Rubymotion developers often seem to shy away from using getter methods for instance variables. Instead, they're defined all over the place - as you'd see more in Objective-C.

app_delegate.rb

# This is the approach I see most commonly in Rubymotion code examples.
class AppDelegate
  
  def application(application, didFinishLaunchingWithOptions:launchOptions)
    @mainViewController = MainViewController.alloc.init
    @window = UIWindow.alloc.initWithFrame(UIScreen.mainScreen.applicationFrame)
    @window.rootViewController = mainViewController
    @window.makeKeyAndVisible
    true
  end
  
end

# In my opinion, this approach is cleaner and is more idiomatic OO Code.
class AppDelegate
  
  def mainViewController
    @mainViewController ||= MainViewController.alloc.init
  end
  
  def window
    @window ||= UIWindow.alloc.initWithFrame(UIScreen.mainScreen.applicationFrame)
  end
  
  def application(application, didFinishLaunchingWithOptions:launchOptions)
    window.rootViewController = mainViewController
    window.makeKeyAndVisible
    true
  end
  
end