Dependency Injection for Wordpress

Dependency Injection for WordPress Plugin Development Mike Toppa WordCamp Nashville April 21, 2012 #wcn12www.toppa.com @mtoppa

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, Kais Candy Co● WordPress development for non-profitswww.toppa.com @mtoppa

Code is Poetrywww.toppa.com @mtoppa

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

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

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

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

“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 JSLintwww.toppa.com @mtoppa

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 Codewww.toppa.com @mtoppa

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

Clean code techniques● Using meaningful names● Dont repeat yourself (DRY)● Object oriented principles and patterns● Unit testing, test driven development (TDD)● The “boy scout rule”● Vertical slicing● ...and many morewww.toppa.com @mtoppa

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

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

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

From LosTechies.com

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

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

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-0www.toppa.com @mtoppa

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.mdwww.toppa.com @mtoppa

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

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

Shashins capabilities are shaped by how its objects are wired together, through implementation of the Dependency Inversion Principlewww.toppa.com @mtoppa

From LosTechies.com

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”

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

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(); } }}

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."

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

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

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

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

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

It comes in two flavorswww.toppa.com @mtoppa

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

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

Which to use? It dependswww.toppa.com @mtoppa

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

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

Example from Shashinclass 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

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

Beyond the textbook exampleswww.toppa.com @mtoppa

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

Shashin Exampleclass 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

What if you need a new object inside a loop, but cant know the subtype youll need ahead of time? Let the injection container figure it outwww.toppa.com @mtoppa

Shashin Exampleclass 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

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 subtypeswww.toppa.com @mtoppa

Will this proliferation of objects eat up all my memory? Nowww.toppa.com @mtoppa

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

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

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

But dont overdo it:avoid needless complexity

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Dependency Injection for Wordpress

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