How to help Capybara wait for a node to appear as a result of some asynchronous operation

buying_steps.rb

include CapybaraAccessories

# This step occurs after an item was added to the cart through XHR
# It must wait for the call to return and for the UI to update.
# A badge must become visible and contain the number of items in cart.
# Without this kind of wait code Capybara's waiting features (2.2.1)
# still miss the update and fail the test.
  
Then(/^I see a caddie badge with (\d+) item\(s\)$/) do |count|
  wait_for{page.find('#count')}.should have_content(count)
end

env.rb

module CapybaraAccessories
  def wait_for
    res = nil
    Timeout::timeout(5) {
      begin
        break if res=yield
      rescue Capybara::ElementNotFound
        sleep 0.01
        retry
      end
    }
    res
  end
end