rails--rails/actionmailer/lib/action_mailer/preview.rb

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

      # :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

  class Preview
    extend ActiveSupport::DescendantsTracker

    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)
        preview = self.new
        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

      protected
        def load_previews #:nodoc:
          if preview_path
            Dir["#{preview_path}/**/*_preview.rb"].each{ |file| require_dependency file }
          end
        end

        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