Dependency Injection, Service Locators, Testing and Life

@dbhurley OpenWest 2014
DI, SL, Testing, Life
A Pragmatic Approach

WELCOME!
This session is not intimidating. This session is
not intimidating. This session is not intimidating.
not inti

WHO?
David Hurley
Open Source Evangelist
Entrepreneur / Co-Founder
Joomla! Community Manager
Production Leadership Team

TERMS
Dependency Injection
An injection is the passing of a dependency (a service)
to a dependent object (a client).
Service Locator
A design pattern which uses a central registry and on
request returns the information necessary to a client.
Inversion of Control
A design in which portions of a computer program
receive flow of control from a generic, reusable library

3 TYPES OF DEPENDENCY INJECTION
Constructor Setter Interface
class carPicker
{
function carPicker ( Finder $finder )
{
$this->finder = $finder;

SERVICE LOCATORS
Most often seen used as a singleton registry*
class serviceLocator {
public static function carFinder ()
{
return static::finder;
}
class carPicker {
function carPicker() {
$finder = serviceLocator::carFinder();

WHAT WE’RE NOT DISCUSSING

WHAT WE ARE DISCUSSING

THE LAW OF DEMETER
Each unit should have only limited knowledge about
other units: only units "closely" related to the current unit.
Each unit should only talk to its friends; don't talk to
strangers.
Only talk to your immediate friends.

WHAT’S THAT MEAN?
Question
How do you pay your tab at a restaurant?

APPLY SL
class Car {
protected $seats
protected $seatbelt
function buildCar (Locator $locator) {
$this->seats = $locator->getSeats();
$this->seatbelt = $locator->getSeatbelt();
...
}
}

APPLY DI
class carFactory {
function buildCar() {
$seatbelt = new SeatBelt();
$seats = new Seats($seatbelt);
return new Car($seats);
}
}

LOOKING DEEPER
class Car {
function _constructor(Seats $seats) {
...
}
}
class Seats {
function _constructor(Seatbelt $seatbelt, Fabric
$fabric) {
…
}

RETURN TO FACTORY
class carFactory {
function buildCar() {
$seatbelt = new SeatBelt();
$fabric = new Fabric();
$seats = new Seats($seatbelt, $fabric);
return new Car($seats);
}
}

KEY CONCEPT?
A Factory is used for all the objects of
the same lifetime.
Constructor Injection: Only inject items whose lifetime is
equal or greater than the injectee
(i.e. The seat is alive at least as long as the car)
Method Parameter: At execution for shorter lifetimes.

TESTING
Service Locators are often difficult to test.
Mixed responsibilities / Law of Demeter partially broken
Dependency Injection allows for easy testing.
Clear to follow / Law of Demeter obeyed
Side note: Null checking
Pre-condition checking is hard to test. Rather check that
things work as expected rather than checking against
nulls.

THE GOAL
The goal is to write clean, concise, easily-
testable code which accomplishes a
purpose and allows others to improve it.

THE OUTCOME
A strong codebase which can be easily read
and understood by others.
“Always code as if the guy who ends
up maintaining your code will be a violent
psychopath who knows where you live.”

QUESTIONS?
David Hurley
@dbhurley (twitter, facebook, linkedin)
me@dbhurley.com

Dependency Injection, Service Locators, Testing and Life

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