Assets Helpers

These helpers are HTML5 generators that target specific assets features.

They are following the settings of the application that uses them. For instance, if we have a project with two applications Web and Admin mounted at / and /admin, respectively, all the asset URLs will respect these prefixes.

They also respect Fingerprint mode and CDN mode for each application.

The following helpers are available for views and templates:

Usage

javascript

It generates a <script> tag for the given source(s). A source can be the file name without the extension, or an absolute URL.

<%= javascript 'https://code.jquery.com/jquery-2.1.1.min.js', 'application' %>
<script src="https://code.jquery.com/jquery-2.1.1.min.js" type="text/javascript"></script>
<script src="/assets/application.js" type="text/javascript"></script>

Alternatively, it accepts only one source and a Hash to represent HTML attributes.

<%= javascript 'application', async: true %>
<script src="/assets/application.js" type="text/javascript" async="true"></script>

stylesheet

It generates a <link> tag for the given source(s). A source can be the file name without the extension, or an absolute URL.

<%= stylesheet 'reset', 'grid', 'main' %>
<link href="/assets/reset.css" type="text/css" rel="stylesheet">
<link href="/assets/grid.css" type="text/css" rel="stylesheet">
<link href="/assets/main.css" type="text/css" rel="stylesheet">

Alternatively, it accepts only one source and a Hash to represent HTML attributes.

<%= stylesheet 'application', crossorigin: 'anonymous' %>
<link href="/assets/application.css" type="text/css" rel="stylesheet" crossorigin="anonymous">

favicon

It generates a <link> tag for the application favicon.

By default it looks at /assets/favicon.ico.

<%= favicon %>
<link href="/assets/favicon.ico" rel="shortcut icon" type="image/x-icon">

We can specify a source name and HTML attributes.

<%= favicon 'favicon.png', rel: 'icon', type: 'image/png' %>
<link rel="icon" type="image/png" href="/assets/favicon.png">

image

It generates a <img> tag for the given source. A source can be the file name with the extension, or an absolute URL.

The alt attribute is set automatically from file name.

<%= image 'logo.png' %>
<img src="/assets/logo.png" alt="Logo">

We can specify arbitrary HTML attributes.

<%= image 'logo.png', alt: 'Application Logo', id: 'logo', class: 'background' %>
<img src="/assets/logo.png" alt="Application Logo" id="logo" class="background">

video

It generates a <video> tag for the given source. A source can be the file name with the extension, or an absolute URL.

<%= video 'movie.mp4' %>
<video src="/assets/movie.mp4"></video>

We can specify arbitrary HTML attributes.

<%= video 'movie.mp4', autoplay: true, controls: true %>
<video autoplay="autoplay" controls="controls" src="/assets/movie.mp4"></video>

It accepts a block for fallback message.

<%=
  video('movie.mp4') do
    "Your browser does not support the video tag"
  end
%>
<video src="/assets/movie.mp4">
  Your browser does not support the video tag
</video>

It accepts a block to specify tracks.

<%=
  video('movie.mp4') do
    track kind: 'captions', src: asset_path('movie.en.vtt'), srclang: 'en', label: 'English'
  end
%>
<video src="/assets/movie.mp4">
  <track kind="captions" src="/assets/movie.en.vtt" srclang="en" label="English">
</video>

It accepts a block to specify multiple sources.

<%=
  video do
    text "Your browser does not support the video tag"
    source src: asset_path('movie.mp4'), type: 'video/mp4'
    source src: asset_path('movie.ogg'), type: 'video/ogg'
  end
%>
<video>
  Your browser does not support the video tag
  <source src="/assets/movie.mp4" type="video/mp4">
  <source src="/assets/movie.ogg" type="video/ogg">
</video>

audio

It generates a <audio> tag for the given source. A source can be the file name with the extension, or an absolute URL.

<%= audio 'song.ogg' %>
<audio src="/assets/song.ogg"></audio>

We can specify arbitrary HTML attributes.

<%= audio 'song.ogg', autoplay: true, controls: true %>
<audio autoplay="autoplay" controls="controls" src="/assets/song.ogg"></audio>

It accepts a block for fallback message.

<%=
  audio('song.ogg') do
    "Your browser does not support the audio tag"
  end
%>
<audio src="/assets/song.ogg">
  Your browser does not support the audio tag
</audio>

It accepts a block to specify tracks.

<%=
  audio('song.ogg') do
    track kind: 'captions', src: asset_path('movie.en.vtt'), srclang: 'en', label: 'English'
  end
%>
<audio src="/assets/song.ogg">
  <track kind="captions" src="/assets/movie.en.vtt" srclang="en" label="English">
</audio>

It accepts a block to specify multiple sources.

<%=
  audio do
    text "Your browser does not support the audio tag"
    source src: asset_path('song.ogg'), type: 'audio/mp4'
    source src: asset_path('movie.ogg'), type: 'audio/ogg'
  end
%>
<audio>
  Your browser does not support the audio tag
  <source src="/assets/song.ogg" type="audio/mp4">
  <source src="/assets/movie.ogg" type="audio/ogg">
</audio>

asset_path

Returns a relative URL for the given asset name.

This is used internally by the other helpers, so the following rules apply to all of them.

Asset Name

When the argument is a relative name, it returns a relative URL.

<%= asset_path 'application.js' %>
/assets/application.js

Absolute URL

When an absolute URL is given, it's always returned as it is, even if Fingerprint or CDN mode are on.

<%= asset_path 'https://code.jquery.com/jquery-2.1.1.min.js' %>
https://code.jquery.com/jquery-2.1.1.min.js

Fingerprint Mode

When Fingerprint Mode is on (usually in production env), the relative URL contains a checksum suffix.

<%= asset_path 'application.css' %>
/assets/application-9ab4d1f57027f0d40738ab8ab70aba86.css

CDN Mode

When CDN Mode is on (usually in production env), it returns an absolute URL that reference the CDN settings.

<%= asset_path 'application.css' %>
https://123.cloudfront.net/assets/application-9ab4d1f57027f0d40738ab8ab70aba86.css

In the example above we have a checksum suffix, because CDN vendors suggest to use this strategy while using their product.

asset_url

Returns an absolute URL for the given asset name.

To build the URL it uses scheme, host, port from your application settings in combination of CDN settings.

Asset Name

When the argument is a relative name, it returns an absolute URL.

<%= asset_url 'application.js' %>
https://bookshelf.org/assets/application.js

Absolute URL

When an absolute URL is given, it's always returned as it is, even if Fingerprint or CDN mode are on.

<%= asset_url 'https://code.jquery.com/jquery-2.1.1.min.js' %>
https://code.jquery.com/jquery-2.1.1.min.js

Fingerprint Mode

When Fingerprint Mode is on (usually in production env), the relative URL contains a checksum suffix.

<%= asset_url 'application.css' %>
https://bookshelf.org/assets/application-9ab4d1f57027f0d40738ab8ab70aba86.css

CDN Mode

When CDN Mode is on (usually in production env), it returns an absolute URL that reference the CDN settings.

<%= asset_url 'application.css' %>
https://123.cloudfront.net/assets/application-9ab4d1f57027f0d40738ab8ab70aba86.css

In the example above we have a checksum suffix, because CDN vendors suggest to use this strategy while using their product.

Content Security Policy

When using absolute URLs as sources or when we use a CDN, we can run into errors because our browser refute to load assets due to security restrictions (aka "Content Security Policy").

To fix them, we need to adjust security settings in our application.

# apps/web/application.rb
module Web
  class Application < Hanami::Application
    configure do
      # ...

      # If we're using our own CDN
      security.content_security_policy "default-src https://123.cloudfront.net;"

      # Or if we're just using jQuery public CDN
      # security.content_security_policy "default-src none; script-src 'self' https://code.jquery.com;"
    end
  end
end

To learn more about Content Security Policy usage, please read:


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