example.rb

class MyViewController < UIViewController
  def viewDidLoad
      @search_bar = UISearchBar.alloc.initWithFrame CGRectZero
      self.navigationController.rac_liftSelector "setNavigationBarHidden:animated:", withSignalsFromArray: [@search_bar.rac_editing, RACSignal.return(true)]
      # Add search bar to table view, etc.
  end
end

ui_search_bar.rb

class UISearchBar
  def rac_editing
    self.delegate = self
    protocol = NSProtocolFromString("UISearchBarDelegate") 
    @editing ||= RACSignal.merge([
      rac_signalForSelector("searchBarTextDidBeginEditing:", fromProtocol: protocol).mapReplace(true),
      rac_signalForSelector("searchBarTextDidEndEditing:", fromProtocol: protocol).mapReplace(false)]
    ).startWith(false).takeUntil(rac_willDeallocSignal)
  end
end