# module ActionView::Helpers::AssetTagHelper

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#{Digest::MD5.hexdigest(source).to_i(16) % 2 + 1}.example.com"
    }
    image_tag("rails.png")
    # => <img alt="Rails" src="http://assets1.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" />

The example above generates “assets1.example.com” and “assets2.example.com”. 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"
    )

### Customizing the asset path¶ ↑

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.

This strategy works well enough for most server setups and requires the least configuration, but if you deploy several application servers at different times - say to handle a temporary spike in load - then the asset time stamps will be out of sync. In a setup like this you may want to set the way that asset paths are generated yourself.

Altering the asset paths that Rails generates can be done in two ways. The easiest is to define the RAILS_ASSET_ID environment variable. The contents of this variable will always be used in preference to calculated timestamps. A more complex but flexible way is to set `ActionController::Base.config.asset_path` to a proc that takes the unmodified asset path and returns the path needed for your asset caching to work. Typically you’d do something like this in `config/environments/production.rb`:

    # Normally you'd calculate RELEASE_NUMBER at startup.
    RELEASE_NUMBER = 12345
    config.action_controller.asset_path = proc { |asset_path|
      "/release-#{RELEASE_NUMBER}#{asset_path}"
    }

This example would cause the following behavior on all servers no matter when they were deployed:

    image_tag("rails.png")
    # => <img alt="Rails" src="/release-12345/images/rails.png" />
    stylesheet_link_tag("application")
    # => <link href="/release-12345/stylesheets/application.css?1232285206" media="screen" rel="stylesheet" type="text/css" />

Changing the asset_path does require that your web servers have knowledge of the asset template paths that you rewrite to so it’s not suitable for out-of-the-box use. To use the example given above you could use something like this in your Apache VirtualHost configuration:

    <LocationMatch "^/release-\d+/(images|javascripts|stylesheets)/.*$">
      # Some browsers still send conditional-GET requests if there's a
      # Last-Modified header or an ETag header even if they haven't
      # reached the expiry date sent in the Expires header.
      Header unset Last-Modified
      Header unset ETag
      FileETag None

      # Assets requested using a cache-busting filename should be served
      # only once and then cached for a really long time. The HTTP/1.1
      # spec frowns on hugely-long expiration times though and suggests
      # that assets which never expire be served with an expiration date
      # 1 year from access.
      ExpiresActive On
      ExpiresDefault "access plus 1 year"
    </LocationMatch>

    # We use cached-busting location names with the far-future expires
    # headers to ensure that if a file does change it can force a new
    # request. The actual asset filenames are still the same though so we
    # need to rewrite the location from the cache-busting location to the
    # real asset location so that we can serve it.
    RewriteEngine On
    RewriteRule ^/release-\d+/(images|javascripts|stylesheets)/(.*)$ /$1/$2 [L]

### Public Instance Methods

audio_path(source) click to toggle source

Computes the path to an audio asset in the public audios directory. Full paths from the document root will be passed through. Used internally by `audio_tag` to build the audio path.

#### Examples¶ ↑

    audio_path("horse")                                            # => /audios/horse
    audio_path("horse.wav")                                        # => /audios/horse.wav
    audio_path("sounds/horse.wav")                                 # => /audios/sounds/horse.wav
    audio_path("/sounds/horse.wav")                                # => /sounds/horse.wav
    audio_path("http://www.example.com/sounds/horse.wav")          # => http://www.example.com/sounds/horse.wav

    # File lib/action_view/helpers/asset_tag_helper.rb, line 304
    def audio_path(source)
      asset_paths.compute_public_path(source, 'audios')
    end

Also aliased as: path_to_audio
audio_tag(source, options = {}) click to toggle source

Returns an html audio tag for the `source`. The `source` can be full path or file that exists in your public audios directory.

#### Examples¶ ↑

    audio_tag("sound")  # =>
      <audio src="/audios/sound" />
    audio_tag("sound.wav")  # =>
      <audio src="/audios/sound.wav" />
    audio_tag("sound.wav", :autoplay => true, :controls => true)  # =>
      <audio autoplay="autoplay" controls="controls" src="/audios/sound.wav" />

    # File lib/action_view/helpers/asset_tag_helper.rb, line 444
    def audio_tag(source, options = {})
      options.symbolize_keys!
      options[:src] = path_to_audio(source)
      tag("audio", options)
    end

auto_discovery_link_tag(type = :rss, url_options = {}, tag_options = {}) click to toggle source

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" />

    # File lib/action_view/helpers/asset_tag_helper.rb, line 221
    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

favicon_link_tag(source='/favicon.ico', options={}) click to toggle source

    <%= favicon_link_tag %>

generates

    <link href="/favicon.ico" rel="shortcut icon" type="image/vnd.microsoft.icon" />

You may specify a different file in the first argument:

    <%= favicon_link_tag '/myicon.ico' %>

That’s passed to `path_to_image` as is, so it gives

    <link href="/myicon.ico" rel="shortcut icon" type="image/vnd.microsoft.icon" />

The helper accepts an additional options hash where you can override “rel” and “type”.

For example, Mobile Safari looks for a different LINK tag, pointing to an image that will be used if you add the page to the home screen of an iPod Touch, iPhone, or iPad. The following call would generate such a tag:

    <%= favicon_link_tag 'mb-icon.png', :rel => 'apple-touch-icon', :type => 'image/png' %>

    # File lib/action_view/helpers/asset_tag_helper.rb, line 253
    def favicon_link_tag(source='/favicon.ico', options={})
      tag('link', {
        :rel  => 'shortcut icon',
        :type => 'image/vnd.microsoft.icon',
        :href => path_to_image(source)
      }.merge(options.symbolize_keys))
    end

font_path(source) click to toggle source

Computes the path to a font asset in the public fonts directory. Full paths from the document root will be passed through.

#### Examples¶ ↑

    font_path("font")                                           # => /fonts/font
    font_path("font.ttf")                                       # => /fonts/font.ttf
    font_path("dir/font.ttf")                                   # => /fonts/dir/font.ttf
    font_path("/dir/font.ttf")                                  # => /dir/font.ttf
    font_path("http://www.example.com/dir/font.ttf")            # => http://www.example.com/dir/font.ttf

    # File lib/action_view/helpers/asset_tag_helper.rb, line 318
    def font_path(source)
      asset_paths.compute_public_path(source, 'fonts')
    end

Also aliased as: path_to_font
image_alt(src) click to toggle source

    # File lib/action_view/helpers/asset_tag_helper.rb, line 377
    def image_alt(src)
      File.basename(src, '.*').sub(/-[[:xdigit:]]{32}\z/, '').capitalize
    end

image_path(source) click to toggle 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:

    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.example.com/img/edit.png")          # => "http://www.example.com/img/edit.png"

If you have images as application resources this method may conflict with their named routes. The alias `path_to_image` is provided to avoid that. Rails uses the alias internally, and plugin authors are encouraged to do so.

    # File lib/action_view/helpers/asset_tag_helper.rb, line 274
    def image_path(source)
      source.present? ? asset_paths.compute_public_path(source, 'images') : ""
    end

Also aliased as: path_to_image
image_tag(source, options = {}) click to toggle source

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" />

    # File lib/action_view/helpers/asset_tag_helper.rb, line 356
    def image_tag(source, options = {})
      options.symbolize_keys!

      src = options[:src] = path_to_image(source)

      unless src =~ /^(?:cid|data):/ || src.blank?
        options[:alt] = options.fetch(:alt){ image_alt(src) }
      end

      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='#{path_to_image(mouseover)}'"
        options[:onmouseout]  = "this.src='#{src}'"
      end

      tag("img", options)
    end

path_to_audio(source)
Alias for: audio_path
path_to_font(source)
Alias for: font_path
path_to_image(source)
Alias for: image_path
path_to_video(source)
Alias for: video_path
video_path(source) click to toggle source

Computes the path to a video asset in the public videos directory. Full paths from the document root will be passed through. Used internally by `video_tag` to build the video path.

#### Examples¶ ↑

    video_path("hd")                                            # => /videos/hd
    video_path("hd.avi")                                        # => /videos/hd.avi
    video_path("trailers/hd.avi")                               # => /videos/trailers/hd.avi
    video_path("/trailers/hd.avi")                              # => /trailers/hd.avi
    video_path("http://www.example.com/vid/hd.avi")             # => http://www.example.com/vid/hd.avi

    # File lib/action_view/helpers/asset_tag_helper.rb, line 289
    def video_path(source)
      asset_paths.compute_public_path(source, 'videos')
    end

Also aliased as: path_to_video
video_tag(sources, options = {}) click to toggle source

Returns an html video tag for the `sources`. If `sources` is a string, a single video tag will be returned. If `sources` is an array, a video tag with nested source tags for each source will be returned. The `sources` can be full paths or files that exists in your public videos directory.

#### Options¶ ↑

You can add HTML attributes using the `options`. The `options` supports two additional keys for convenience and conformance:

- `:poster` - Set an image (like a screenshot) to be shown before the video loads. The path is calculated like the `src` of `image_tag`.

- `: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.

#### Examples¶ ↑

    video_tag("trailer")  # =>
      <video src="/videos/trailer" />
    video_tag("trailer.ogg")  # =>
      <video src="/videos/trailer.ogg" />
    video_tag("trailer.ogg", :controls => true, :autobuffer => true)  # =>
      <video autobuffer="autobuffer" controls="controls" src="/videos/trailer.ogg" />
    video_tag("trailer.m4v", :size => "16x10", :poster => "screenshot.png")  # =>
      <video src="/videos/trailer.m4v" width="16" height="10" poster="/images/screenshot.png" />
    video_tag("/trailers/hd.avi", :size => "16x16")  # =>
      <video src="/trailers/hd.avi" width="16" height="16" />
    video_tag("/trailers/hd.avi", :height => '32', :width => '32') # =>
      <video height="32" src="/trailers/hd.avi" width="32" />
    video_tag(["trailer.ogg", "trailer.flv"]) # =>
      <video><source src="trailer.ogg" /><source src="trailer.ogg" /><source src="trailer.flv" /></video>
    video_tag(["trailer.ogg", "trailer.flv"] :size => "160x120") # =>
      <video height="120" width="160"><source src="trailer.ogg" /><source src="trailer.flv" /></video>

    # File lib/action_view/helpers/asset_tag_helper.rb, line 414
    def video_tag(sources, options = {})
      options.symbolize_keys!

      options[:poster] = path_to_image(options[:poster]) if options[:poster]

      if size = options.delete(:size)
        options[:width], options[:height] = size.split("x") if size =~ %r{^\d+x\d+$}
      end

      if sources.is_a?(Array)
        content_tag("video", options) do
          sources.map { |source| tag("source", :src => source) }.join.html_safe
        end
      else
        options[:src] = path_to_video(sources)
        tag("video", options)
      end
    end