module ActionDispatch::Http::MimeNegotiation

def accepts

Returns the accepted MIME type for the request. 
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

def content_mime_type

The MIME type of the HTTP request, such as Mime[:xml]. 
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

def format(_view_path = nil)


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

Returns the MIME type for the \format used in the request. 
def format(_view_path = nil)
  formats.first || Mime::NullType.instance
end

def format=(extension)

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

before_action :adjust_format_for_iphone
class ApplicationController < ActionController::Base

that are not controlled by the extension.
Sets the \format by string extension, which can be used to force custom formats 
def format=(extension)
  parameters[:format] = extension.to_s
  set_header "action_dispatch.request.formats", [Mime::Type.lookup_by_extension(parameters[:format])]
end

def format_from_path_extension 

def format_from_path_extension
  path = get_header("action_dispatch.original_path") || get_header("PATH_INFO")
  if match = path && path.match(/\.(\w+)\z/)
    Mime[match.captures.first]
  end
end

def formats 

def formats
  fetch_header("action_dispatch.request.formats") do |k|
    v = if params_readable?
      Array(Mime[parameters[:format]])
    elsif use_accept_header && valid_accept_header
      accepts.dup
    elsif extension_format = format_from_path_extension
      [extension_format]
    elsif xhr?
      [Mime[:js]]
    else
      [Mime[:html]]
    end
    v.select! do |format|
      format.symbol || format.ref == "*/*"
    end
    set_header k, v
  end
end

def formats=(extensions)

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

before_action :adjust_format_for_iphone_with_html_fallback
class ApplicationController < ActionController::Base

to the +:html+ format.
In this example, the +:iphone+ format will be used if it's available, otherwise it'll fallback

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

def has_content_type? # :nodoc:

:nodoc: 
def has_content_type? # :nodoc:
  get_header "CONTENT_TYPE"
end

def negotiate_mime(order)

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

def params_readable? 

def params_readable?
  parameters[:format]
rescue *RESCUABLE_MIME_FORMAT_ERRORS
  false
end

def return_only_media_type_on_content_type 

def return_only_media_type_on_content_type
  ActionDispatch.deprecator.warn(
    "`config.action_dispatch.return_only_request_media_type_on_content_type` is deprecated and will" \
    " be removed in Rails 7.2."
  )
end

def return_only_media_type_on_content_type=(value) 

def return_only_media_type_on_content_type=(value)
  ActionDispatch.deprecator.warn(
    "`config.action_dispatch.return_only_request_media_type_on_content_type` is deprecated and will" \
      " be removed in Rails 7.2."
  )
end

def should_apply_vary_header? 

def should_apply_vary_header?
  !params_readable? && use_accept_header && valid_accept_header
end

def use_accept_header 

def use_accept_header
  !self.class.ignore_accept_header
end

def valid_accept_header 

def valid_accept_header
  (xhr? && (accept.present? || content_mime_type)) ||
    (accept.present? && !accept.match?(BROWSER_LIKE_ACCEPTS))
end

def variant 

def variant
  @variant ||= ActiveSupport::ArrayInquirer.new
end

def variant=(variant)

Sets the \variant for template. 
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