Add the ability to intercept emails before previewing

To support the ability for tools like CSS style inliners to operate on emails
being previewed this commit adds a hook in a similar fashion to the existing
delivery interceptor hook, e.g:

  class CSSInlineStyler
    def self.previewing_email(message)
      # inline CSS styles
    end
  end

  ActionMailer::Base.register_preview_interceptor CSSInlineStyler

Fixes #13622.

2 files changed, 52 insertions, 2 deletions

diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index 41db62cc58..5af0217973 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -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
diff --git a/actionmailer/lib/action_mailer/preview.rb b/actionmailer/lib/action_mailer/preview.rb
index ecceaf8c70..0efd702e4d 100644
--- a/actionmailer/lib/action_mailer/preview.rb
+++ b/actionmailer/lib/action_mailer/preview.rb
@@ -10,6 +10,31 @@ module ActionMailer
       #     config.action_mailer.preview_path = "#{Rails.root}/lib/mailer_previews"
       #
       class_attribute :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
@@ -68,6 +97,12 @@ module ActionMailer
         def preview_path? #:nodoc:
           Base.preview_path?
         end
+
+        def inform_preview_interceptors(message) #:nodoc:
+          Base.preview_interceptors.each do |interceptor|
+            interceptor.previewing_email(message)
+          end
+        end
     end
   end
 end