1 files changed, 143 insertions, 0 deletions
diff --git a/actionmailer/lib/action_mailer/preview.rb b/actionmailer/lib/action_mailer/preview.rb
new file mode 100644
index 0000000000..500b3bede0
--- /dev/null
+++ b/actionmailer/lib/action_mailer/preview.rb
@@ -0,0 +1,143 @@
+# frozen_string_literal: true
+
+require "active_support/descendants_tracker"
+
+module ActionMailer
+  module Previews #:nodoc:
+    extend ActiveSupport::Concern
+
+    included do
+      # Set the location of mailer previews through app configuration:
+      #
+      #     config.action_mailer.preview_path = "#{Rails.root}/lib/mailer_previews"
+      #
+      mattr_accessor :preview_path, instance_writer: false
+
+      # Enable or disable mailer previews through app configuration:
+      #
+      #     config.action_mailer.show_previews = true
+      #
+      # Defaults to +true+ for development environment
+      #
+      mattr_accessor :show_previews, instance_writer: false
+
+      # :nodoc:
+      mattr_accessor :preview_interceptors, instance_writer: false, default: [ActionMailer::InlinePreviewInterceptor]
+    end
+
+    module ClassMethods
+      # 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
+
+      # Unregister one or more previously registered Interceptors.
+      def unregister_preview_interceptors(*interceptors)
+        interceptors.flatten.compact.each { |interceptor| unregister_preview_interceptor(interceptor) }
+      end
+
+      # Register an 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 constantized.
+      def register_preview_interceptor(interceptor)
+        preview_interceptor = interceptor_class_for(interceptor)
+
+        unless preview_interceptors.include?(preview_interceptor)
+          preview_interceptors << preview_interceptor
+        end
+      end
+
+      # Unregister a previously registered Interceptor.
+      # Either a class or a string can be passed in as the Interceptor. If a
+      # string is passed in it will be constantized.
+      def unregister_preview_interceptor(interceptor)
+        preview_interceptors.delete(interceptor_class_for(interceptor))
+      end
+
+      private
+
+        def interceptor_class_for(interceptor)
+          case interceptor
+          when String, Symbol
+            interceptor.to_s.camelize.constantize
+          else
+            interceptor
+          end
+        end
+    end
+  end
+
+  class Preview
+    extend ActiveSupport::DescendantsTracker
+
+    attr_reader :params
+
+    def initialize(params = {})
+      @params = params
+    end
+
+    class << self
+      # Returns all mailer preview classes.
+      def all
+        load_previews if descendants.empty?
+        descendants
+      end
+
+      # 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, params = {})
+        preview = new(params)
+        message = preview.public_send(email)
+        inform_preview_interceptors(message)
+        message
+      end
+
+      # Returns all of the available email previews.
+      def emails
+        public_instance_methods(false).map(&:to_s).sort
+      end
+
+      # Returns +true+ if the email exists.
+      def email_exists?(email)
+        emails.include?(email)
+      end
+
+      # Returns +true+ if the preview exists.
+      def exists?(preview)
+        all.any? { |p| p.preview_name == preview }
+      end
+
+      # Find a mailer preview by its underscored class name.
+      def find(preview)
+        all.find { |p| p.preview_name == preview }
+      end
+
+      # Returns the underscored name of the mailer preview without the suffix.
+      def preview_name
+        name.sub(/Preview$/, "").underscore
+      end
+
+      private
+        def load_previews
+          if preview_path
+            Dir["#{preview_path}/**/*_preview.rb"].sort.each { |file| require_dependency file }
+          end
+        end
+
+        def preview_path
+          Base.preview_path
+        end
+
+        def show_previews
+          Base.show_previews
+        end
+
+        def inform_preview_interceptors(message)
+          Base.preview_interceptors.each do |interceptor|
+            interceptor.previewing_email(message)
+          end
+        end
+    end
+  end
+end