Trying out some new RSpec `shared_example` ideas

spec-domain-exportable_widget.rb

require 'domain/exportable_widget'
require 'support/preconditions'

RSpec.describe ExportableWidget do

  # We don't know yet if we will have multiple exportable objects, but this
  # is simply the documentation of our intended shared protocol if we do later.
  shared_examples_for "an exportable" do
    required_attribute :exportable

    it "has an authorization token" do
      expect(exportable.auth_token).to be_a String
    end

    it "has a last exported at timestamp" do
      expect(exportable.last_exported_at).to be_a DateTime
    end

    it "can have a nil last exported timestamp if it was never exported" do
      expect{
        expect{
          exportable.last_exported_at = nil
        }.to change(exportable, :last_exported_at).to(nil)
      }.to change(exportable, :exported?).from(true).to(false)
      # The following matcher will be available / fixed in 3.1.0 possibly 3.0.3
      #}.to change(exportable, :exported?).to(true)
      #  .and change(exportable, :last_exported_at).to(nil)
    end
  end

  subject(:any_exportable_widget) {
    ExportableWidget.new(
      widget_id: a_sample_widget_id,
      auth_token: SecureRandom.uuid,
      last_exported_at: 1.day.ago,
    )
  }

  let(:a_sample_widget_id) { 1 }

  it_behaves_like "an exportable" do
    subject(:exportable) { any_exportable_widget }
  end

  it "has a widget id" do
    expect(any_exportable_widget.widget_id).to eq a_sample_widget_id
  end

end

spec-support-preconditions.rb

RequiredAttributeError = Class.new(RuntimeError)

module MyApp
  module RSpec
    module Preconditions

      def required_attribute(bareword)
        # TODO: Figure out how to get the calling line in the error and not
        #       the `fail` line below
        before do
          bareword = bareword.intern
          unless respond_to?(bareword) && !send(bareword).nil?
            fail RequiredAttributeError, "No `#{bareword}` object provided"
          end
        end
      end

    end
  end
end

RSpec.configure do |config|
  config.extend MyApp::RSpec::Preconditions
end