Dependency Injection and the Symfony2 Service Container

Richard Miller ● http://www.limethinking.co.uk ● http://miller.limethinking.co.uk ● @mr_r_miller

What is Dependency Injection?

class ProductController { public function addAction() { //validate the POST variables $productSaver = new ProductSaver(); $productSaver->save($product); } //-- }

class ProductSaver { public function save($product) { $mapper = new DataMapper(); $mapper->save($product); $emailNotifier = new EmailNotifier(); $emailNotifier->notify($product); } }

class EmailNotifier { public function notify($product) { $mail = new Mail(); $mail->addTo('productmanager@example.com'); $mail->addCC('deputyproductmanager@example.com'); //create message body using $product $mailer = new Mailer(); $mailer->send($mail); } }

Problems with EmailNotifier ● Not reusable ● Change of email addresses ● Change of mailer ● Impossible to unit test ● Any test will create an actual Mailer and send the email

public function notify($product) { $mail = new Mail(); $mail->addTo($this->toAddress); $mail->addCC('deputyproductmanager@example.com'); //create message body using $product $mailer = new Mailer(); $mailer->send($mail); } protected $toAddress; public function __construct($toAddress) { $this->toAddress = $toAddress; } $mail->addTo($this->toAddress);

protected $toAddress; protected $ccAddresses = array(); public function __construct($toAddress) { $this->toAddress = $toAddress; } public function setCcAddress($ccAddress) { $this->ccAddresses[] = $ccAdresss; } public function notify($product) { $mail = new Mail(); $mail->addTo($this->toAddress); foreach($this->ccAddresses as $ccAddress) { $mail->addCC($ccAddress); } //create message body using $product $mailer = new Mailer(); $mailer->send($mail); } protected $ccAddresses = array(); public function addCcAddress($ccAddress) { $this->ccAddresses[] = $ccAdresss; } foreach($this->ccAddresses as $ccAddress) { $mail->addCC($ccAddress); }

Constructor Injection ● Fixed at instantiation ● Guaranteed to have been injected

Setter Injection ● Good for optional dependencies ● Good for collections of dependencies

protected $toAddress; protected $ccAddresses = array(); protected $mailer; public function __construct($toAddress, MailerInterface $mailer) { $this->toAddress = $toAddress; $this->mailer = $mailer; } public function addCcAddress($ccAddress) { $this->ccAddresses[] = $ccAdresss; } public function notify($product) { $mail = new Mail(); $mail->addTo($this->toAddress); foreach($this->ccAddresses as $ccAddress) { $mail->addCC($ccAddress); } //create message body using $product $this->mailer->send($mail); } protected $mailer; MailerInterface $mailer $this->mailer = $mailer; $this->mailer->send($mail);

$settings = Settings::fetch(); switch($settings->get('mailer)) { case 'smtp': $mailer = new SMTPMailer(); break; case 'sendmail': $mailer = new SendmailMailer(); break; case 'test': $mailer = new TestMailer(); break; case 'basic': default: $mailer = new Mailer(); break; } $mailer->send($mail);

protected $toAddress; protected $ccAddresses = array(); protected $mailer; public function __construct($toAddress, MailerInterface $mailer) { $this->toAddress = $toAddress; $this->mailer = $mailer; } public function addCcAddress($ccAddress) { $this->ccAddresses[] = $ccAdresss; } public function notify($product) { $mail = new Mail(); $mail->addTo($this->toAddress); foreach($this->ccAddresses as $ccAddress) { $mail->addCC($ccAddress); } //create message body using $product $this->mailer->send($mail); } protected $mailer; MailerInterface $mailer $this->mailer = $mailer; $this->mailer->send($mail);

Type Hinting ● Ensures object with correct interface injected ● Hint should be Interface not Implementation ● Helps IDE with auto-completion

protected $toAddress; protected $ccAddresses = array(); protected $mailer; public function __construct($toAddress, MailerInterface $mailer) { $this->toAddress = $toAddress; $this->mailer = $mailer; } public function addCcAddress($ccAddress) { $this->ccAddresses[] = $ccAdresss; } public function notify($product) { $mail = new Mail(); $mail->addTo($this->toAddress); foreach($this->ccAddresses as $ccAddress) { $mail->addCC($ccAddress); } //create message body using $product $this->mailer->send($mail); }

protected $toAddress; protected $ccAddresses = array(); protected $mailer; public function __construct($toAddress, MailerInterface $mailer) { $this->toAddress = $toAddress; $this->mailer = $mailer; } public function addCcAddress($ccAddress) { $this->ccAddresses[] = $ccAdresss; } public function notify($product) { $mail = MailFactory::get(); $mail->addTo($this->toAddress); foreach($this->ccAddresses as $ccAddress) { $mail->addCC($ccAddress); } //create message body using $product $this->mailer->send($mail); } $mail = MailFactory::get();

//-- protected $mailFactory; public function __construct($toAddress, MailerInterface $mailer, MailFactoryInterface $mailFactory ) { $this->toAddress = $toAddress; $this->mailer = $mailer; $this->mailFactory = $mailFactory; } //-- public function notify($product) { $mail = $this->mailFactory->get(); $mail->addTo($this->toAddress); foreach($this->ccAddresses as $ccAddress) { $mail->addCC($ccAddress); } //create message body using $product $this->mailer->send($mail); } protected $mailFactory; MailFactoryInterface $mailFactory $this->mailFactory = $mailFactory; $mail = $this->mailFactory->get();

Where does the creation of dependencies go ?

class ProductSaver { public function save($product) { $mapper = new DataMapper(); $mapper->save($product); $emailNotifier = new EmailNotifier(); $emailNotifier->notify($product); } }

public function save($product) { $mapper = new DataMapper(); $mapper->save($product); $emailNotifier = new EmailNotifier('productmanager@example.com', new Mailer, new MailFactory ); $emailNotifier->setCcAddress('deputyproductmanager@example.com'); $emailNotifier->setCcAddress('salesteam@example.com'); $emailNotifier->notify($product); } $emailNotifier = new EmailNotifier('productmanager@example.com', new Mailer, new MailFactory ); $emailNotifier->addCcAddress('deputyproductmanager@example.com'); $emailNotifier->addCcAddress('salesteam@example.com');

protected $notifiers = array(); public function addNotifier(NotifierInterface $notifier) { $this->notifiers[] = $notifier; } public function save($product) { $mapper = new DataMapper(); $mapper->save($product); foreach($this->notifiers as $notifier) { $notifier->notify($product); } } protected $notifiers = array(); public function addNotifier(NotifierInterface $notifier) { $this->notifiers[] = $notifier; } foreach($this->notifiers as $notifier) { $notifier->notify($product); }

protected $notifiers = array(); protected $mapper; public function __construct(DataMapperInterface $mapper) { $this->mapper = $mapper; } //-- public function save($product) { $this->mapper->save($product); foreach($this->notifiers as $notifier) { $notifier->notify($product); } } protected $mapper; $this->mapper->save($product); public function __construct(DataMapperInterface $mapper) { $this->mapper = $mapper; }

Problem solved?

class ProductController { public function addAction() { //validate the POST variables $productSaver = new ProductSaver(); $productSaver->save($product); } //-- }

public function addAction() { //validate the POST variables $emailNotifier = new EmailNotifier('productmanager@example.com', new Mailer, new MailFactory ); $emailNotifier->setCcAddress('deputyproductmanager@example.com'); $emailNotifier->setCcAddress('salesteam@example.com'); $mapper = new DataMapper(); $productSaver = new ProductSaver($emailNotifier, $mapper); $productSaver->setNotifier($emailNotifier); $productSaver->save($product); } $emailNotifier = new EmailNotifier('productmanager@example.com', new Mailer, new MailFactory ); $emailNotifier->addCcAddress('deputyproductmanager@example.com'); $emailNotifier->addCcAddress('salesteam@example.com'); $mapper = new DataMapper(); $productSaver = new ProductSaver($mapper); $productSaver->addNotifier($emailNotifier);

protected $productSaver; public function __construct($productSaver) { $this->productSaver = $productSaver; } public function addAction() { //validate the POST variables $this->productSaver->save($product); }

$emailNotifier = new EmailNotifier('productmanager@example.com', new Mailer, new MailFactory ); $emailNotifier->addCcAddress('deputyproductmanager@example.com'); $emailNotifier->addCcAddress('salesteam@example.com'); $mapper = new DataMapper(); $productSaver = new ProductSaver($mapper); $productSaver->addNotifier($emailNotifier); $controller = new ProductController($productSaver);

Advantages to separate wiring ● Set up config differently for each app ● No config code in classes ● No need to maintain different versions for different apps ● Just inject object with different functionality

Disadvantages to manual wiring ● Large unwieldy bootstrap file ● Everything set up whether used or not ● Repetitive code

The Symfony2 Service Container to the rescue ● Creates and configures services ● Allows configuration with XML, YAML and PHP ● Only creates used services

productmanager@example.com

productmanager@example.com deputyproductmanager@example.com salesteam@example.com deputyproductmanager@example.com salesteam@example.com

services: emailNotifer: class: NameSpace\Of\EmailNotifer arguments: [productmanager@example.com, @mailer, @mailFactory] calls: - [ addCcAddress, [ deputyproductmanager@example.com ] ] - [ addCcAddress, [ salesteam@example.com ] ] mailer: class: NameSpace\Of\Mailer mailFactory: class: NameSpace\Of\MailFactory

$container->setDefinition('emailNotifer', new Definition( 'NameSpace\Of\EmailNotifer', array( 'productmanager@example.com', new Reference('mailer'), new Reference('mailFactory') ) ))->addMethodCall('addCcAddress', array( 'deputyproductmanager@example.com' ))->addMethodCall('addCcAddress', array( 'salesteam@example.com' )); $container->setDefinition('mailer', new Definition( 'NameSpace\Of\Mailer' )); $container->setDefinition('mailFactory', new Definition( 'NameSpace\Of\MailFactory' ));

How do we actually use the container?

$productController = $container->get('productController');

class ProductController extends Controller { public function addAction() { //validate the POST variables $productSaver = $this->container->get('productSaver'); $productSaver->save($product); } //-- }

A Criticism of Dependency Injection ● It's difficult to navigate code

Tools to help ● container:debug command ● JMSDebuggingBundle

php app/console container:debug [container] Public services Service Id Class Name doctrine Symfony\Bundle\DoctrineBundle\Registry doctrine.dbal.default_connection Doctrine\DBAL\Connection doctrine.odm.mongodb.cache.array Doctrine\Common\Cache\ArrayCache #--

php app/console container:debug assetic.asset_manager [container] Information for service assetic.asset_manager Service Id assetic.asset_manager Class Assetic\Factory\LazyAssetManager Tags - Scope container Public yes

JMSDebuggingBundle ● http://github.com/schmittjoh/JMSDebuggingBun dle

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Conclusion ● Class level advantages ● Promotes re-usability ● Allows true unit testing ● App level advantages ● Allows easy configuration of application ● Easy to implement new features by dropping in new objects ● No need to maintain multiple code bases

More ● Cookbook articles - http://symfony.com/doc/current/cookbook/ ● http://miller.limethinking.co.uk ● @mr_r_miller ● http://joind.in/3704