We are setting an instance variable in a stub, easy but a little bit doubtful.

ivar_setting_in_stub_spec.rb

# This is an adapted example from http://relishapp.com/rspec/rspec-mocks/v/2-1/dir/stubs/stub-with-substitute-implementation
describe "a stubbed implementation" do
  it "works" do
    object = Object.new
    object.stub(:foo) do |arg|
      if arg == :set then
        @x = 42
      elsif arg == :inc
        @x += 1
      end
    end
    
    object.foo(:set).should == 42
    object.foo(:inc).should == 43
  end
end