Test Driven Development (TDD) is a software development practice that involves writing a failing test first, then code to pass that test, and refactoring the code. Behavior Driven Development (BDD) builds on TDD and describes how the end user will interact with the software using scenarios and steps. BDD tools like Cucumber use regular expressions to match steps in scenarios to code.

1. Test Driven Design
Behavior Driven Design
Jason Noble
instructor@jasonnoble.org

2. Test Driven Design
• TDD is a software development practice that
relies on the repetition of a very short
development cycle
– Write a failing test that defines a desired
improvement or new function
– Write the minimum code possible to make the
test pass
– Refactor the code to acceptable standards

3. Red Green Refactor
• Write a failing test (Red)
• Make the test pass (Green)
• Refactor tests or code (NOT both!)

4. Why use TDD?
• Helps you break problems down into small
manageable tasks
• Writing tests takes the fear out of
programming
• Writing tests helps you communicate what
your code SHOULD do/accomplish

5. TDD Example
• What can a calculator
do?
– Add – Square Root
– Subtract – On / Off
– Memory Clear
– Multiply
– Memory Set
– Divide
– Memory Recall
– Equals
– Negative numbers
– Percent – Decimals

6. Write a failing test (Red)
• Write a failing test
# spec/calculator_spec.rb
require ’./calculator’
describe Calculator do
end
• Failure:
spec/calculator_spec.rb:1:in `require': cannot load such file –
./calculator (LoadError)

7. Make the test pass (Green)
• Write code to make the test pass
touch calculator.rb
• Test Failure
spec/calculator_spec.rb:3:in `<top (required)>': uninitialized
constant Calculator (NameError)

8. Make the test pass (Green)
• Write code to make the test pass
# calculator.rb
class Calculator
end
• Test Success
No examples found.
Finished in 0.00005 seconds
0 examples, 0 failures

9. What did we do?
• Write a very simple test
• Got a LoadError
• Fixed the LoadError
• Got a NameError
• Fixed the NameError
• Tests are green!
• Refactor?

10. Add a test
# spec/calculator_spec.rb
let(:calculator) { Calculator.new }
it "can be created" do
calculator.should be_a(Calculator)
end
Tests are still green!
Running: spec/calculator_spec.rb
Calculator
can be created
Finished in 0.00127 seconds
1 example, 0 failures

11. What did we do?
• Add a very simple test
• Got no errors
• Tests are green!
• Refactor?

12. Add a test
# spec/calculator_spec.rb
describe "#total" do
it "returns 0 by default" do
calculator.total.should == 0
end
end
Tests are red!

13. Make the tests pass
• Add the minimum code to make the test fail in
a different way (or succeed)
# calculator.rb
def total
end

14. Make the tests pass
• Write the MINIMUM code necessary to make
the test pass
# calculator.rb
def total
0
end
• Tests are GREEN!
• Refactor?

15. What did we do?
• Write a very simple test (total.should == 0)
• Got a NoMethod error
• Fixed the NoMethodError
• Got an unexpected result returned
• Fixed the result to always return 0
• Tests are green!
• Refactor?

16. Behavior Driven Development
• Uses the basis of TDD, domain driven design
and object oriented design to provide
software developers and business analysts
with shared tools in order to collaborate on
software development
• TDD describes how the software works
• BDD describes how the end user uses the
software

17. BDD Example
Scenario: User adds two positive numbers
Given I turn the calculator on,
Then I should see zero on the screen
When I add 5
And I add 7
Then I should see 12 on the screen

18. Ruby Cucumber
• Uses regular expressions to match steps
Given /I turn the calculator on/ do
@calculator = Calculator.new
@calculator.on
end
Then /I should see (d+) on the screen/ do |expected_value|
@calculator.total.should == expected_value
end
When /I add (d+) / do |number|
@calculator.add(number)
end

19. Further Readings
• Test Driven Development: By Example by Kent
Beck
• Everyday Rails Testing with Rspec by Aaron
Sumner
• The Cucumber Book by Matt Wynne and Aslak
Hellesoy