Refactor ThumbnailPathService

This makes it easier to override parts. Also added more documentation

app/services/curation_concerns/thumbnail_path_service.rb

@@ -12,36 +12,47 @@ def call(object)
         if thumb.audio?
           audio_image
         elsif thumbnail?(thumb)
-          Rails.application.routes.url_helpers.download_path(object.thumbnail_id, file: 'thumbnail')
+          thumbnail_path(thumb)
         else
           default_image
         end
       end
 
-      def fetch_thumbnail(object)
-        return object if object.thumbnail_id == object.id
-        ::ActiveFedora::Base.find(object.thumbnail_id)
-      rescue ActiveFedora::ObjectNotFoundError
-        Rails.logger.error("Couldn't find thumbnail #{object.thumbnail_id} for #{object.id}")
-        nil
-      end
+      protected
 
-      def default_image
-        ActionController::Base.helpers.image_path 'default.png'
-      end
+        def fetch_thumbnail(object)
+          return object if object.thumbnail_id == object.id
+          ::ActiveFedora::Base.find(object.thumbnail_id)
+        rescue ActiveFedora::ObjectNotFoundError
+          Rails.logger.error("Couldn't find thumbnail #{object.thumbnail_id} for #{object.id}")
+          nil
+        end
 
-      def audio_image
-        ActionController::Base.helpers.image_path 'audio.png'
-      end
+        # @return the network path to the thumbnail
+        # @param [FileSet] thumbnail the object that is the thumbnail
+        def thumbnail_path(thumbnail)
+          Rails.application.routes.url_helpers.download_path(thumbnail.id,
+                                                             file: 'thumbnail')
+        end
 
-      # @return true if there a file on disk for this object, otherwise false
-      def thumbnail?(thumb)
-        File.exist?(thumbnail_filepath(thumb))
-      end
+        def default_image
+          ActionController::Base.helpers.image_path 'default.png'
+        end
 
-      def thumbnail_filepath(thumb)
-        CurationConcerns::DerivativePath.derivative_path_for_reference(thumb, 'thumbnail')
-      end
+        def audio_image
+          ActionController::Base.helpers.image_path 'audio.png'
+        end
+
+        # @return true if there a file on disk for this object, otherwise false
+        # @param [FileSet] thumbnail the object that is the thumbnail
+        def thumbnail?(thumb)
+          File.exist?(thumbnail_filepath(thumb))
+        end
+
+        # @param [FileSet] thumbnail the object that is the thumbnail
+        def thumbnail_filepath(thumb)
+          CurationConcerns::DerivativePath.derivative_path_for_reference(thumb, 'thumbnail')
+        end
     end
   end
 end