readme.md

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email_previews.rb

# app/admin/email_previews.rb
ActiveAdmin.register_page 'Email Previews' do

  content do
    div '.'
  end

  sidebar 'Mail Previews' do
    Dir['app/mailer_previews/*_preview.rb'].each do |preview_path|
      preview_mailer = File.basename(preview_path, '.rb')
      mailer = preview_mailer.chomp('_preview')
      div { strong { mailer } }
      preview_mailer_class = preview_mailer.camelize.constantize
      mails = preview_mailer_class.public_instance_methods(false).map(&:to_s).sort
      mails.each do |mail|
        div { link_to mail, {action: :preview, mailer: mailer, mail: mail} }
      end
      br
    end
  end

  page_action :preview do
    mailer_preview_class = "#{params[:mailer]}_preview".camelize.constantize
    @email = mailer_preview_class.new.send(params[:mail])
    ActionMailer::InlinePreviewInterceptor.previewing_email(@email)

    part = @email.find_first_mime_type('text/html') || @email
    @email_content = part.decoded
  end

end

preview.haml

-# app/views/admin/email_previews/preview.haml
%p
  != "<b>From:</b> #{h @email.header['from']}"
  != "<br><b>Reply-To:</b> #{h @email.header['reply-to']}" if @email.reply_to
  != "<br><b>To:</b> #{h @email.header['to']}"
  != "<br><b>CC:</b> #{h @email.header['cc']}" if @email.cc
  != "<br><b>Subject:</b> #{h @email.subject}"
  - if @email.attachments.present?
    %br
    %b Attachments:
    - @email.attachments.each do |a|
      - filename = a.respond_to?(:original_filename) ? a.original_filename : a.filename
      = link_to filename, "data:application/octet-stream;charset=utf-8;base64,#{Base64.encode64(a.body.to_s)}", download: filename

%iframe(srcdoc="#{@email_content}" style="width: 100%;height: 500px;border: 2px solid lightgrey;")

user_mailer_preview.rb

# app/mailer_previews/user_mailer_preview.rb
class UserMailerPreview

  def some_email
    UserMailer.some_email(User.find(123).id)
  end

end