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:

      image_tag("rails.png")
      # => <img alt="Rails src="/images/rails.png?1230601161" />
      stylesheet_link_tag("application")
      # => <link href="/stylesheets/application.css?1232285206" media="screen" rel="stylesheet" type="text/css" />

### 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 asset server by setting ActionController::Base.asset_host in the application configuration, typically in `config/environments/production.rb`. For example, you‘d define `assets.example.com` to be your asset host this way:

      ActionController::Base.asset_host = "assets.example.com"

Helpers take that into account:

      image_tag("rails.png")
      # => <img alt="Rails" src="http://assets.example.com/images/rails.png?1230601161" />
      stylesheet_link_tag("application")
      # => <link href="http://assets.example.com/stylesheets/application.css?1232285206" media="screen" rel="stylesheet" type="text/css" />

Browsers typically open at most two simultaneous connections to a single host, which means your assets often have to wait for other assets to finish downloading. You can alleviate this by using a `%d` wildcard in the `asset_host`. For example, "assets%d.example.com". If that wildcard is present Rails distributes asset requests among the corresponding four hosts "assets0.example.com", …, "assets3.example.com". With this trick browsers will open eight simultaneous connections rather than two.

      image_tag("rails.png")
      # => <img alt="Rails" src="http://assets0.example.com/images/rails.png?1230601161" />
      stylesheet_link_tag("application")
      # => <link href="http://assets2.example.com/stylesheets/application.css?1232285206" media="screen" rel="stylesheet" type="text/css" />

To do this, you can either setup four 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 like this:

      ActionController::Base.asset_host = Proc.new { |source|
        "http://assets#{rand(2) + 1}.example.com"
      }
      image_tag("rails.png")
      # => <img alt="Rails" src="http://assets0.example.com/images/rails.png?1230601161" />
      stylesheet_link_tag("application")
      # => <link href="http://assets1.example.com/stylesheets/application.css?1232285206" media="screen" rel="stylesheet" type="text/css" />

The example above generates "assets1.example.com" and "assets2.example.com" randomly. This option is useful for example if you need fewer/more than four hosts, custom host names, etc.

As you see the proc takes a `source` parameter. That‘s a string with the absolute path of the asset with any extensions and timestamps in place, for example "/images/rails.png?1230601161".

       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 alt="Rails" src="http://images.example.com/images/rails.png?1230601161" />
      stylesheet_link_tag("application")
      # => <link href="http://assets.example.com/stylesheets/application.css?1232285206" media="screen" rel="stylesheet" type="text/css" />

Alternatively you may ask for a second parameter `request`. That one is particularly useful 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. If you don‘t have SSL certificates for each of the asset hosts this technique allows you to avoid warnings in the client about mixed media.

      ActionController::Base.asset_host = Proc.new { |source, request|
        if request.ssl?
          "#{request.protocol}#{request.host_with_port}"
        else
          "#{request.protocol}assets.example.com"
        end
      }

You can also implement a custom asset host object that responds to `call` and takes either one or two parameters just like the proc.

      config.action_controller.asset_host = AssetHostingWithMinimumSsl.new(
        "http://asset%d.example.com", "https://asset1.example.com"
      )

### Using asset timestamps

By default, Rails appends asset‘s timestamps to all asset paths. 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 drifts out of sync, you‘ll see different timestamps at random and the cache won‘t work. In that case 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.

### Methods

auto_discovery_link_tag cache_asset_timestamps cache_asset_timestamps= 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'].freeze unless const_defined?(:JAVASCRIPT_DEFAULT_SOURCES)

### Public Class methods

cache_asset_timestamps()
cache_asset_timestamps=(value)

You can enable or disable the asset tag timestamps cache. With the cache enabled, the asset tag helper methods will make fewer expense file system calls. However this prevents you from modifying any asset files while the server is running.

      ActionView::Helpers::AssetTagHelper.cache_asset_timestamps = false

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.

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 =>
        <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>

The `:recursive` option is also available for caching:

      javascript_include_tag :all, :cache => true, :recursive => true

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 stylesheets 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" />

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 =>
        <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" />

The `:recursive` option is also available for caching:

      stylesheet_link_tag :all, :cache => true, :recursive => true

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

[Validate]