问如何在测试过程中对帮助程序中的函数进行存根

EN

def show_welcome_banner?
  (controller_name == 'competition' && action_name == 'index') ||
  (controller_name == 'submissions' && action_name == 'show')
end

describe PageHelper do
  it "should know when the welcome banner is to be shown" do
    helper.stub!(:controller_name).and_return('index')
    show_welcome_banner?.should == false
  end
end

controller.stub!(:controller_name).and_return('index')

  1) PageHelper should know when the welcome banner is to be shown
     Failure/Error: show_welcome_banner?.should == false
     NameError:
       undefined local variable or method `controller_name' for #<RSpec::Core::ExampleGroup::Nested_1:0x1059a10b8>
     # ./app/helpers/page_helper.rb:16:in `show_welcome_banner?'
     # ./spec/helpers/page_helper_spec.rb:7