This class provides a TestCase for testing generators. To set up, you need just to configure the destination and set which generator is being tested:

class AppGeneratorTest < Rails::Generators::TestCase
  tests AppGenerator
  destination File.expand_path("../tmp", __dir__)
end

If you want to ensure your destination root is clean before running each test, you can set a setup callback:

class AppGeneratorTest < Rails::Generators::TestCase
  tests AppGenerator
  destination File.expand_path("../tmp", __dir__)
  setup :prepare_destination
end