Session Code: NE.17

Introduction in the TDD Mantra

Dennis van der Stelt

Introduction

• Class-A– Kennisprovider– Microsoft development– Training / Coaching– www.class-a.nl

• Dennis van der Stelt– Trainer / Coach– Weblog at http://bloggingabout.net/blogs/dennis/

Agenda

• A quick introduction into unit testing• Test Driven Development mantra• Refactoring• Benefits of Test Driven Development• Demo• Key rules of Test Driven Development• Mocking / Stubbing• Legacy code

A quick introduction

Unit testing

1. Automated testing

2. Facilitates change

3. Simplifies integration

4. Documentation

unit testing isn’t about just testing code

it‘s about allowing you to

determine when behavior changes

TDD Mantra

• Write the test• Write the code• Refactor• Repeat

REDRED GREENGREEN REFACTORREFACTOR

I’M TEST-DRIVENI’M TEST-DRIVEN

TDD Mantra

Where to begin?• Work from specification (or notes, or napkin)• Create a list of tests• Pick the simplest one• Make test, make code• Refactor

• Find a bug? Create the test that would’vecaught it!

Test Driven Development

...is not about testing.

• It’s about design• It’s about unambigious requirements

• It’s also about simplicity and YAGNI

Simplicity is more complicated then you think. But it’s well worth it.Ron Jeffries, Extreme Programming Installed, 2001

Test Driven Development

...is about simple design that

1. Runs all tests

2. Reveals all intention

3. Has no duplication

4. Has the fewest number of classes and methods

Test Driven Development“The act of writing a unit test is more an act of design then of

verification”

- Robert C. Martin

“Test-driven development seeks specification, not validation, letting you think through your design before you write your functional code”

- Scott Ambler

“Test-Driven Development is a powerful way to produce well designed code with fewer defects”

- Martin Fowler

“Fewer defects, less debugging, more confidence, better design, and higher productivity in my programming practice”

- Kent Beck

Benefits of TDD (1/3)

Benefits on your design & code• Exposes design flaws & invalid assumptions• Simpler class hierarchies• Less code, smaller functions• Less conditional code• Faster development time

• Tests are both design & documentation

Benefits of TDD (2/3)

Benefits on your tests & bugs• Tests cover *everything*• Bugs found earlier & faster• Bugs reproduced more easily• Less (difficult) debugging

Benefits of TDD (3/3)

Cost of change

WaterfallTest-Driven Development embraces change!

Cons of TDD

• Needs practice, but you’ll learn to love it!• Needs discipline• Need to know OODP• Scares away managers

Bowling gameFirst a quick design session…

Bowling game

• 10 frames, each holds 2 throws• Spare: 10 pins 2nd try, next throw added extra• Strike: 10 pins in 1st try, score next frame added• 10th frame spare or strike,

player can make extra balls to complete frame.

404

4210

3417

630

2335 55

965

0065

0974

2783

4 + (2+4) 17 + (6+4+3) 35 + (10+9+1)55 + (9 + 1 + 0)

Quick design sessions

+roll(pins : int)+score() : int

Game Frame

We clearly need the Game class.

A game has ten frames.

-pins : int

Roll

A frame has 1 or 2 roles.

tenth frame

The tenth frame has two or three roles.

It is different from all the other frames.

+score() : int

Frame

The score function must iteratethrough all the frames, andcalculate all their scores.

next frame

The score for spare or strikedepends on the frame’s

succesor

10 1..2

1

Bowling gameFinally some code!

Key rules of TDD

A test is no unit test if…• …it talks to a database• …it communicates across the network• …it touches the file system• …it can’t run at the same time as other tests• …you have to perform actions to run the tests

(ex. configuration change)

How to isolate your code

Unit testProduction

code

Logging code

Data access code

Web service code

How to isolate your code

Unit testProduction

code

Logging code

Data access code

Web service code

!! !database logging

serviceremoteserver

How to isolate your code

Unit testProduction

code

Logging code

Data access code

Web service code

FakeData

access

FakeLogging

FakeWeb

service

Mocks & Stubs

A mock or stub is a stunt double for…• …an object outside your unit• …an external object (database)• …a complex object(s)• …a not yet developed object

Mocking & Stubbing are different things!

Isolation framework[TestMethod]public void Does_AllowDependencyInjection_When_CreatingInstance(){  // Arrange  var fake = Isolate.Fake.Instance<ICustomerRepository>();  Customer cust = new Customer();   // Act  BusinessAction ba = new BusinessAction(fake);  ba.PerformSomeSortOfAction(cust);   // Assert  Isolate.Verify. WasCalledWithExactArguments(() => fake.SaveChanges(cust));}

Legacy code

Why is legacy code bad?• The previous developer isn’t there to

explain it.• It takes time to learn the code.• It’s not easy to change the code.• You might break some part of the

application you didn’t even touch.

Wikipedia : Legacy code is source code that relates to a no-longer supported or manufactured operating system or other computer system.

What is legacy code?

Legacy codeNow imagine...

• You’re developer on a project,• You get sick for two weeks,• Your code needs to be completed,• Another developer comes in.

- The previous developer isn’t there to explain it.- It takes time to learn the code.- It’s not easy to change the code.- You might break some part of the application you didn’t even touch.

What if you have unit tests?

You have tests to explain the codeYou have tests to learn the codeYou have tests to verify what you did works,and did not break any other part of the application.

Without unit tests,you’re writing legacy code as we speak!

© Michael Feathers – Working Effectively with Legacy Code

WRITE UNIT TESTS!Conclusion:

and do it the right way using Test-Driven Development.

Thank you for your attention

• Dennis van der Stelt– Dennis@BloggingAbout.NET– http://twitter.com/dvdstelt/– http://bloggingabout.net/blogs/dennis/

Resources to more information can be found here, incl. this slidedeck.

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Session Code: NE.17