TDD, BDD and mocks

Test-
Behaviour- } Driven Development
Kerry Buckley

Clearing up some
Misconceptions

TDD does not mean
handing acceptance
tests to developers

Software design is
emergent, and happens
during development

Why TDD?

• Makes you think about required behaviour
• Reduces speculative code
• Provides documentation
• Improves quality

Automated Testing
class Adder
def add a, b
a + b
end
end

class AdderTest < Test::Unit::TestCase
def test_add
adder = Adder.new
assert_equal 4, adder.add(2, 2)
assert_equal 2, adder.add(4, -2)
end
end

Are You Really Testing
Your Code?
class Adder
def add a, b
a + b
end
end

def test_add
assert_equal 4, 2 + 2
end
end

Test-First Development

Write Write
tests code

Start Failing Done
tests

Test-Driven
Development
Clean Failing
code test

Refactor

All tests pass

State-Based
class DongleTest < Test::Unit::TestCase
def test_wibble
# Set up test inputs
dongle = Dongle.new
dongle.addString(quot;fooquot;)
dongle.addRemoteResource(quot;http://foo.com/barquot;)

# Exercise functionality under test
dongle.wibble!

# Verify results are as expected
assert_equal(42, dongle.answer)
end
end

Behaviour-Driven
Development
Veriﬁcation Speciﬁcation

State-based Interaction-based

Bottom-up Outside-in

Testing tool Design tool

Invention Discovery

More Descriptive Test
Names
def test_should_add_two_positive_numbers
assert_equal 4, Adder.new.add(2, 2)
end

def test_should_add_a_positive_and_a_negative_number
assert_equal 2, Adder.new.add(4, -2)
end
end

RSpec

describe quot;An adderquot; do
it quot;should add two positive numbersquot; do
Adder.new.add(2, 2).should == 4
end

it quot;should add a positive and a negative numberquot; do
Adder.new.add(4, -2).should == 2
end
end

Generated
Documentation
$ spec -f s adder_spec.rb

An adder
- should add two positive numbers
- should add a positive and a negative number

Finished in 0.005493 seconds

2 examples, 0 failures

Matchers (RSpec)

@string.should == quot;fooquot;

@array.should_not be_empty

@hash.should have_key(:foo)

@object.should be_an_instance_of String

lambda { @stack.pop }.should raise_error(StackUnderflowError)

Matchers (HamCrest)

assertThat(string, equalTo(quot;fooquot;));

assertThat(array, hasItem(quot;barquot;));

assertThat(obj, instanceOf(String.class));

assertThat(number, greaterThan(42));

Describing Features

Feature: Transferring money between two accounts

Scenario: Simple transfer
Given an account called 'source' containing £100
And an account called 'destination' containing £50
When I transfer £20 from source to destination
Then the 'source' account should contain £80
And the 'destination' account should contain £70

Describing Features
Given /^an account called '(w*)' containing £(d*)$/ do |name, amount|
@@accounts ||= {}
@@accounts[name] = Account.new(amount.to_i)
end

When /^I transfer £(d*) from (w*) to (w*)$/ do |amount, from, to|
AccountController.new.transfer @@accounts[from], @@accounts[to], amount.to_i
end

Then /^the '(w*)' account should contain £(d*)$/ do |name, amount|
@@accounts[name].balance.should == amount.to_i
end

Mock Objects

• Stand-ins for collaborating objects
• Mock the interface, not a speciﬁc object
• Verify that expected calls are made
• Not stubs!
• For your code only!

Mocking Patterns

• Record and playback
• Specify expectations before running
• Check expectations after running

Mocking Patterns
class AccountController {
public void transfer(Account from, Account to, int amount) {
from.debit(amount);
to.credit(amount);
}
}

class AccountController
def transfer from, to, amount
from.debit amount
to.credit amount
end
end

EasyMock
public void testTransferShouldDebitSourceAccount() {
AccountController controller = new AccountController();

Account from = createMock(Account.class);
Account to = createNiceMock(Account.class);
from.debit(42);

replay(from);
replay(to);

controller.transfer(from, to, 42);

verify(from);
}

JMock
Mockery context = new Mockery();


Account from = context.mock(Account.class);
Account to = context.mock(Account.class);

context.checking(new Expectations() {{
oneOf(from).debit(42);
}});


context.assertIsSatisfied();
}

Mockito

Account from = mock(Account.class);
Account to = mock(Account.class);


verify(from).debit(42);
}

RSpec
describe 'Making a transfer' do
it 'should debit the source account' do
controller = AccountController.new

from = stub_everything 'from'
to = stub_everything 'to'

from.should_receive(:debit).with 42

controller.transfer from, to, 42
end
end

Not-a-Mock
describe 'Making a transfer' do
it 'should debit the source account' do
controller = AccountController.new
from = Account.new
to = Account.new

from.stub_method :debit => nil
to.stub_method :credit => nil

controller.transfer from, to, 42

from.should_have_received(:debit).with(42)
end
end

Other Mock Features
• Stubs (for when you don’t care)
• Mock class/static methods
• Specify return values
• Specify expected number of calls
• Specify method ordering
• Raise exceptions on method calls

Good Practices
• Test behaviour, not implementation
• One expectation per test
• Don’t test private methods
• Don’t mock everything
• Stub queries; mock actions
• Tell, don’t ask
• Listen to test smells!

TDD/BDD Summary
• Never write any code without a failing test
• Start from the outside, with acceptance tests
• Drive design inwards using mock objects
• Tests should be descriptive speciﬁcations
• Red – Green – Refactor
• YAGNI
• TATFT!

Further Reading
Introducing BDD (Dan North)
http://dannorth.net/introducing-bdd

BDD Introduction
http://behaviour-driven.org/Introduction

Mock Roles, Not Objects
(Freeman, Mackinnon, Pryce, Walnes)
http://www.jmock.org/oopsla2004.pdf

BDD in Ruby (Dave Astels)
http://blog.daveastels.com/ﬁles/BDD_Intro.pdf

Test all the F***in Time (Brian Liles, video)
http://www.icanhaz.com/tatft

TDD, BDD and mocks

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