Object oriented programming for WordPress
plugin development Mike Toppa WordCamp NYC June 9, 2012 www.toppa.com @mtoppa

www.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, Kai's Candy Co ● WordPress development for non-profits www.toppa.com @mtoppa

Overview ● Nuts and bolts
of classes and objects in PHP ● The single responsibility principle ● The dependency inversion principle ● When to prefer OO to procedural programming www.toppa.com @mtoppa

www.toppa.com @mtoppa

Simple example class Lamp {
// property declaration private $maxSafeWatts = 100; // method declaration public function getMaxSafeWatts() { return $this->maxSafeWatts; } } --------------------------------------------------------------- // instantiate and assign $myLamp = new Lamp(); echo $myLamp->getMaxSafeWatts();

Rule of thumb: avoid public
properties class Lamp { public $maxSafeWatts = 100; } --------------------------------------------- $myLamp = new Lamp(); // dangerous to allow this! $myLamp->maxSafeWatts = 'a string to wreck your math';

Prefer private properties class Lamp
{ private $maxSafeWatts = 100; public setMaxSafeWatts($watts) { if (!is_numeric($watts) || $watts > 125 || $watts < 1) { throw New Exception('invalid value for watts'); } $this->maxSafeWatts = $watts; return $this->maxSafeWatts; } } ------------------------------------------------------------------- $myLamp = new Lamp(); $myLamp->setMaxSafeWatts(75);

Constructors class Lamp { private
$bulb; public function __construct($bulb) { $this->bulb = $bulb; } } --------------------------------------------- $myLamp = new Lamp('3 way');

Type Hinting class Lamp {
private $bulb; public function __construct(Bulb $bulb) { $this->bulb = $bulb; } } --------------------------------------------- $myBulb = new Bulb(); $myLamp = new Lamp($bulb);

Organizing your classes
www.toppa.com @mtoppa

Initializing your OO plugin //
this is the start.php file for a “Lamp” WP plugin... // require or autoload the “main” class file for your plugin // and then... $lamp = new Lamp(); $lamp->run(); // that's it!

Abstract classes and inheritance Lamp
FloorLamp DeskLamp HangingLamp

Abstract classes and methods abstract
class Lamp { protected $color; protected $maxSafeWatts; public function setColor($color) { $this->color = $color; return $this->color; } abstract public function setMaxSafeWatts($watts); }

Implementing abstract classes class FloorLamp
extends Lamp { public function setMaxSafeWatts($watts) { /* if numeric and less than 150... */ $this->maxSafeWatts = $watts; return $this->maxSafeWatts; } } ------------------------------------------------------------------ class DeskLamp extends Lamp { public function setMaxSafeWatts($watts) { /* if numeric and less than 100... */ $this->maxSafeWatts = $watts; return $this->maxSafeWatts; } }

Interfaces

Using interfaces: the facade pattern
Functions A PHP facade application interface WordPress Drupal Some other facade facade facade implementation implementation implementation

Interface Example interface FunctionsFacade {
public function enqueueStylesheet($handle); } ------------------------------------------------------------------------ class FunctionsFacadeWp implements FunctionsFacade { public function enqueueStylesheet($handle) { return wp_enqueue_style($handle); } } // we'll look at how to actually use the interface in a few minutes...

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

From LosTechies.com

The purpose is to reduce
the complexity and fragility of a class www.toppa.com @mtoppa

But what does it mean
to do “one thing”? www.toppa.com @mtoppa

Cohesion
www.toppa.com @mtoppa

Only one reason to change
www.toppa.com @mtoppa

Classes Example ShashinDisplayer.php ShashinAlbumDisplayer.php ShashinAlbumDisplayerPicasa.php
ShashinAlbumDisplayerTwitpic.php ShashinAlbumDisplayerFlickr.php ShashinPhotoDisplayer.php ShashinPhotoDisplayerPicasa.php ShashinPhotoDisplayerTwitpic.php ShashinPhotoDisplayerFlickr.php

Methods Example class ShashinInstall {
public function run() { $this->createAlbumTable(); $this->verifyAlbumTable(); $this->createPhotoTable(); $this->verifyPhotoTable(); $this->updateSettings(); return true; } // ... }

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

SwitchableDevice and Lamp interface SwitchableDevice
{ public function turnOn(); public function turnOff(); } -------------------------------------------------------------- class Lamp implements SwitchableDevice { public function turnOn() { // code } public function turnOff() { // code } }

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

Using the Button // require
or autoload the class files, then... $lamp = new Lamp(); $buttonForLamp = new Button($lamp); $buttonforLamp->poll(); $motor = new Motor(); $buttonForMotor = new Button($motor); $buttonForMotor->poll();

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

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

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."

But don't overdo it: avoid
needless complexity

When to prefer OOP to
procedural programming www.toppa.com @mtoppa

Object oriented programming for WordPress
plugin development Mike Toppa WordCamp NYC June 9, 2012 www.toppa.com @mtoppa Skill level Theory and practice Won't be an expert We will be looking at code

www.toppa.com @mtoppa What I'll cover
is not specific to WordPress When you write a plugin you are writing software Your software should be organized around its use cases, it should not be organized around WordPress' architecture That is, you should not start with something like a “plugin” class. Your classes should be organized around the business problem they are trying to solved, not around the details of WordPress.

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

Overview ● Nuts and bolts
of classes and objects in PHP ● The single responsibility principle ● The dependency inversion principle ● When to prefer OO to procedural programming www.toppa.com @mtoppa

www.toppa.com @mtoppa

Simple example class Lamp {
// property declaration private $maxSafeWatts = 100; // method declaration public function getMaxSafeWatts() { return $this->maxSafeWatts; } } --------------------------------------------------------------- // instantiate and assign $myLamp = new Lamp(); echo $myLamp->getMaxSafeWatts(); A class consists of properties, and methods which perform actions, by manipulating those properties Encapsulation of related methods and properties. This is what a class really is. Properties and methods have a visibility (public, private, or protected) An object is created by instantiating a class (calling new). Then you typically, assign it to a variable, and call its methods as needed

Rule of thumb: avoid public
properties class Lamp { public $maxSafeWatts = 100; } --------------------------------------------- $myLamp = new Lamp(); // dangerous to allow this! $myLamp->maxSafeWatts = 'a string to wreck your math'; Public properties can be used and abused by external code at any time.

Prefer private properties class Lamp
{ private $maxSafeWatts = 100; public setMaxSafeWatts($watts) { if (!is_numeric($watts) || $watts > 125 || $watts < 1) { throw New Exception('invalid value for watts'); } $this->maxSafeWatts = $watts; return $this->maxSafeWatts; } } ------------------------------------------------------------------- $myLamp = new Lamp(); $myLamp->setMaxSafeWatts(75); By requiring them to be set through a method call, you can control what types of values are valid, and what ranges are valid.

Constructors class Lamp { private
$bulb; public function __construct($bulb) { $this->bulb = $bulb; } } --------------------------------------------- $myLamp = new Lamp('3 way'); The constructor is a special method, used for initializing a class. It's optional – is called when you call “new” A constructor does not return anything It should be used for getting a class into a valid initial state A common design mistake is to put a lot of complex logic in the constructor, or call it to execute the object's functionality.

Type Hinting class Lamp {
private $bulb; public function __construct(Bulb $bulb) { $this->bulb = $bulb; } } --------------------------------------------- $myBulb = new Bulb(); $myLamp = new Lamp($bulb);

Organizing your classes www.toppa.com @mtoppa
One class per file, and the file name should match the class name Give the class a meaningful name and its methods meaningful names Class names and property names should be nouns or noun phrases Method names should be verbs or verb phrases Get a real IDE that autocompletes variable names and method names

Initializing your OO plugin //
this is the start.php file for a “Lamp” WP plugin... // require or autoload the “main” class file for your plugin // and then... $lamp = new Lamp(); $lamp->run(); // that's it!

Abstract classes and inheritance Lamp
FloorLamp DeskLamp HangingLamp

Abstract classes and methods abstract
class Lamp { protected $color; protected $maxSafeWatts; public function setColor($color) { $this->color = $color; return $this->color; } abstract public function setMaxSafeWatts($watts); } An abstract class cannot be implemented directly It can also have abstract methods, which must be implemented by the child class Protected methods and properties are essentially private, but can be used by child classes

Implementing abstract classes class FloorLamp
extends Lamp { public function setMaxSafeWatts($watts) { /* if numeric and less than 150... */ $this->maxSafeWatts = $watts; return $this->maxSafeWatts; } } ------------------------------------------------------------------ class DeskLamp extends Lamp { public function setMaxSafeWatts($watts) { /* if numeric and less than 100... */ $this->maxSafeWatts = $watts; return $this->maxSafeWatts; } }

Interfaces An electrical outlet is
a great example of an interface. It can power anything designed to plug into it. It doesn't need to know or care about exactly what it's connected to. Interfaces define a set of methods a class must implement. It's similar to abstract classes in this way, but there is no inheritance.

Using interfaces: the facade pattern
Functions A PHP facade application interface WordPress Drupal Some other facade facade facade implementation implementation implementation A different implementation of the facade would allow the PHP application to work outside of WordPress, without touching the application's code When you write a class to implement an interface, it can interact with other classes that know how to talk to that interface, without those other classes having to know anything about your particular implementation of the interface

Interface Example interface FunctionsFacade {
public function enqueueStylesheet($handle); } ------------------------------------------------------------------------ class FunctionsFacadeWp implements FunctionsFacade { public function enqueueStylesheet($handle) { return wp_enqueue_style($handle); } } // we'll look at how to actually use the interface in a few minutes...

The SOLID Principles ● Single
Responsibility (SRP) ● Open-Closed (OCP) ● Liskov Substitution (LSP) ● Interface Segregation (ISP) ● Dependency Inversion (DIP) www.toppa.com @mtoppa Introduced by Bob Martin in his book “Agile Software Development”

From LosTechies.com Applied to classes
and methods Do one thing, do it well, do it only For methods, this typically means changing the value of only one variable If you are passing more than 2 or 3 arguments into a method, you are probably doing more than one thing For classes, it means having a single conceptual responsibility

The purpose is to reduce
the complexity and fragility of a class www.toppa.com @mtoppa We want code that is flexible and easy to understand, not brittle and mind-numbing to read When a method is manipulating multiple properties and invoking lots of other methods, it can be very hard to test, debug, and change. This is a common reason why developers feel fear when making a change – they don't know what might break When a class has many methods and multiple responsibilities, it can be hard to understand and difficult to refactor

But what does it mean
to do “one thing”? www.toppa.com @mtoppa

Cohesion
www.toppa.com @mtoppa

Only one reason to change
www.toppa.com @mtoppa A typical example is when business logic is entangled with the user interface. If you want to develop an RSS feed for a web page, and can't create the feed without tearing apart or copying-and-pasting code that's woven into your HTML, then you've got code that has more than one reason to change.

Classes Example ShashinDisplayer.php ShashinAlbumDisplayer.php ShashinAlbumDisplayerPicasa.php
ShashinAlbumDisplayerTwitpic.php ShashinAlbumDisplayerFlickr.php ShashinPhotoDisplayer.php ShashinPhotoDisplayerPicasa.php ShashinPhotoDisplayerTwitpic.php ShashinPhotoDisplayerFlickr.php You can start to see the power of the OO approach here I can add support for a new photos service by creating a new subclass, instead of having to touch code all over the place, adding a bunch of “if” statements

Methods Example class ShashinInstall {
public function run() { $this->createAlbumTable(); $this->verifyAlbumTable(); $this->createPhotoTable(); $this->verifyPhotoTable(); $this->updateSettings(); return true; } // ... }

From LosTechies.com It's common to
see code that hard-wires together all the parts, when those connections could be made more flexible and extensible

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” } This solution violates the DIP ● Button depends directly on Lamp ● Button is not reusable ● It can't control, for example, a Motor

Dependency inversion applied <<interface>> Button
SwitchableDevice + poll() + turnOn() + turnOff() Lamp This is the Abstract Server pattern What it means Neither Button nor Lamp “own” the interface Buttons can now control any device that implements SwitchableDevice Lamps and other SwitchableDevices can now be controlled by any object that accepts a SwitchableDevice

SwitchableDevice and Lamp interface SwitchableDevice
{ public function turnOn(); public function turnOff(); } -------------------------------------------------------------- class Lamp implements SwitchableDevice { public function turnOn() { // code } public function turnOff() { // code } }

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

Using the Button // require
or autoload the class files, then... $lamp = new Lamp(); $buttonForLamp = new Button($lamp); $buttonforLamp->poll(); $motor = new Motor(); $buttonForMotor = new Button($motor); $buttonForMotor->poll();

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

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

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." Author: Steve Freeman and Nat Pryce

But don't overdo it: avoid
needless complexity The complexity of having 44 classes in Shashin is justified by its need for flexibility You can support a new viewer or a new photo service simply by creating a new subclass. This is much more maintainable and extensible than a single huge file with deeply nested conditionals and unclear dependencies But if I knew I would never need that kind of flexibility, there would be no justification for creating these layers of abstraction – they would just be needless complexity

When to prefer OOP to
procedural programming www.toppa.com @mtoppa This can be subjective. For me, it hurts my brain to program procedurally, even for small projects. For a very small project, OO will usually involve more code If you can classify the components of an application into objects with properties and methods, and you have an idea of what kind of changes will come in the future, an OO approach is very powerful. Also, if you want to do unit testing, you need OO code