module ActionDispatch::Http::MimeNegotiation

# File lib/action_dispatch/http/mime_negotiation.rb, line 42
def accepts
  fetch_header("action_dispatch.request.accepts") do |k|
    header = get_header("HTTP_ACCEPT").to_s.strip

    v = if header.empty?
      [content_mime_type]
    else
      Mime::Type.parse(header)
    end
    set_header k, v
  rescue ::Mime::Type::InvalidMimeType => e
    raise InvalidType, e.message
  end
end

Returns the accepted MIME type for the request.

# File lib/action_dispatch/http/mime_negotiation.rb, line 24
def content_mime_type
  fetch_header("action_dispatch.request.content_type") do |k|
    v = if get_header("CONTENT_TYPE") =~ /^([^,;]*)/
      Mime::Type.lookup($1.strip.downcase)
    else
      nil
    end
    set_header k, v
  rescue ::Mime::Type::InvalidMimeType => e
    raise InvalidType, e.message
  end
end

The MIME type of the HTTP request, such as Mime.

# File lib/action_dispatch/http/mime_negotiation.rb, line 68
def format(_view_path = nil)
  formats.first || Mime::NullType.instance
end

Returns the MIME type for the format used in the request.

# GET /posts/5.xml
request.format # => Mime[:xml]

# GET /posts/5.xhtml
request.format # => Mime[:html]

# GET /posts/5
request.format # => Mime[:html] or Mime[:js], or request.accepts.first

# File lib/action_dispatch/http/mime_negotiation.rb, line 174
def format=(extension)
  parameters[:format] = extension.to_s
  set_header "action_dispatch.request.formats", [Mime::Type.lookup_by_extension(parameters[:format])]
end

Sets the format by string extension, which can be used to force custom formats that are not controlled by the extension.

class ApplicationController < ActionController::Base
  before_action :adjust_format_for_iphone

  private
    def adjust_format_for_iphone
      request.format = :iphone if request.env["HTTP_USER_AGENT"][/iPhone/]
    end
end

# File lib/action_dispatch/http/mime_negotiation.rb, line 194
def formats=(extensions)
  parameters[:format] = extensions.first.to_s
  set_header "action_dispatch.request.formats", extensions.collect { |extension|
    Mime::Type.lookup_by_extension(extension)
  }
end

Sets the formats by string extensions. This differs from format= by allowing you to set multiple, ordered formats, which is useful when you want to have a fallback.

In this example, the :iphone format will be used if it’s available, otherwise it’ll fall back to the :html format.

class ApplicationController < ActionController::Base
  before_action :adjust_format_for_iphone_with_html_fallback

  private
    def adjust_format_for_iphone_with_html_fallback
      request.formats = [ :iphone, :html ] if request.env["HTTP_USER_AGENT"][/iPhone/]
    end
end

# File lib/action_dispatch/http/mime_negotiation.rb, line 202
def negotiate_mime(order)
  formats.each do |priority|
    if priority == Mime::ALL
      return order.first
    elsif order.include?(priority)
      return priority
    end
  end

  order.include?(Mime::ALL) ? format : nil
end

Returns the first MIME type that matches the provided array of MIME types.

# File lib/action_dispatch/http/mime_negotiation.rb, line 159
def variant
  @variant ||= ActiveSupport::ArrayInquirer.new
end

Returns the variant for the response template as an instance of ActiveSupport::ArrayInquirer.

request.variant = :phone
request.variant.phone?  # => true
request.variant.tablet? # => false

request.variant = [:phone, :tablet]
request.variant.phone?                  # => true
request.variant.desktop?                # => false
request.variant.any?(:phone, :desktop)  # => true
request.variant.any?(:desktop, :watch)  # => false

# File lib/action_dispatch/http/mime_negotiation.rb, line 137
def variant=(variant)
  variant = Array(variant)

  if variant.all?(Symbol)
    @variant = ActiveSupport::ArrayInquirer.new(variant)
  else
    raise ArgumentError, "request.variant must be set to a Symbol or an Array of Symbols."
  end
end

Sets the variant for the response template.

When determining which template to render, Action View will incorporate all variants from the request. For example, if an ArticlesController#index action needs to respond to request.variant = [:ios, :turbo_native], it will render the first template file it can find in the following list:

• app/views/articles/index.html+ios.erb

• app/views/articles/index.html+turbo_native.erb

• app/views/articles/index.html.erb

Variants add context to the requests that views render appropriately. Variant names are arbitrary, and can communicate anything from the request’s platform (:android, :ios, :linux, :macos, :windows) to its browser (:chrome, :edge, :firefox, :safari), to the type of user (:admin, :guest, :user).

Note: Adding many new variant templates with similarities to existing template files can make maintaining your view code more difficult.

Parameters¶ ↑

• variant - a symbol name or an array of symbol names for variants used to render the response template

Examples¶ ↑

class ApplicationController < ActionController::Base
  before_action :determine_variants

  private
    def determine_variants
      variants = []

      # some code to determine the variant(s) to use

      variants << :ios if request.user_agent.include?("iOS")
      variants << :turbo_native if request.user_agent.include?("Turbo Native")

      request.variant = variants
    end
end