user_creation_spec.rb

require "user_creation_service"
require "spec"
require "rr"

Spec::Runner.configure do |config|
  config.mock_with RR::Adapters::Rspec
end

describe UserCreationService do
  before do
    @user = stub
    @params = { :name => "Matz", :language => "Ruby" }

    mock(@user_class = Object.new).create(@params) { @user }
    mock(@log_class = Object.new).new_user(@user)

    @service = UserCreationService.new(@user_class, @log_class)
    @service.create(@params)
  end

  it "creates the user with the supplied parameters" do 
    @user_class.should have_received.create(@params)
  end

  it "logs the creation of the user" do
    @log_class.should have_received.new_user(@user)
  end
end