Add Request#formats=(extensions) that lets you set multiple formats directly in a prioritized order

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Add Request#formats=(extensions) that lets you set multiple formats directly in a prioritized order *DHH*
+
+    Example of using this for custom iphone views with an HTML fallback:
+
+      class ApplicationController < ActionController::Base
+        before_filter :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
+
+
 *   Add Routing Concerns to declare common routes that can be reused inside
     others resources and routes.
 
@@ -28,7 +42,7 @@
 
         resources :posts, concerns: [:commentable, :image_attachable]
 
-    *David Heinemeier Hansson + Rafael Mendonça França*
+    *DHH + Rafael Mendonça França*
 
 *   Add start_hour and end_hour options to the select_hour helper. *Evan Tann*
 

actionpack/lib/action_dispatch/http/mime_negotiation.rb

@@ -80,6 +80,27 @@ def format=(extension)
         @env["action_dispatch.request.formats"] = [Mime::Type.lookup_by_extension(parameters[:format])]
       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 fallback
+      # to the :html format.
+      #
+      #   class ApplicationController < ActionController::Base
+      #     before_filter :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
+      def formats=(extensions)
+        parameters[:format] = extensions.first.to_s
+        @env["action_dispatch.request.formats"] = extensions.collect do |extension|
+          Mime::Type.lookup_by_extension(extension)
+        end
+      end
+
       # Receives an array of mimes and return the first user sent mime that
       # matches the order array.
       #