1.
Dependency Injection for
WordPress Plugin Development
Mike Toppa
WordCamp Nashville
April 21, 2012
#wcn12
www.toppa.com @mtoppa

2.
Mike Toppa
● Director of Development, WebDevStudios
● 17 years of experience in web development,
project management, and team management
● Universities: Georgetown, Stanford, Penn
● Dot coms: E*Trade, Ask Jeeves
● Start-ups: Finexa, Kai's Candy Co
● WordPress development for non-profits
www.toppa.com @mtoppa

3.
Code is Poetry
www.toppa.com @mtoppa

4.
We want code to be...
● Easy to understand
● Easy to change
● Reusable (DRY)
● Bug free
www.toppa.com @mtoppa

5.
So why is it usually like this?
“O.P.C” http://abstrusegoose.com/432

6.
It happens because we are always in a hurry,
and requirements are always changing.
www.toppa.com @mtoppa

7.
But coding fast and dirty
only makes us slower in the long run.
www.toppa.com @mtoppa

8.
“We like to think we spend our time power typing,
but we actually spend most of our time
staring into the abyss.”
- Douglas Crockford
principal discoverer of JSON,
Creator of JSLint
www.toppa.com @mtoppa

9.
The ratio of time spent reading code versus
writing is well over 10 to 1.
Therefore, making code easy to read
makes it easier to write.
- Bob Martin
Paraphrased from his book Clean Code
www.toppa.com @mtoppa

10.
Clean code saves time, saves $$
www.toppa.com @mtoppa

11.
Clean code techniques
● Using meaningful names
● Don't repeat yourself (DRY)
● Object oriented principles and patterns
● Unit testing, test driven development (TDD)
● The “boy scout rule”
● Vertical slicing
● ...and many more
www.toppa.com @mtoppa

12.
Clean code techniques
● Using meaningful names
● Don't repeat yourself (DRY)
● Object oriented principles and patterns
● Unit testing, test driven development (TDD)
● The “boy scout rule”
● Vertical slicing
● ...and many more
www.toppa.com @mtoppa

13.
The SOLID Principles
● Single Responsibility (SRP)
● Open-Closed (OCP)
● Liskov Substitution (LSP)
● Interface Segregation (ISP)
● Dependency Inversion (DIP)
www.toppa.com @mtoppa

14.
The SOLID Principles
● Single Responsibility (SRP)
● Open-Closed (OCP)
● Liskov Substitution (LSP)
● Interface Segregation (ISP)
● Dependency Inversion (DIP)
www.toppa.com @mtoppa

15.
Shashin
My plugin for displaying albums, photos, and
videos from Picasa, Twitpic, and YouTube
(and others coming soon)
www.toppa.com @mtoppa

16.
I used the new version as a test case for applying
clean code principles to WordPress plugins
www.toppa.com @mtoppa

17.
From LosTechies.com

18.
My Shashin plugin consists of 44 classes, each of
which has a meaningful name, follows the SRP,
and “does one thing”
PicasaPhotoDisplayer
SettingsMenu
YouTubeSynchronizer
Uninstall
etc.
www.toppa.com @mtoppa

19.
Shashin example
class Admin_ShashinInstall {
// ...
public function run() {
$this->createAlbumTable();
$this->verifyAlbumTable();
$this->createPhotoTable();
$this->verifyPhotoTable();
$this->updateSettings();
return true;
}
// ...
}
www.toppa.com @mtoppa

20.
Class Autoloading
● The problem:
● PHP lacks an equivalent statement to “import” or
“use” for classes
● Having to hardcode a bunch of file paths is extra
work, and makes code harder to change
● The solution:
● PHP 5.1.2 and spl_autoload_register()
● PHP Standards Working Group: PSR-0
www.toppa.com @mtoppa

21.
PSR-0: Official Description
● Each "_" character in the CLASS NAME is
converted to a DIRECTORY_SEPARATOR
● The fully-qualified namespace and class is suffixed
with ".php" when loading from the file system
● Alphabetic characters... may be of any
combination of lower case and upper case
● ...And other rules about namespaces
● Source: https://github.com/php-fig/fig-
standards/blob/master/accepted/PSR-0.md
www.toppa.com @mtoppa

22.
PSR-0 Example
Admin_ShashinInstall
is translated to
Admin/ShashinInstall.php
www.toppa.com @mtoppa

23.
Toppa Plugin Libraries Autoloader
// this is at the top of the main file for my Shashin plugin
require_once dirname(__FILE__) . '/../toppa-plugin-libraries-for-
wordpress/ToppaAutoLoaderWp.php';
$shashinAutoLoader = new ToppaAutoLoaderWp('/shashin');
// that's it! I can now call “new” on any class under the shashin folder
// and its class file will be automatically loaded
www.toppa.com @mtoppa

24.
Shashin's capabilities are shaped by how its
objects are wired together, through
implementation of the
Dependency Inversion Principle
www.toppa.com @mtoppa

25.
From LosTechies.com

26.
Naïve model of a button and lamp
Lamp
Button
+ turnOn()
+ poll()
+ turnOff()
class Button {
private $lamp;
public function __construct(Lamp $lamp) {
$this->lamp = $lamp;
}
public function poll() {
if (/* some condition */) {
$this->lamp->turnOn();
}
}
}
Example from “Agile Software Development”

27.
Dependency inversion applied
<<interface>>
Button SwitchableDevice
+ poll() + turnOn()
+ turnOff()
Lamp
This is the Abstract Server pattern

28.
class Lamp implements SwitchableDevice {
public function turnOn() {
// code
}
public function turnOff() {
// code
}
}
class Button {
private $switchableDevice;
public function __construct(SwitchableDevice $switchableDevice) {
$this->switchableDevice = $switchableDevice;
}
public function poll() {
if (/* some condition */) {
$this->switchableDevice->turnOn();
}
}
}

29.
A web of collaborating objects
● The SRP and DIP together drive a
“composition” approach to OO design
● From Growing Object Oriented Software,
Guided by Tests:
"An object oriented system is a web of
collaborating objects... The behavior of the
system is an emergent property of the
composition of the objects - the choice of
objects and how they are connected... Thinking
of a system in terms of its dynamic
communication structure is a significant mental
shift from the static classification that most of us
learn when being introduced to objects."

30.
Composition
(“Has a...”)
vs.
Inheritance
(“Is a...”)
www.toppa.com @mtoppa

31.
The SRP is about objects that do one thing
The DIP is about how to wire them together
to create working, flexible software
www.toppa.com @mtoppa

32.
Never, ever do this
class Button {
private $lamp;
public function __construct() {
$this->lamp = new Lamp();
}
//...
}
www.toppa.com @mtoppa

33.
Instead, you want to
inject the object dependency
Dependency injection!
www.toppa.com @mtoppa

34.
Dependency injection is one of many
design patterns
for implementing the
Dependency inversion principle
www.toppa.com @mtoppa

35.
It comes in two flavors
www.toppa.com @mtoppa

36.
Constructor injection
public function __construct(SwitchableDevice $switchableDevice) {
$this->switchableDevice = $switchableDevice;
}
www.toppa.com @mtoppa

37.
Setter injection
public function setSwitchableDevice(SwitchableDevice $switchableDevice) {
$this->switchableDevice = $switchableDevice;
}
www.toppa.com @mtoppa

38.
Which to use?
It depends
www.toppa.com @mtoppa

39.
Dependency chains
If class A depends on class B,
and class B depends on class C,
class A should be blissfully unaware of class C
www.toppa.com @mtoppa

40.
To do this without going insane,
you need an injection container
www.toppa.com @mtoppa

41.
Example from Shashin
class Lib_ShashinContainer {
// …
public function getPhotoRefData() {
if (!isset($this->photoRefData)) {
$this->photoRefData = new Lib_ShashinPhotoRefData();
}
return $this->photoRefData;
}
public function getClonablePhoto() {
if (!isset($this->clonablePhoto)) {
$this->getPhotoRefData();
$this->clonablePhoto = new Lib_ShashinPhoto($this->photoRefData);
}
return $this->clonablePhoto;
}
www.toppa.com @mtoppa

42.
Example continued
public function getClonablePhotoCollection() {
if (!isset($this->photoCollection)) {
$this->getClonablePhoto();
$this->getSettings();
$this->clonablePhotoCollection = new Lib_ShashinPhotoCollection();
$this->clonablePhotoCollection->setClonableDataObject($this->clonablePhoto);
$this->clonablePhotoCollection->setSettings($this->settings);
}
return $this->clonablePhotoCollection;
}
www.toppa.com @mtoppa

43.
Beyond the textbook examples
www.toppa.com @mtoppa

44.
What to do when you need
a new object inside a loop
One solution is cloning
www.toppa.com @mtoppa

45.
Shashin Example
class Admin_ShashinSynchronizerPicasa extends Admin_ShashinSynchronizer {
// …
public function syncAlbumPhotos(array $decodedAlbumData) {
// …
foreach ($decodedAlbumData['feed']['entry'] as $entry) {
$photoData = $this->extractFieldsFromDecodedData($entry, $photoRefData,
'picasa');
// ...
$photo = clone $this->clonablePhoto;
$photo->set($photoData);
$photo->flush();
}
www.toppa.com @mtoppa

46.
What if you need a new object inside a loop,
but can't know the subtype you'll need
ahead of time?
Let the injection container figure it out
www.toppa.com @mtoppa

47.
Shashin Example
class Public_ShashinLayoutManager {
// ...
public function setTableBody() {
// …
for ($i = 0; $i < count($this->collection); $i++) {
// ...
$dataObjectDisplayer = $this->container->getDataObjectDisplayer(
$this->shortcode,
$this->collection[$i]
);
$this->tableBody .= $dataObjectDisplayer->run();
// ...
}
www.toppa.com @mtoppa

48.
Dependency injection: key benefits
● Makes your object dependencies adaptable to
emerging needs
● With an injection container, simplifies managing
dependency chains
● Supports “preferring polymorphism to
conditionals”, making it easy to add new class
subtypes
www.toppa.com @mtoppa

49.
Will this proliferation of objects
eat up all my memory?
No
www.toppa.com @mtoppa

50.
Use dependency injection to instantiate
only the objects you need,
when you need them
www.toppa.com @mtoppa

51.
Make immutable objects into
properties of the container,
so you only need to instantiate them once
www.toppa.com @mtoppa

52.
PHP 5 has improved memory
management
“In PHP 5, the infrastructure of the object model was
rewritten to work with object handles. Unless you
explicitly clone an object by using the clone keyword you
will never create behind the scene duplicates of your
objects. In PHP 5, there is neither a need to pass objects
by reference nor assigning them by reference.”
From http://devzone.zend.com/article/1714
www.toppa.com @mtoppa

53.
But don't overdo it:
avoid needless complexity