ActionMailer RSpec helpers. Makes mailer test more similar to controller tests in that you can test that instance variables are assigned for rendering

example_mailer_spec.rb

require "spec_helper"

describe ExampleMailer do

  describe :new_user do
    let(:user) { FactoryGirl.create :user }

    it "sends to the correct addresses" do
      # :mail is similar to the controller specs 'get', 'post' etc methods
      mail :new_user, user
      
      # :response is similar to controller specs, it returns the mail message
      expect(response.from).to include 'from@example.com'
      expect(response.to).to   include 'to@example.com'
    end

    it "assigns @user" do
      mail :new_user, user
      
      # :assigns is similar to controller specs, it checks the view_assigns of the mailer renderer
      expect(assigns(:user)).to eq user
    end

  end

end

mailer_helpers.rb

module MailerHelpers

  def mail(method, *arguments)
    @_mail_called = true
    @_mailer_method = method
    @_mailer_arguments = arguments
  end

  def mailer
    raise "You must use mail(:method_name) before calling :mailer" unless @_mail_called
    @mailer ||= described_class.send :new, @_mailer_method, *@_mailer_arguments
  end

  def response
    raise "You must use mail(:method_name) before calling :response" unless @_mail_called
    @response ||= mailer.message
  end

  def assigns(method)
    raise "You must use mail(:method_name) before calling :assigns" unless @_mail_called
    mailer.view_assigns.with_indifferent_access[method]
  end

end

RSpec.configure do |config|
  config.include MailerHelpers, type: :mailer
end

This appears to be broken in Rails 5 😞

      Failure/Error: @mailer ||= described_class.send :new, @_mailer_method, *@_mailer_arguments

      ArgumentError:
        wrong number of arguments (given 3, expected 0)

Looks like this fixes it in Rails 5:

module MailerHelpers

  def mail(method, *arguments)
    @_mail_called = true
    @_mailer_method = method
    @_mailer_arguments = arguments
  end

  def mailer
    raise "You must use mail(:method_name) before calling :mailer" unless @_mail_called
    @mailer ||= described_class.new.tap do |mailer|
      mailer.process(@_mailer_method, *@_mailer_arguments)
    end
  end

  def response
    raise "You must use mail(:method_name) before calling :response" unless @_mail_called
    @response ||= mailer.message
  end

  def assigns(method)
    raise "You must use mail(:method_name) before calling :assigns" unless @_mail_called
    mailer.view_assigns.with_indifferent_access[method]
  end

end