Example of mocking and stubing with dependency injection

example.rb

module Example
  class Dog
    attr_reader :stomach

    def initialize(stomach)
      @stomach = stomach
    end

    def hungry?
      stomach.empty?
    end
  end
end

module Example
  describe Dog do
    subject { Dog.new(stomach)}

    describe "#new" do
      let(:stomach) { double(:stomach) }

      it "should take a stomach" do
        subject.stomach.should == stomach
      end
    end

    describe "#hungry?" do
      context "when stomach is empty" do
        let(:stomach) { double(:stomach) }

        before do
          stomach.stub(:empty?) { true }
        end

        it "is hungry" do
          subject.hungry?.should be_true
        end
      end
    end
  end
end