Add a preview interceptor

ActionMailer lets us intercept messages before they are delivered or
previewed, this is the ideal time to fetch our preview from the
Notifications API and update the message with it.

Read more in the 'Previewing' section:

https://api.rubyonrails.org/classes/ActionMailer/Base.html

lib/mail/notify.rb

@@ -9,6 +9,7 @@
 require "mail/notify/mailer"
 require "mail/notify/message"
 require "mail/notify/mail_notify_previews_controller"
+require "mail/notify/mail_notify_preview_interceptor"
 
 Mail::Message.include Mail::Notify::Message
 

lib/mail/notify/engine.rb

@@ -1,12 +1,12 @@
 # frozen_string_literal: true
 
-require "rails/mailers_controller"
-
 module Mail
   module Notify
     class Engine < Rails::Engine
       initializer "mail-notify.add_delivery_method", before: "action_mailer.set_configs" do
         ActionMailer::Base.add_delivery_method(:notify, Mail::Notify::DeliveryMethod)
+
+        config.action_mailer.preview_interceptors = [:mail_notify_preview_interceptor]
       end
     end
   end

lib/mail/notify/mail_notify_preview_interceptor.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+##
+# This is a ActionMailer interceptor class for previews from the Notifications API
+#
+# See ActionMailer::Base:
+#
+# https://github.com/rails/rails/blob/v5.2.8.1/actionmailer/lib/action_mailer/base.rb#L367
+
+class MailNotifyPreviewInterceptor
+  ##
+  # ActionMailer call back when a preview is being generated.
+  #
+  # Transforms the content of the passed in message.
+
+  def self.previewing_email(message)
+    new(message).transform!
+  end
+
+  ##
+  # Creates a new MailNotifyPreviewInterceptor ready for use.
+
+  def initialize(message)
+    @message = message
+  end
+
+  ##
+  # Transforms the content of the message to that from the Notifications API preview.
+  #
+  # The html is wrapped in a layout and rendered by the MailNotifyPreviewsControllers renderer.
+
+  def transform!
+    # the messages delivery method will be our Mail::Notify::DeliveryMethod and have the `preview` method.
+    preview = @message.delivery_method.preview(@message)
+
+    @message.subject = preview.subject
+    @message.html_part.body = renderer.render html: preview.html.html_safe, layout: "govuk_notify_layout"
+    @message.text_part.body = preview.body
+
+    @message
+  end
+
+  private
+
+  def renderer
+    # rendering in Rails without a controller gets far too complicated, instead
+    # we rely on this empty controller to do it for us.
+
+    MailNotifyPreviewsController.renderer
+  end
+end

spec/mail/notify/mail_notify_preview_interceptor_spec.rb

@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe MailNotifyPreviewInterceptor do
+  let(:preview_response_double) do
+    double(
+      "preview message response",
+      subject: "Preview subject",
+      html: "<p>Preview body</p>",
+      body: "Preview body"
+    )
+  end
+
+  let(:delivery_method_double) do
+    double("delivery method", preview: preview_response_double)
+  end
+
+  let(:message_double) do
+    message = Mail::Message.new(subject: "Original subject")
+
+    message.add_part(Mail::Part.new(content_type: "text/html"))
+    message.add_part(Mail::Part.new(content_type: "text/plain"))
+
+    allow(message).to receive(:delivery_method).and_return(delivery_method_double)
+    message
+  end
+
+  describe ".previewing_email" do
+    it "transforms the message" do
+      transformed_message = MailNotifyPreviewInterceptor.previewing_email(message_double)
+
+      expect(transformed_message.subject).to eql("Preview subject")
+      expect(transformed_message.text_part.body.raw_source).to eql("Preview body")
+      expect(transformed_message.html_part.body).to include("<p>Preview body</p>")
+    end
+  end
+
+  describe "#transform!" do
+    it "sets the subject of the message" do
+      message = message_double
+
+      described_class.new(message).transform!
+
+      expect(message.subject).to eql preview_response_double.subject
+    end
+
+    it "sets the plain text body  of the message" do
+      message = message_double
+
+      described_class.new(message).transform!
+
+      expect(message.text_part.body.raw_source).to eql preview_response_double.body
+    end
+
+    it "sets the html body of the message" do
+      message = message_double
+
+      described_class.new(message).transform!
+
+      expect(message.html_part.body.raw_source).to include(preview_response_double.html)
+    end
+  end
+end