iOS

Android

class AppDelegate
  def application(application, didFinishLaunchingWithOptions:launchOptions)
    alert = UIAlertView.new
    alert.message = 'Hello World'
    alert.show
    true
  end
end

class MainActivity < Android::App::Activity
  def onCreate(savedInstanceState)
    puts "Hello World!"
    super
    view = Android::Widget::TextView.new(self)
    view.text = "Hello World!"
    self.contentView = view
  end
end