Dependency injection with Symfony 2

Dependency Injection and Symfony

Introduction

Cameron MandersonDeveloper, Director of Flint, Melbourne Symfony2 Organiser

[email protected]: @cammandersonwww.linkedin.com/in/cameronmanderson

Topic Highlight

• More in-depth look at the background of “Dependency Injection”

• Aiming to uncover in Symfony– Modularity in our implementation– Loose decoupling of dependencies– Managing scope and access to dependencies

Background

Some terminology

• Dependency– Something that is required by another object to

fulfill its function• Dependent– A client object that needs a dependency in order

to perform its function

Shown easiest like this..

Dependent Dependency

Emailer Spell Checker

Like…

Example in code

This seems fine.. Right?

What about English or French?

Can we test it was called? – no…

We need an approach to handling these dependencies.

We need looser coupling.

Dependency Injection

So what is Dependency Injection?

Dependency Injection

Dependency Injection“A design pattern whose purpose is to improve the testability of, and simplify deployment of components”

Essentially something that takes care of creating and linking our objects

Involves 3 parts• Our 2 objects; dependency and dependent• And.. An Injector to instantiate objects at runtime

Example

Injector configures our objects at runtime, figuring out the order of creation

Dependent Dependency

InjectorInjector reads the dependencies, and creates and stiches them together

So.. Why dependency injection?

• Why do we need better management?– Applications are full of objects and dependencies• We want automation of construction• We want easy management of objects• We don’t want hundreds of Factories

– Hard to manage those dependencies• Applications become tightly coupled to one-another• Difficult to swap out implementations• Manage configuration of those dependencies

Pre-dependency injection

• How do we currently do it?– Hand-written construction– Using Factory patterns

Hand-construction

Factory

Enter Dependency Injection

• Offset the instantiation of those objects to something else, as well as the dependencies– Some behind the scenes magic– Read a definition of how these dependencies

should be handled– Create the objects and associate their

dependencies• Scope (talk about later)

Injector/idioms

• Simple logic:– Can it create the object yet? If there is a

dependency, go create it.. Loop etc.• Essentially… a role to..– Creating the “new” object when needed– Managing the dependencies • Constructor Injection• Setter Injection

Injector Configuration

Plain PHP files..

Constructor Injection

• Constructor injection has dependencies in the constructor– Need to create the dependencies for the

dependent first– Inject them through the constructor

Setter Injection

• Create the object first and inject the dependents through methods (setters)

Some stuff to be careful of…

• Circular dependencies– Oops. Be careful. (There are ways to handle.. Not

yet in Symfony)

C B

A

So how does this help?

• We are all about building modular applications– Decoupling dependencies– Making clear what the intent of the module is– Separation of concerns– Improve testability

Decoupling

• Reduce the specific hard-value reliance– Facilitate swapping out new implementations – Reduce that complexity– Hopefully reduce the risk..

Make clear what it is

• We want clear intent to our application– Make it easy to use and modify– “Program to contract”• Implement interfaces that clearly defines the intent of

our code

Separation of concerns

• We want to better encourage design to separate the concerns in our application– Discreet features

• Application Logic vs Infrastructure Logic

Application

Infrastructure

Testability

• Keep it clean– Avoiding static calls or singletons– Out of container testing easy– Easy to test concerns individually and manually

define dependencies and mocks

We are lazy

• It is much easier utilising a dependency injector to handle creating objects and handle those dependencies than to worry about implementing patterns and configuration yourself.

Basics… check!

• That’s the theory– Dependencies exist in our application– Dependency injection assists creating and

managing those objects– Makes our life easier

• Let’s talk about Symfony and Services..

Symfony Service Architecture

• Symfony is an Event Driven application• Also provides a dependency injector and

service locator

The Service

Service– An object that performs a well defined function

when called upon– Separated from other parts of the application– Implements an action• e.g. Template Renderer, Mail, etc

Configuring the Injector

Service Locator (Service Container)

• Assists by providing a way to access services by “key”

Finding Keys

• Might be a problem?• Symfony makes it easy, use the CLI

Further stuff to read…

• Topics– Aliasing– Public and Private– Specifying parameters in your config.yml

• http://symfony.com/doc/current/book/service_container.html

Wrapping it up

• My experience with it has been that:– You are encouraged to write services that others

can use and extend– Your applications are much easier to test– Scope is much easier to manage– It encourages others to implement their own

domain logic when needed, thanking you how easy it is to implement their own part. ;-)

Conclusion

• Covered the concepts of dependency injection– The problem– Using dependency injection to remove (code)

• Showed how it applies in Symfony– Better decoupling, modularity and reuse