The art of Mocking

What mocks are, how they fit the OOP world and how to use them in Ruby.

Transcript

1. The art of Mocking
2. 1. How mocks fit in the OOP world 2. Mock objects in practice 3. Rules on how to use them 4. Common misconceptions
4. Domain logic 3rd party
 integrations
5. A B Query
6. A B Command
7. A B Query C
8. A B Command C
9. A B C Domain logic 3rd party
 integrations
10. Testing objects with collaborators in isolation
11. class ApiClient ... def is_it_up? Net::HTTP.head("...") true rescue NetworkError false end end
12. class ApiClient # @http_client from the constructor def is_it_up? @http_client.head("...") true rescue NetworkError false end end ApiClient.new(HTTPClient.new).is_it_up? # => false
13. describe ApiClient do let(:http_client) { instance_double(HTTPClient) } subject { described_class.new(http_client) } ... end Verifying double (aka mock)
14. describe ApiClient do ... describe "#is_it_up?" do context "when HEAD request succeeds" do before do expect(http_client) .to_receive(:head) .with("...") end it "returns true" do expect(subject.is_it_up?).to be true end end end end
15. describe ApiClient do ... describe "#is_it_up?" do context "when HEAD request fails" do before do expect(http_client) .to_receive(:head) .and_raise(NetworkError) end it "returns false" do expect(subject.is_it_up?).to be false end end end end
16. Mock objects mimic the same interface as the real object
17. Rules
18. 1. Only mock objects you own and control.
19. 2. Mock behavior, not data.
20. 3. Always write complete
 expectations.
21. 4. Only mock immediate neighbors.
22. Misconceptions
23. 1. Stubs =/= Mocks.
24. Stubs are a dummy implementation of production code.
25. Mocks act as stubs, but also include assertions on interactions.
26. 2. Mocks are not only useful on systems’ boundaries (I/O).
27. 3. Mocks do not inhibit refactoring.
28. Questions, and hopefully answers.