TDD in Practice

Alan Christensen @christensena

Preamble

•Who am I?

•My TDD journey

Overview

•TDD to drive out design (live coding)

•Two approaches to TDD

•Bit on architecture and design

TDD for Design

•TDD is supposed to improve your design

•How does that work?

Worked example

•Order confirmation (think shopping cart)

•Payment has already been taken

•What next?

Order Confirmation

•Flag order as Ready to Ship

•Notify customer, e.g. confirmation email

•Arrange shipment

coding time!

OrderConfirmationEmailBuilderOrderConfirmer

Test Test

OrderConfirmationEmailBuilder

Fake

Email TemplateEngine

Fake

Review•Try to keep the Red Green Refactor

discipline

•NCrunch is wonderful but costs $$$

•Use test naming to document behaviour

•Asserts should match test naming (so typically one assert per test)

•Arrange/Act/Assert helps with readability

Review: Mocking

•Prefer state based tests

•Prefer manual (static) fakes over dynamic

•Prefer stubs if using dynamic mocks

•Use adapters over untestable code

•Use adapters/facades to make application code more expressive of intent

Review: TDD Design

•Starting with test helps focus on good API

•When test setups (Arrange) get large, it usually tells us we need to factor code into smaller pieces (classes, modules whatever)

•Tests help us focus on one small component at a time

•Often end up with more general, re-usable components (with rigorous tests)

Lots of tiny classes!

•Is this a bad thing?

•Best use a DI container

•used right they fade into the background

•Use IDE code navigation tools

•e.g. Go to Definition, Find Usages, Ctrl-T in R#

•Use keyboard shortcuts

Disadvantages

•Any problems with this approach?

Two TDD approaches

•“Unit testing is out, vertical slice testing is in” - Sebastian Lambla (2013)

Two TDD Approaches

“A lot of the bad advice about TDD has come from believing that it means testing a module in isolation from other modules, so that you essentially can only have one class under test and you have to have everything else mocked out.” - Ian Cooper (NDC in Oslo 2013 TDD: where did it all go wrong?)

OrderConfirmationEmailBuilderOrderConfirmer

Test Test

OrderConfirmationEmailBuilder

Fake

Mail TemplateEngine

Fake

What is a unit test?

•All about isolation, but from what?

•Other classes or methods?

•Other systems (e.g. file system, email, database)?

•Other tests?

Isolation approach

•Focused test suites, specify behaviour of small units (e.g. classes)

•Substitute out all dependencies

•Tests inevitably specify implementation details to some extent

OrderConfirmationEmailBuilderOrderConfirmer

Test Test

OrderConfirmationEmailBuilder

Fake

Mail Template Engine

Fake

isolated: the good

•Focused tests allow us to focus on one thing at a time

•When tests fail, the source of problem is usually easy to identify

•Set ups are small, tests always fast

•Encourages well-factored code

•Tests relatively easy to write

isolated: the bad

•Tends to lock in to a specific implementation

•Tests can become a liability, slowing down refactoring

•Can lead to focus on implementation details over business value

vertical slice approach

•Tests talk to public API only

•Tests verify expected results, but not how they are arrived at

• Isolation is between tests, not between “units”.

•Test suite per feature instead of per class

•Still don’t touch external systems (i.e. still fast and stable)

MailSenderOrderConfirmer

Test

MailSenderOrderConfirmation

EmailBuilderOrderConfirmer

Test

Email TemplatingEngine

vertical slices: the good

•More expressive tests

•More able to change implementation without breaking tests

vertical slices: the bad

•Harder to drive out your design

•Easy to “get lost” in the implementation without close guidance of your tests

•Harder to pinpoint bugs when tests fail

•Can be more difficult to write tests

http://www.woodwrightschool.com/ship-in-a-bottle-w-jim/http://www.woodwrightschool.com/ship-in-a-bottle-w-jim/

So which to use?

•The answer of course: it depends...

•Isolation when “discovering” your design

•Replace them as appropriate with vertical slice tests

•Delete brittle tests! Use code coverage tools to ensure you still have coverage

Other tips

•Use “builder” pattern for making test data

•Refactor test code like “real” code. Keep it clean!

•There is no “right” way to do things

Traditional architecture

UI

Services

DB

Hexagonal Architecture

http://matteo.vaccari.name/blog/archives/154

Hexagonal Architecture - Alastair Cockburn

Links

• Source code and link to these slides including this one! https://github.com/christensena/TDDIntro

• TDD, where did it all go wrong? (Ian Cooper, NDC 2013) http://vimeo.com/68375232

• http://codebetter.com/sebastienlambla/2013/07/11/unit-testing-is-out-vertical-slice-testing-is-in/

• http://martinfowler.com/articles/mocksArentStubs.html

• http://alistair.cockburn.us/Hexagonal+architecture

Questions?

Reruns

•Repeating this talk 29 August at Christchurch APN

Alan Christensen @christensena