def on(screen_class, &block)
      return super(screen_class, &block) unless screen_class.ancestors.include? TE3270
      raise '@emulator instance variable must be available to use the ScreenFactory methods' unless @emulator
      @current_screen = screen_class.new @emulator
      block.call @current_screen if block
      @current_screen
    end
def on(screen_class, &block)
      return super(screen_class, &block) unless screen_class.ancestors.include? TE3270
      raise '@emulator instance variable must be available to use the ScreenFactory methods' unless @emulator
      @current_screen = screen_class.new @emulator
      block.call @current_screen if block
      @current_screen
    end
Create a screen object.  Also sets an instance variable +@current_screen

 @param [Class]  screen_class a class that has included the TE3270 module
 @param [block]  an optional block to be called
 @return [ScreenObject] the newly created screen object