What is Dependency Injection? Shawn Stratton 2nd June 2011—

What I’ll Cover 2 I will cover:

What I’ll Cover 2 I will cover: • What DI isn’t

What I’ll Cover 2 I will cover: • What DI isn’t • What DI is

What I’ll Cover 2 I will cover: • What DI isn’t • What DI is • Why you should use DI

What I’ll Cover 2 I will cover: • What DI isn’t • What DI is • Why you should use DI I won’t cover:

What I’ll Cover 2 I will cover: • What DI isn’t • What DI is • Why you should use DI I won’t cover: • How to implement DI in your project

What I’ll Cover 2 I will cover: • What DI isn’t • What DI is • Why you should use DI I won’t cover: • How to implement DI in your project • Differences in available containers

What I’ll Cover 2 I will cover: • What DI isn’t • What DI is • Why you should use DI I won’t cover: • How to implement DI in your project • Differences in available containers • How to cook awesome bacon (ask Jeff)

What is Dependency Injection not?

New 4 Figure 1: http://martinfowler.com/articles/injection.html

Magic 5 Figure 2: Merlin by One Luck Guy (Flickr)

Complex 6 Figure 3: Complexity 3 by Michael Heiss (Flickr)

So what is it?

A Design Style 8 db = $db ; $this−>dependency = $dependency ; } public function somefunc ( ) { / / Use Dependencies } }

Easy 9 defineSingleton( ’PDO ’ , $pdo) ; $dependant = $sl−>get( ’ Dependant ’ ) ; / / Dependant i s type of Dependant

About Components

Service Locator

Service Locator 12 Service Location is like ordering with substitutions, and having the waiter completely ignore the substitutions; you get what’s on the menu, nothing more, nothing less. Figure 4: Matthew Weier O‘Phinney on Service Locators

Service Locators Detail 13 • It’s a fancy registry. • Inject the locator into the class via contstructor, call the the locator to find your services. • Works, but it’s not foolproof

Containers

Another Analogy 15 Dependency Injection is like ordering off the menu – but specifying things like, ”I’d like to substitute portabella mushrooms for the patties, please.” The waiter then goes and brings your dish, which has portabella mushrooms instead of the hamburger patties listed on the menu. Figure 5: Matthew Weier O‘Phinney on DI Containers

DI Container Detail 16 • Still a fancy registry, basically just a Service Locator. • Instantiates new classes by resolving and injecting their dependencies. • Very Clean in regards to separation of concerns. • Not required to run the system (you can do this manually, trust me)

What are the benefits of Dependency Injection?

Makes Testing Easy 18 getMock( ’ Dependency ’ , array ( ’ someFunction ’ ) ) ; $dependency−>expects( $this−>once ( ) )−>method( ’ someFunction ’ ) ; $this−>dependant = new Dependant($pdo , $dependency) ; } }

Easy Extension 19 Steps to extend and use a class:

Easy Extension 19 Steps to extend and use a class: 1. Create class b and have it extend class a

Easy Extension 19 Steps to extend and use a class: 1. Create class b and have it extend class a 2. Change Mapping

Easy Extension 19 Steps to extend and use a class: 1. Create class b and have it extend class a 2. Change Mapping 3. Profit!

What are the costs?

Enforces Interfaces 21

Mapping Files 22 array ( ’ class ’ => ’ Zend Foo ’ , ’ arguments ’ => array ( ’ c o n s t r u c t ’ => ’ComponentA ’ ) , ) , ’ComponentA ’ => array ( ’ class ’ => ’ Zend Foo Component A ’ , ’ instanceof ’ => ’ Zend Foo Component Interface ’ , ) ) ; Figure 6: Zend DI Proposal by Frederic Cargnelutti (mod- ified)

Thank You 23 More Resources: • Martin Fowler on Inversion of Control - http://martinfowler.com/articles/injectio • Ralph Schindler on Learning Dependency Injection - http://bit.ly/php-di • Sebastian Bergmann has an awesome book called Real-World Solutions for Developing High-Quality PHP Frameworks and Applications Ask Luke Allison about his Amazing Horse!