"Cat Burglar" by Amber Grundy

http://www.flickr.com/photos/abear23/1444321123/

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Test Driven Development

• What is TDD?
• Why should I care?
• How does it work?

Requirement

As a customer
in order to gather the products I want to purchase
I want to add items to my shopping cart

Example

Scenario: add an item to the shopping cart
  Given an article with the name "Nintendo Wii"
  When I am on the homepage
  And I follow "Nintendo Wii"
  And I press "Add to cart"
  Then I should see "Item was added to cart"

Scenario: add an item to the shopping cart
  Given an article with the name "Nintendo Wii"
  When I am on the homepage
  And I follow "Nintendo Wii"
  And I press "Add to cart"
  Then I should see "Item was added to cart"
 
 
 

Scenario: add an item to the shopping cart
  Given an article with the name "Nintendo Wii"
  When I am on the homepage
  And I follow "Nintendo Wii"
  And I press "Add to cart"
  Then I should see "Item was added to cart"

Unknown class "Article"
1 scenario (1 failed)

class Article
  attr_accessor :name
end

Scenario: add an item to the shopping cart
  Given an article with the name "Nintendo Wii"
  When I am on the homepage
  And I follow "Nintendo Wii"
  And I press "Add to cart"
  Then I should see "Item was added to cart"

Error retrieving page (404 not found)
1 scenario (1 failed)

Scenario: add an item to the shopping cart
  Given an article with the name "Nintendo Wii"
  When I am on the homepage
  And I follow "Nintendo Wii"
  And I press "Add to cart"
  Then I should see "Item was added to cart"

No such link "Nintendo Wii"
1 scenario (1 failed)

Scenario: add an item to the shopping cart
  Given an article with the name "Nintendo Wii"
  When I am on the homepage
  And I follow "Nintendo Wii"
  And I press "Add to cart"
  Then I should see "Item was added to cart"

No such button "Add to cart"
1 scenario (1 failed)

Scenario: add an item to the shopping cart
  Given an article with the name "Nintendo Wii"
  When I am on the homepage
  And I follow "Nintendo Wii"
  And I press "Add to cart"
  Then I should see "Item was added to cart"

Expected to see "Item was added to cart"
1 scenario (1 failed)

Scenario: add an item to the shopping cart
  Given an article with the name "Nintendo Wii"
  When I am on the homepage
  And I follow "Nintendo Wii"
  And I press "Add to cart"
  Then I should see "Item was added to cart"

1 scenario (1 passed)
 

What just happened?

• I'm confident my code fulfills the requirement.
• I pretty much switched off my brain during implementation.
• I wrote the minimum amount of code required to make the example pass.

View the rest of this chapter here:
why-expanded.with-fonts.pdf

Hack #1

Choose the right kind of test

• Unit test
• Integration test
• System test ("full-stack integration test")
• System integration test

Full-stack integration test

Scenario: add an item to the shopping cart
  Given an article with the name "Nintendo Wii"
  When I am on the homepage
  And I follow "Nintendo Wii"
  And I press "Add to cart"
  Then I should see "Item was added to cart"

Unit test

describe Hero do
  describe '#attack' do
    it "should damage the target for the hero's strength" do
      monster = Monster.create(:hitpoints => 100)
      hero = Hero.create(:strength => 70)
      hero.attack(monster)
      monster.hitpoints.should == 30
    end
  end
end

Headless browsers

• HtmlUnit
• Test::Unit
• capybara-webkit

Tools to script real browsers

• Selenium
• Watir

visit '/login'
fill_in 'E-mail', :with => 'user@example.com'
fill_in 'Password', :with => 'secret'
click_link 'Sign in'
page.should have_content('Signed in successfully')

Hack #3

Use a separate database for running tests

• Great when your model is coupled to your database through O/R mapping magic
• Code using the database can be tested without changes
• Also tests that your O/R mapping is configured as expected
• Removes the need for most test implementations in typical applications

Cloning a database schema

mysqldump -uroot -psecret -d hero_development |↲
mysql -uroot -psecret hero_test

• Tests usually require some state to work on, e.g. a shopping cart with some items to test shipping cost calculations.
• We call these example states "scenarios".

• You will eventually have many test scenarios.
• Using one giant scenario for everything is dependency hell.
• In order to isolate tests from each other, each test should create its own scenario from scratch.
• In practice this means cleaning out the database after each test.

Transactions can help you clean up

1. 1. Clone the development database
2. 2. Start a transaction
3. 3. Run test #1
4. 4. Rollback the transaction
5. 5. Start a transaction
6. 6. Run test #2
7. 7. Rollback the transaction
8. ...

"The RSS feed should include the latest forum comment"

This is horrible:

describe Feed, '.to_rss' do
  it "should include the latest forum comment" do
    forum = Forum.create(:name => "Example Forum")
    user = User.create(:email => "foo@bar.com",
                       :name => "Hans Dampf")
    topic = Topic.create(:forum => forum,
                         :subject => "Example Topic",
                         :author => user)
    Comment.create(:topic => topic,
                   :author => user,
                   :text => "Hi world!")
    Feed.to_rss.should include("Hi world!")
  end
end

User.blueprint do
  name "John Doe"
  email "john@doe.com"
  password 'secret'
end

Forum.blueprint do
  name "Example Forum"
end

Topic.blueprint do
  forum
  subject "Example Topic"
end

Comment.blueprint do
  topic
  author
  text "Lorem ipsum dolor sit amet"
end

This is expressive:

describe Feed, '.to_rss' do
  it "should include the latest forum comment" do
    Comment.make(:text => "Hi world!")
    Feed.to_rss.should include("Hi world!")
  end
end

PayPal.capture_payment("RE-23400123")

GoogleMaps.fetch_geocode("Augsburg")

NuclearMissile.new.launch

system("rm -rf /")

class Order

  def initialize(number)
    @number = number
    @paid = false
  end

  def finish
    if PayPal.capture_payment(@number)
      @paid = true
    end
  end

  def paid?
    @paid
  end

end

describe Order, '#finish' do
  it "should mark the order as paid" do
    order = Order.new("ON-5512")
    order.finish
    order.should be_paid
  end
end

Order finish should mark the order as paid FAILED   
 
 
 

describe Order, '#finish' do
  it "should mark the order as paid" do
    order = Order.new("ON-5512")
    order.finish
    order.should be_paid
  end
end

Order finish should mark the order as paid FAILED   
PayPalError: Unknown transaction reference "ON-5512"

1 example, 1 failure

describe Order, '#finish' do
  it "should mark the order as paid" do
    order = Order.new("ON-5512")
    order.finish
    order.should be_paid
  end
end

Order finish should mark the order as paid FAILED   
HTTPError: Could not resolve host api.paypal.com

1 example, 1 failure

describe Order, '#finish' do
  it "should mark the order as paid" do
    PayPal.stub(:capture_payment).and_return(true)
    order = Order.new("ON-5512")
    order.finish
    order.should be_paid
  end
end

1 example, 0 failures
Order finish should mark the order as paid FAILED   
 

• While we want to mute incovenient side effects, we might still want to test that a side effect would have been triggered.
• Like stubbing, mocking replaces method implementations by scripted behavior.
• In addition, you can use mocking to observe interaction between objects.

describe Order, 'finish' do
  it "should capture the payment through PayPal" do
    # ???
  end
end

describe Order, 'finish' do
  it "should capture the payment through PayPal" do
    PayPal
      .should_receive(:capture_payment)↲
      .with("ON-5512")↲
      .and_return(true)
    order = Order.new("ON-5512")
    order.finish
  end
end

• Use mocking to observe interaction outside the scope of your example.
• Mocking lets you reuse test coverage from other examples without repeating yourself.
• No need to rewrite tests for libraries that you know are tested.

Getting started

• Find a popular testing framework for your technology
• Full-stack integration tests give a quick fix
• Test the happy path

end

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