Undefined local variable or method `window_size'

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1

Error message is:
in <module:Android>': undefined local variable or methodwindow_size’ for Xinchengshe::Android:Module (NameError)
I already required rubygems and appium_lib, but why it still said undefined local variable or method?

following is my script

require 'rubygems'
require 'appium_lib'

APP_PATH = 'D:/appium/adt-bundle-windows-x86_64-20130717/sdk/xcf.apk'


opts = { caps: { platformName: 'Android', deviceName: 'iPhone Simulator', app:  APP_PATH },
  appium_lib: { sauce_username: nil, sauce_access_key: nil, debug: true} }

Appium::Driver.new(opts).start_driver

module Xinchengshe
  module Android
 
   sizes = window_size
    puts sizes
    # Quit when you're done!
     driver_quit
     puts 'Tests Succeeded!'
  end
end
2

window_size is method of Appium::Driver instance:

$driver.window_size