delete("cache")
recursive = options.delete("recursive")
if ActionController::Base.perform_caching && cache
joined_javascript_name = (cache == true ? "all" : cache) + ".js"
joined_javascript_path = File.join(JAVASCRIPTS_DIR, joined_javascript_name)
unless File.exists?(joined_javascript_path)
JavaScriptSources.create(self, @controller, sources, recursive).write_asset_file_contents(joined_javascript_path)
end
javascript_src_tag(joined_javascript_name, options)
else
JavaScriptSources.create(self, @controller, sources, recursive).expand_sources.collect { |source|
javascript_src_tag(source, options)
}.join("\n")
end
end
# Register one or more javascript files to be included when symbol
# is passed to javascript_include_tag. This method is typically intended
# to be called from plugin initialization to register javascript files
# that the plugin installed in public/javascripts.
#
# ActionView::Helpers::AssetTagHelper.register_javascript_expansion :monkey => ["head", "body", "tail"]
#
# javascript_include_tag :monkey # =>
#
#
#
def self.register_javascript_expansion(expansions)
JavaScriptSources.expansions.merge!(expansions)
end
# Register one or more stylesheet files to be included when symbol
# is passed to stylesheet_link_tag. This method is typically intended
# to be called from plugin initialization to register stylesheet files
# that the plugin installed in public/stylesheets.
#
# ActionView::Helpers::AssetTagHelper.register_stylesheet_expansion :monkey => ["head", "body", "tail"]
#
# stylesheet_link_tag :monkey # =>
#
#
#
def self.register_stylesheet_expansion(expansions)
StylesheetSources.expansions.merge!(expansions)
end
# Register one or more additional JavaScript files to be included when
# javascript_include_tag :defaults is called. This method is
# typically intended to be called from plugin initialization to register additional
# .js files that the plugin installed in public/javascripts.
def self.register_javascript_include_default(*sources)
JavaScriptSources.expansions[:defaults].concat(sources)
end
def self.reset_javascript_include_default #:nodoc:
JavaScriptSources.expansions[:defaults] = JAVASCRIPT_DEFAULT_SOURCES.dup
end
# Computes the path to a stylesheet asset in the public stylesheets directory.
# If the +source+ filename has no extension, .css will be appended.
# Full paths from the document root will be passed through.
# Used internally by +stylesheet_link_tag+ to build the stylesheet path.
#
# ==== Examples
# stylesheet_path "style" # => /stylesheets/style.css
# stylesheet_path "dir/style.css" # => /stylesheets/dir/style.css
# stylesheet_path "/dir/style.css" # => /dir/style.css
# stylesheet_path "http://www.railsapplication.com/css/style" # => http://www.railsapplication.com/css/style.css
# stylesheet_path "http://www.railsapplication.com/css/style.js" # => http://www.railsapplication.com/css/style.css
def stylesheet_path(source)
StylesheetTag.create(self, @controller, source).public_path
end
alias_method :path_to_stylesheet, :stylesheet_path # aliased to avoid conflicts with a stylesheet_path named route
# Returns a stylesheet link tag for the sources specified as arguments. If
# you don't specify an extension, .css will be appended automatically.
# You can modify the link attributes by passing a hash as the last argument.
#
# ==== Examples
# stylesheet_link_tag "style" # =>
#
#
# stylesheet_link_tag "style.css" # =>
#
#
# stylesheet_link_tag "http://www.railsapplication.com/style.css" # =>
#
#
# stylesheet_link_tag "style", :media => "all" # =>
#
#
# stylesheet_link_tag "style", :media => "print" # =>
#
#
# stylesheet_link_tag "random.styles", "/css/stylish" # =>
#
#
#
# You can also include all styles in the stylesheets directory using :all as the source:
#
# stylesheet_link_tag :all # =>
#
#
#
#
# If you want Rails to search in all the subdirectories under stylesheets, you should explicitly set :recursive:
#
# stylesheet_link_tag :all, :recursive => true
#
# == Caching multiple stylesheets into one
#
# You can also cache multiple stylesheets into one file, which requires less HTTP connections and can better be
# compressed by gzip (leading to faster transfers). Caching will only happen if ActionController::Base.perform_caching
# is set to true (which is the case by default for the Rails production environment, but not for the development
# environment). Examples:
#
# ==== Examples
# stylesheet_link_tag :all, :cache => true # when ActionController::Base.perform_caching is false =>
#
#
#
#
# stylesheet_link_tag :all, :cache => true # when ActionController::Base.perform_caching is true =>
#
#
# stylesheet_link_tag "shop", "cart", "checkout", :cache => "payment" # when ActionController::Base.perform_caching is false =>
#
#
#
#
# stylesheet_link_tag "shop", "cart", "checkout", :cache => "payment" # when ActionController::Base.perform_caching is true =>
#
#
# The :recursive option is also available for caching:
#
# stylesheet_link_tag :all, :cache => true, :recursive => true
def stylesheet_link_tag(*sources)
options = sources.extract_options!.stringify_keys
cache = options.delete("cache")
recursive = options.delete("recursive")
if ActionController::Base.perform_caching && cache
joined_stylesheet_name = (cache == true ? "all" : cache) + ".css"
joined_stylesheet_path = File.join(STYLESHEETS_DIR, joined_stylesheet_name)
unless File.exists?(joined_stylesheet_path)
StylesheetSources.create(self, @controller, sources, recursive).write_asset_file_contents(joined_stylesheet_path)
end
stylesheet_tag(joined_stylesheet_name, options)
else
StylesheetSources.create(self, @controller, sources, recursive).expand_sources.collect { |source|
stylesheet_tag(source, options)
}.join("\n")
end
end
# Computes the path to an image asset in the public images directory.
# Full paths from the document root will be passed through.
# Used internally by +image_tag+ to build the image path.
#
# ==== Examples
# image_path("edit") # => /images/edit
# image_path("edit.png") # => /images/edit.png
# image_path("icons/edit.png") # => /images/icons/edit.png
# image_path("/icons/edit.png") # => /icons/edit.png
# image_path("http://www.railsapplication.com/img/edit.png") # => http://www.railsapplication.com/img/edit.png
def image_path(source)
ImageTag.create(self, @controller, source).public_path
end
alias_method :path_to_image, :image_path # aliased to avoid conflicts with an image_path named route
# Returns an html image tag for the +source+. The +source+ can be a full
# path or a file that exists in your public images directory.
#
# ==== Options
# You can add HTML attributes using the +options+. The +options+ supports
# three additional keys for convenience and conformance:
#
# * :alt - If no alt text is given, the file name part of the
# +source+ is used (capitalized and without the extension)
# * :size - Supplied as "{Width}x{Height}", so "30x45" becomes
# width="30" and height="45". :size will be ignored if the
# value is not in the correct format.
# * :mouseover - Set an alternate image to be used when the onmouseover
# event is fired, and sets the original image to be replaced onmouseout.
# This can be used to implement an easy image toggle that fires on onmouseover.
#
# ==== Examples
# image_tag("icon") # =>
#
# image_tag("icon.png") # =>
#
# image_tag("icon.png", :size => "16x10", :alt => "Edit Entry") # =>
#
# image_tag("/icons/icon.gif", :size => "16x16") # =>
#
# image_tag("/icons/icon.gif", :height => '32', :width => '32') # =>
#
# image_tag("/icons/icon.gif", :class => "menu_icon") # =>
#
# image_tag("mouse.png", :mouseover => "/images/mouse_over.png") # =>
#
# image_tag("mouse.png", :mouseover => image_path("mouse_over.png")) # =>
#
def image_tag(source, options = {})
options.symbolize_keys!
options[:src] = path_to_image(source)
options[:alt] ||= File.basename(options[:src], '.*').split('.').first.to_s.capitalize
if size = options.delete(:size)
options[:width], options[:height] = size.split("x") if size =~ %r{^\d+x\d+$}
end
if mouseover = options.delete(:mouseover)
options[:onmouseover] = "this.src='#{image_path(mouseover)}'"
options[:onmouseout] = "this.src='#{image_path(options[:src])}'"
end
tag("img", options)
end
private
def javascript_src_tag(source, options)
content_tag("script", "", { "type" => Mime::JS, "src" => path_to_javascript(source) }.merge(options))
end
def stylesheet_tag(source, options)
tag("link", { "rel" => "stylesheet", "type" => Mime::CSS, "media" => "screen", "href" => html_escape(path_to_stylesheet(source)) }.merge(options), false, false)
end
module ImageAsset
DIRECTORY = 'images'.freeze
def directory
DIRECTORY
end
def extension
nil
end
end
module JavaScriptAsset
DIRECTORY = 'javascripts'.freeze
EXTENSION = 'js'.freeze
def public_directory
JAVASCRIPTS_DIR
end
def directory
DIRECTORY
end
def extension
EXTENSION
end
end
module StylesheetAsset
DIRECTORY = 'stylesheets'.freeze
EXTENSION = 'css'.freeze
def public_directory
STYLESHEETS_DIR
end
def directory
DIRECTORY
end
def extension
EXTENSION
end
end
class AssetTag
extend ActiveSupport::Memoizable
Cache = {}
CacheGuard = Mutex.new
def self.create(template, controller, source, include_host = true)
CacheGuard.synchronize do
key = if controller.respond_to?(:request)
[self, controller.request.protocol,
ActionController::Base.asset_host,
ActionController::Base.relative_url_root,
source, include_host]
else
[self, ActionController::Base.asset_host, source, include_host]
end
Cache[key] ||= new(template, controller, source, include_host).freeze
end
end
ProtocolRegexp = %r{^[-a-z]+://}.freeze
def initialize(template, controller, source, include_host = true)
# NOTE: The template arg is temporarily needed for a legacy plugin
# hook that is expected to call rewrite_asset_path on the
# template. This should eventually be removed.
@template = template
@controller = controller
@source = source
@include_host = include_host
end
def public_path
compute_public_path(@source)
end
memoize :public_path
def asset_file_path
File.join(ASSETS_DIR, public_path.split('?').first)
end
memoize :asset_file_path
def contents
File.read(asset_file_path)
end
def mtime
File.mtime(asset_file_path)
end
private
def request
@controller.request
end
def request?
@controller.respond_to?(:request)
end
# Add the the extension +ext+ if not present. Return full URLs otherwise untouched.
# Prefix with /dir/ if lacking a leading +/+. Account for relative URL
# roots. Rewrite the asset path for cache-busting asset ids. Include
# asset host, if configured, with the correct request protocol.
def compute_public_path(source)
source += ".#{extension}" if missing_extension?(source)
unless source =~ ProtocolRegexp
source = "/#{directory}/#{source}" unless source[0] == ?/
source = rewrite_asset_path(source)
source = prepend_relative_url_root(source)
end
source = prepend_asset_host(source)
source
end
def missing_extension?(source)
extension && (File.extname(source).blank? || File.exist?(File.join(ASSETS_DIR, directory, "#{source}.#{extension}")))
end
def prepend_relative_url_root(source)
relative_url_root = ActionController::Base.relative_url_root
if request? && @include_host && source !~ %r{^#{relative_url_root}/}
"#{relative_url_root}#{source}"
else
source
end
end
def prepend_asset_host(source)
if @include_host && source !~ ProtocolRegexp
host = compute_asset_host(source)
if request? && !host.blank? && host !~ ProtocolRegexp
host = "#{request.protocol}#{host}"
end
"#{host}#{source}"
else
source
end
end
# Pick an asset host for this source. Returns +nil+ if no host is set,
# the host if no wildcard is set, the host interpolated with the
# numbers 0-3 if it contains %d (the number is the source hash mod 4),
# or the value returned from invoking the proc if it's a proc.
def compute_asset_host(source)
if host = ActionController::Base.asset_host
if host.is_a?(Proc)
case host.arity
when 2
host.call(source, request)
else
host.call(source)
end
else
(host =~ /%d/) ? host % (source.hash % 4) : host
end
end
end
# Use the RAILS_ASSET_ID environment variable or the source's
# modification time as its cache-busting asset id.
def rails_asset_id(source)
if asset_id = ENV["RAILS_ASSET_ID"]
asset_id
else
path = File.join(ASSETS_DIR, source)
if File.exist?(path)
File.mtime(path).to_i.to_s
else
''
end
end
end
# Break out the asset path rewrite in case plugins wish to put the asset id
# someplace other than the query string.
def rewrite_asset_path(source)
if @template.respond_to?(:rewrite_asset_path)
# DEPRECATE: This way to override rewrite_asset_path
@template.send(:rewrite_asset_path, source)
else
asset_id = rails_asset_id(source)
if asset_id.blank?
source
else
"#{source}?#{asset_id}"
end
end
end
end
class ImageTag < AssetTag
include ImageAsset
end
class JavaScriptTag < AssetTag
include JavaScriptAsset
end
class StylesheetTag < AssetTag
include StylesheetAsset
end
class AssetCollection
extend ActiveSupport::Memoizable
Cache = {}
CacheGuard = Mutex.new
def self.create(template, controller, sources, recursive)
CacheGuard.synchronize do
key = [self, sources, recursive]
Cache[key] ||= new(template, controller, sources, recursive).freeze
end
end
def initialize(template, controller, sources, recursive)
# NOTE: The template arg is temporarily needed for a legacy plugin
# hook. See NOTE under AssetTag#initialize for more details
@template = template
@controller = controller
@sources = sources
@recursive = recursive
end
def write_asset_file_contents(joined_asset_path)
FileUtils.mkdir_p(File.dirname(joined_asset_path))
File.open(joined_asset_path, "w+") { |cache| cache.write(joined_contents) }
mt = latest_mtime
File.utime(mt, mt, joined_asset_path)
end
private
def determine_source(source, collection)
case source
when Symbol
collection[source] || raise(ArgumentError, "No expansion found for #{source.inspect}")
else
source
end
end
def validate_sources!
@sources.collect { |source| determine_source(source, self.class.expansions) }.flatten
end
def all_asset_files
path = [public_directory, ('**' if @recursive), "*.#{extension}"].compact
Dir[File.join(*path)].collect { |file|
file[-(file.size - public_directory.size - 1)..-1].sub(/\.\w+$/, '')
}.sort
end
def tag_sources
expand_sources.collect { |source| tag_class.create(@template, @controller, source, false) }
end
def joined_contents
tag_sources.collect { |source| source.contents }.join("\n\n")
end
# Set mtime to the latest of the combined files to allow for
# consistent ETag without a shared filesystem.
def latest_mtime
tag_sources.map { |source| source.mtime }.max
end
end
class JavaScriptSources < AssetCollection
include JavaScriptAsset
EXPANSIONS = { :defaults => JAVASCRIPT_DEFAULT_SOURCES.dup }
def self.expansions
EXPANSIONS
end
APPLICATION_JS = "application".freeze
APPLICATION_FILE = "application.js".freeze
def expand_sources
if @sources.include?(:all)
assets = all_asset_files
((defaults.dup & assets) + assets).uniq!
else
expanded_sources = validate_sources!
expanded_sources << APPLICATION_JS if include_application?
expanded_sources
end
end
memoize :expand_sources
private
def tag_class
JavaScriptTag
end
def defaults
determine_source(:defaults, self.class.expansions)
end
def include_application?
@sources.include?(:defaults) && File.exist?(File.join(JAVASCRIPTS_DIR, APPLICATION_FILE))
end
end
class StylesheetSources < AssetCollection
include StylesheetAsset
EXPANSIONS = {}
def self.expansions
EXPANSIONS
end
def expand_sources
@sources.first == :all ? all_asset_files : validate_sources!
end
memoize :expand_sources
private
def tag_class
StylesheetTag
end
end
end
end
end
# stylesheet_link_tag("application")
# =>
#
# This is useful since browsers typically open at most two connections to a single host,
# which means your assets often wait in single file for their turn to load. You can
# alleviate this by using a %d wildcard in asset_host (for example, "assets%d.example.com")
# to automatically distribute asset requests among four hosts (e.g., "assets0.example.com" through "assets3.example.com")
# so browsers will open eight connections rather than two.
#
# image_tag("rails.png")
# => 
# stylesheet_link_tag("application")
# =>
#
# To do this, you can either setup 4 actual hosts, or you can use wildcard DNS to CNAME
# the wildcard to a single asset host. You can read more about setting up your DNS CNAME records from
# your ISP.
#
# Note: This is purely a browser performance optimization and is not meant
# for server load balancing. See http://www.die.net/musings/page_load_time/
# for background.
#
# Alternatively, you can exert more control over the asset host by setting asset_host to a proc
# that takes a single source argument. This is useful if you are unable to setup 4 actual hosts or have
# fewer/more than 4 hosts. The example proc below generates http://assets1.example.com and
# http://assets2.example.com randomly.
#
# ActionController::Base.asset_host = Proc.new { |source| "http://assets#{rand(2) + 1}.example.com" }
# image_tag("rails.png")
# => 
# stylesheet_link_tag("application")
# =>
#
# The proc takes a source parameter (which is the path of the source asset) and an optional
# request parameter (which is an entire instance of an ActionController::AbstractRequest
# subclass). This can be used to generate a particular asset host depending on the asset path and the particular
# request.
#
# ActionController::Base.asset_host = Proc.new { |source|
# if source.starts_with?('/images')
# "http://images.example.com"
# else
# "http://assets.example.com"
# end
# }
# image_tag("rails.png")
# => 
# stylesheet_link_tag("application")
# =>
#
# The optional request parameter to the proc is useful in particular for serving assets from an
# SSL-protected page. The example proc below disables asset hosting for HTTPS connections, while still sending
# assets for plain HTTP requests from asset hosts. This is useful for avoiding mixed media warnings when serving
# non-HTTP assets from HTTPS web pages when you don't have an SSL certificate for each of the asset hosts.
#
# ActionController::Base.asset_host = Proc.new { |source, request|
# if request.ssl?
# "#{request.protocol}#{request.host_with_port}"
# else
# "#{request.protocol}assets.example.com"
# end
# }
#
# === Using asset timestamps
#
# By default, Rails will append all asset paths with that asset's timestamp. This allows you to set a cache-expiration date for the
# asset far into the future, but still be able to instantly invalidate it by simply updating the file (and hence updating the timestamp,
# which then updates the URL as the timestamp is part of that, which in turn busts the cache).
#
# It's the responsibility of the web server you use to set the far-future expiration date on cache assets that you need to take
# advantage of this feature. Here's an example for Apache:
#
# # Asset Expiration
# ExpiresActive On
# 
# image_tag("icon.png") # =>
# 
# image_tag("icon.png", :size => "16x10", :alt => "Edit Entry") # =>
# 
# image_tag("/icons/icon.gif", :height => '32', :width => '32') # =>
# 
# image_tag("mouse.png", :mouseover => image_path("mouse_over.png")) # =>
#