A helper to generate mailer previews automatically (or more easily) for each action that exists on the mailer.

mailer_preview_helper.rb

module MailerPreviewHelper
  extend ActiveSupport::Concern

  included do
    # Allows a mailer preview to override `params` method (either class or instance level)
    # to define the hash that will be passed to the mailer by default by the `preview` helper.
    class_attribute :params, default: {}, instance_accessor: false

    mailer.instance_methods(false).each do |action|
      define_method(action) { mailer.with(**params).send action }
    end
  end

  class_methods do
    def mailer
      @mailer ||= preview_name.classify.constantize
    end
  end

  private

  # The associated Mailer class for this Preview
  def mailer
    self.class.mailer
  end

  # Derives the action (message) name from the caller and invokes it on the mailer class
  def preview(*args)
    action = caller_locations(1..1).first.base_label
    mailer.with(**params).send action, *args
  end

  # Explicitly defined instead of leveraging class_attribute instance methods
  # because we need to override the `params` method defined by ActionMailer::Preview.
  # Also, we don't want the writer/predicate or for it to be public
  # b/c that would add them to the mailer preview message listing.
  # TODO make params persist for both header frame and body frame requests
  def params
    self.class.params
  end
end

my_mailer.rb

class MyMailer < ActionMailer::Base
  before_action do
    @user = params[:user]
  end
  
  def welcome
  end

  def welcome_the_second(arg)
  end
  
  def goodbye
  end
end

my_mailer_preview.rb

class MyMailerPreview
  include MailerPreviewHelper
  
  # welcome is defined automatically!
  # same as: MyMailer.with(user: ...).welcome
  
  # goodbye is defined automatically!
  # same as: MyMailer.with(user: ...).goodbye
  
  def welcome_the_second
    preview("but with args")
  end
  
  def completely_different
    mailer.with(something: "different").goodbye
  end
  
  def self.params
    { user: FactoryBot.create(:user) }
  end
end