3 files changed, 73 insertions, 15 deletions

diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index a30e3e65da..eb8cca9ee4 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -51,7 +51,7 @@ module ActionMailer
   # * <tt>mail</tt> - Allows you to specify email to be sent.
   #
   # The hash passed to the mail method allows you to specify any header that a <tt>Mail::Message</tt>
-  # will accept (any valid Email header including optional fields).
+  # will accept (any valid email header including optional fields).
   #
   # The mail method, if not passed a block, will inspect your views and send all the views with
   # the same name as the method, so the above action would send the +welcome.text.erb+ view
@@ -330,6 +330,21 @@ module ActionMailer
   # An overview of all previews is accessible at <tt>http://localhost:3000/rails/mailers</tt>
   # on a running development server instance.
   #
+  # Previews can also be intercepted in a similar manner as deliveries can be by registering
+  # a preview interceptor that has a <tt>previewing_email</tt> method:
+  #
+  #   class CssInlineStyler
+  #     def self.previewing_email(message)
+  #       # inline CSS styles
+  #     end
+  #   end
+  #
+  #   config.action_mailer.register_preview_interceptor :css_inline_styler
+  #
+  # Note that interceptors need to be registered both with <tt>register_interceptor</tt>
+  # and <tt>register_preview_interceptor</tt> if they should operate on both sending and
+  # previewing emails.
+  #
   # = Configuration options
   #
   # These options are specified on the class level, like
@@ -429,18 +444,30 @@ module ActionMailer
       end
 
       # Register an Observer which will be notified when mail is delivered.
-      # Either a class or a string can be passed in as the Observer. If a string is passed in
-      # it will be <tt>constantize</tt>d.
+      # Either a class, string or symbol can be passed in as the Observer.
+      # If a string or symbol is passed in it will be camelized and constantized.
       def register_observer(observer)
-        delivery_observer = (observer.is_a?(String) ? observer.constantize : observer)
+        delivery_observer = case observer
+          when String, Symbol
+            observer.to_s.camelize.constantize
+          else
+            observer
+          end
+
         Mail.register_observer(delivery_observer)
       end
 
       # Register an Interceptor which will be called before mail is sent.
-      # Either a class or a string can be passed in as the Interceptor. If a string is passed in
-      # it will be <tt>constantize</tt>d.
+      # Either a class, string or symbol can be passed in as the Interceptor.
+      # If a string or symbol is passed in it will be camelized and constantized.
       def register_interceptor(interceptor)
-        delivery_interceptor = (interceptor.is_a?(String) ? interceptor.constantize : interceptor)
+        delivery_interceptor = case interceptor
+          when String, Symbol
+            interceptor.to_s.camelize.constantize
+          else
+            interceptor
+          end
+
         Mail.register_interceptor(delivery_interceptor)
       end
 
@@ -737,7 +764,7 @@ module ActionMailer
       m.charset = charset = headers[:charset]
 
       # Set configure delivery behavior
-      wrap_delivery_behavior!(headers.delete(:delivery_method),headers.delete(:delivery_method_options))
+      wrap_delivery_behavior!(headers.delete(:delivery_method), headers.delete(:delivery_method_options))
 
       # Assign all headers except parts_order, content_type and body
       assignable = headers.except(:parts_order, :content_type, :body, :template_name, :template_path)
diff --git a/actionmailer/lib/action_mailer/preview.rb b/actionmailer/lib/action_mailer/preview.rb
index ecceaf8c70..fde0e86f5b 100644
--- a/actionmailer/lib/action_mailer/preview.rb
+++ b/actionmailer/lib/action_mailer/preview.rb
@@ -9,7 +9,32 @@ module ActionMailer
       #
       #     config.action_mailer.preview_path = "#{Rails.root}/lib/mailer_previews"
       #
-      class_attribute :preview_path, instance_writer: false
+      mattr_accessor :preview_path, instance_writer: false
+
+      # :nodoc:
+      mattr_accessor :preview_interceptors, instance_writer: false
+      self.preview_interceptors = []
+
+      # Register one or more Interceptors which will be called before mail is previewed.
+      def register_preview_interceptors(*interceptors)
+        interceptors.flatten.compact.each { |interceptor| register_preview_interceptor(interceptor) }
+      end
+
+      # Register am Interceptor which will be called before mail is previewed.
+      # Either a class or a string can be passed in as the Interceptor. If a
+      # string is passed in it will be <tt>constantize</tt>d.
+      def register_preview_interceptor(interceptor)
+        preview_interceptor = case interceptor
+          when String, Symbol
+            interceptor.to_s.camelize.constantize
+          else
+            interceptor
+          end
+
+        unless preview_interceptors.include?(preview_interceptor)
+          preview_interceptors << preview_interceptor
+        end
+      end
     end
   end
 
@@ -23,10 +48,14 @@ module ActionMailer
         descendants
       end
 
-      # Returns the mail object for the given email name
+      # Returns the mail object for the given email name. The registered preview
+      # interceptors will be informed so that they can transform the message
+      # as they would if the mail was actually being delivered.
       def call(email)
         preview = self.new
-        preview.public_send(email)
+        message = preview.public_send(email)
+        inform_preview_interceptors(message)
+        message
       end
 
       # Returns all of the available email previews
@@ -56,7 +85,7 @@ module ActionMailer
 
       protected
         def load_previews #:nodoc:
-          if preview_path?
+          if preview_path
             Dir["#{preview_path}/**/*_preview.rb"].each{ |file| require_dependency file }
           end
         end
@@ -65,8 +94,10 @@ module ActionMailer
           Base.preview_path
         end
 
-        def preview_path? #:nodoc:
-          Base.preview_path?
+        def inform_preview_interceptors(message) #:nodoc:
+          Base.preview_interceptors.each do |interceptor|
+            interceptor.previewing_email(message)
+          end
         end
     end
   end
diff --git a/actionmailer/lib/action_mailer/railtie.rb b/actionmailer/lib/action_mailer/railtie.rb
index c893ddfef5..8d1e40297b 100644
--- a/actionmailer/lib/action_mailer/railtie.rb
+++ b/actionmailer/lib/action_mailer/railtie.rb
@@ -46,7 +46,7 @@ module ActionMailer
     end
 
     config.after_initialize do
-      if ActionMailer::Base.preview_path?
+      if ActionMailer::Base.preview_path
         ActiveSupport::Dependencies.autoload_paths << ActionMailer::Base.preview_path
       end
     end