Module | ActionView::Helpers::AssetTagHelper
In: | lib/action_view/helpers/asset_tag_helper.rb

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")
        => <img src="http://assets.example.com/images/rails.png" alt="Rails" />
      stylesheet_link_tag("application")
        => <link href="http://assets.example.com/stylesheets/application.css" media="screen" rel="stylesheet" type="text/css" />

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")
        => <img src="http://assets0.example.com/images/rails.png" alt="Rails" />
      stylesheet_link_tag("application")
        => <link href="http://assets3.example.com/stylesheets/application.css" media="screen" rel="stylesheet" type="text/css" />

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 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 assets1.example.com and assets2.example.com randomly.

      ActionController::Base.asset_host = Proc.new { |source| "http://assets#{rand(2) + 1}.example.com" }
      image_tag("rails.png")
        => <img src="http://assets2.example.com/images/rails.png" alt="Rails" />
      stylesheet_link_tag("application")
        => <link href="http://assets1.example.com/stylesheets/application.css" media="screen" rel="stylesheet" type="text/css" />

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")
        => <img src="http://images.example.com/images/rails.png" alt="Rails" />
      stylesheet_link_tag("application")
        => <link href="http://assets.example.com/stylesheets/application.css" media="screen" rel="stylesheet" type="text/css" />

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 <FilesMatch "\.(ico|gif|jpe?g|png|js|css)$">

      ExpiresDefault "access plus 1 year"

</FilesMatch>

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).

### Methods

auto_discovery_link_tag image_path image_tag javascript_include_tag javascript_path path_to_image path_to_javascript path_to_stylesheet register_javascript_expansion register_javascript_include_default register_stylesheet_expansion stylesheet_link_tag stylesheet_path

### Constants

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'] unless const_defined?(:JAVASCRIPT_DEFAULT_SOURCES)

### Public Class methods

register_javascript_expansion(expansions)

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 # =>
        <script type="text/javascript" src="/javascripts/head.js"></script>
        <script type="text/javascript" src="/javascripts/body.js"></script>
        <script type="text/javascript" src="/javascripts/tail.js"></script>

register_javascript_include_default(*sources)

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`.

register_stylesheet_expansion(expansions)

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 # =>
        <link href="/stylesheets/head.css"  media="screen" rel="stylesheet" type="text/css" />
        <link href="/stylesheets/body.css"  media="screen" rel="stylesheet" type="text/css" />
        <link href="/stylesheets/tail.css"  media="screen" rel="stylesheet" type="text/css" />

### Public Instance methods

auto_discovery_link_tag(type = :rss, url_options = {}, tag_options = {})

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 # =>
        <link rel="alternate" type="application/rss+xml" title="RSS" href="http://www.currenthost.com/controller/action" />
     auto_discovery_link_tag(:atom) # =>
        <link rel="alternate" type="application/atom+xml" title="ATOM" href="http://www.currenthost.com/controller/action" />
     auto_discovery_link_tag(:rss, {:action => "feed"}) # =>
        <link rel="alternate" type="application/rss+xml" title="RSS" href="http://www.currenthost.com/controller/feed" />
     auto_discovery_link_tag(:rss, {:action => "feed"}, {:title => "My RSS"}) # =>
        <link rel="alternate" type="application/rss+xml" title="My RSS" href="http://www.currenthost.com/controller/feed" />
     auto_discovery_link_tag(:rss, {:controller => "news", :action => "feed"}) # =>
        <link rel="alternate" type="application/rss+xml" title="RSS" href="http://www.currenthost.com/news/feed" />
     auto_discovery_link_tag(:rss, "http://www.example.com/feed.rss", {:title => "Example RSS"}) # =>
        <link rel="alternate" type="application/rss+xml" title="Example RSS" href="http://www.example.com/feed" />

image_path(source)

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

image_tag(source, options = {})

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")  # =>
       <img src="/images/icon" alt="Icon" />
     image_tag("icon.png")  # =>
       <img src="/images/icon.png" alt="Icon" />
     image_tag("icon.png", :size => "16x10", :alt => "Edit Entry")  # =>
       <img src="/images/icon.png" width="16" height="10" alt="Edit Entry" />
     image_tag("/icons/icon.gif", :size => "16x16")  # =>
       <img src="/icons/icon.gif" width="16" height="16" alt="Icon" />
     image_tag("/icons/icon.gif", :height => '32', :width => '32') # =>
       <img alt="Icon" height="32" src="/icons/icon.gif" width="32" />
     image_tag("/icons/icon.gif", :class => "menu_icon") # =>
       <img alt="Icon" class="menu_icon" src="/icons/icon.gif" />
     image_tag("mouse.png", :mouseover => "/images/mouse_over.png") # =>
       <img src="/images/mouse.png" onmouseover="this.src='/images/mouse_over.png'" onmouseout="this.src='/images/mouse.png'" alt="Mouse" />
     image_tag("mouse.png", :mouseover => image_path("mouse_over.png")) # =>
       <img src="/images/mouse.png" onmouseover="this.src='/images/mouse_over.png'" onmouseout="this.src='/images/mouse.png'" alt="Mouse" />

javascript_include_tag(*sources)

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" # =>
        <script type="text/javascript" src="/javascripts/xmlhr.js"></script>

      javascript_include_tag "xmlhr.js" # =>
        <script type="text/javascript" src="/javascripts/xmlhr.js"></script>

      javascript_include_tag "common.javascript", "/elsewhere/cools" # =>
        <script type="text/javascript" src="/javascripts/common.javascript"></script>
        <script type="text/javascript" src="/elsewhere/cools.js"></script>

      javascript_include_tag "http://www.railsapplication.com/xmlhr" # =>
        <script type="text/javascript" src="http://www.railsapplication.com/xmlhr.js"></script>

      javascript_include_tag "http://www.railsapplication.com/xmlhr.js" # =>
        <script type="text/javascript" src="http://www.railsapplication.com/xmlhr.js"></script>

      javascript_include_tag :defaults # =>
        <script type="text/javascript" src="/javascripts/prototype.js"></script>
        <script type="text/javascript" src="/javascripts/effects.js"></script>
        ...
        <script type="text/javascript" src="/javascripts/application.js"></script>

- = 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 # =>
        <script type="text/javascript" src="/javascripts/prototype.js"></script>
        <script type="text/javascript" src="/javascripts/effects.js"></script>
        ...
        <script type="text/javascript" src="/javascripts/application.js"></script>
        <script type="text/javascript" src="/javascripts/shop.js"></script>
        <script type="text/javascript" src="/javascripts/checkout.js"></script>

Note that the default javascript files will be included first. So Prototype and Scriptaculous are available to all subsequently included files.

## 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 =>
        <script type="text/javascript" src="/javascripts/prototype.js"></script>
        <script type="text/javascript" src="/javascripts/effects.js"></script>
        ...
        <script type="text/javascript" src="/javascripts/application.js"></script>
        <script type="text/javascript" src="/javascripts/shop.js"></script>
        <script type="text/javascript" src="/javascripts/checkout.js"></script>

      javascript_include_tag :all, :cache => true # when ActionController::Base.perform_caching is true =>
        <script type="text/javascript" src="/javascripts/all.js"></script>

      javascript_include_tag "prototype", "cart", "checkout", :cache => "shop" # when ActionController::Base.perform_caching is false =>
        <script type="text/javascript" src="/javascripts/prototype.js"></script>
        <script type="text/javascript" src="/javascripts/cart.js"></script>
        <script type="text/javascript" src="/javascripts/checkout.js"></script>

      javascript_include_tag "prototype", "cart", "checkout", :cache => "shop" # when ActionController::Base.perform_caching is true =>
        <script type="text/javascript" src="/javascripts/shop.js"></script>

javascript_path(source)

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

path_to_image(source)

Alias for image_path

path_to_javascript(source)

Alias for javascript_path

path_to_stylesheet(source)

Alias for stylesheet_path

stylesheet_link_tag(*sources)

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" # =>
        <link href="/stylesheets/style.css" media="screen" rel="stylesheet" type="text/css" />

      stylesheet_link_tag "style.css" # =>
        <link href="/stylesheets/style.css" media="screen" rel="stylesheet" type="text/css" />

      stylesheet_link_tag "http://www.railsapplication.com/style.css" # =>
        <link href="http://www.railsapplication.com/style.css" media="screen" rel="stylesheet" type="text/css" />

      stylesheet_link_tag "style", :media => "all" # =>
        <link href="/stylesheets/style.css" media="all" rel="stylesheet" type="text/css" />

      stylesheet_link_tag "style", :media => "print" # =>
        <link href="/stylesheets/style.css" media="print" rel="stylesheet" type="text/css" />

      stylesheet_link_tag "random.styles", "/css/stylish" # =>
        <link href="/stylesheets/random.styles" media="screen" rel="stylesheet" type="text/css" />
        <link href="/css/stylish.css" media="screen" rel="stylesheet" type="text/css" />

You can also include all styles in the stylesheet directory using `:all` as the source:

      stylesheet_link_tag :all # =>
        <link href="/stylesheets/style1.css"  media="screen" rel="stylesheet" type="text/css" />
        <link href="/stylesheets/styleB.css"  media="screen" rel="stylesheet" type="text/css" />
        <link href="/stylesheets/styleX2.css" media="screen" rel="stylesheet" type="text/css" />

## 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 =>
        <link href="/stylesheets/style1.css"  media="screen" rel="stylesheet" type="text/css" />
        <link href="/stylesheets/styleB.css"  media="screen" rel="stylesheet" type="text/css" />
        <link href="/stylesheets/styleX2.css" media="screen" rel="stylesheet" type="text/css" />

      stylesheet_link_tag :all, :cache => true # when ActionController::Base.perform_caching is true =>
        <link href="/stylesheets/all.css"  media="screen" rel="stylesheet" type="text/css" />

      stylesheet_link_tag "shop", "cart", "checkout", :cache => "payment" # when ActionController::Base.perform_caching is false =>
        <link href="/stylesheets/shop.css"  media="screen" rel="stylesheet" type="text/css" />
        <link href="/stylesheets/cart.css"  media="screen" rel="stylesheet" type="text/css" />
        <link href="/stylesheets/checkout.css" media="screen" rel="stylesheet" type="text/css" />

      stylesheet_link_tag "shop", "cart", "checkout", :cache => "payment" # when ActionController::Base.perform_caching is true =>
        <link href="/stylesheets/payment.css"  media="screen" rel="stylesheet" type="text/css" />

stylesheet_path(source)

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

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