Dependency Injection and the Symfony2 Service Container

Dependency injection and the service container are core parts of Symfony2's philosophy. Whilst it is easy to start getting services from the container, a better grasp of dependency injection will allow developers to make the most of this powerful feature.This talk starts by covering the basics of dependency injection and the different types of injection. It then moves from low level dependency injection onto using the service container for full scale application configuration. It continues with how to configure services using Symfony2's service container and the advantages of doing this in allowing application level wiring together of classes. Finally it takes a look at the tools available in Symfony2 for getting an overview of service use and interaction within in an application.

Transcript

1. Dependency Injection and
   the Symfony2 Service
   Container
   

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

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3. What is Dependency Injection?
   

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

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

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6. 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);
   }
   }
   

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7. 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
   

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8. 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);
   

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9. 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);
   }
   

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

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

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12. 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);
    

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13. $settings = Settings::fetch();
    switch($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);
    

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14. 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);
    

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

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16. 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);
    }
    

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17. 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();
    

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18. //--
    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();
    

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19. Where does the creation of dependencies go ?
    

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

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21. 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');
    

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22. 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);
    }
    

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23. 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;
    }
    

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24. Problem solved?
    

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

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26. 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);
    

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

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28. $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);
    

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29. 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
    

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

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

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32. 
    class="NameSpace\Of\Mailer"/>
    class="NameSpace\Of\MailFactory"/>
    
    

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33. 
    class="NameSpace\Of\EmailNotifer">
    productmanager@example.com
    
    
    
    class="NameSpace\Of\Mailer"/>
    class="NameSpace\Of\MailFactory"/>
    
    

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34. 
    class="NameSpace\Of\EmailNotifer">
    productmanager@example.com
    
    
    
    deputyproductmanager@example.com
    
    
    salesteam@example.com
    
    
    class="NameSpace\Of\Mailer"/>
    class="NameSpace\Of\MailFactory"/>
    
    
    deputyproductmanager@example.com
    
    
    salesteam@example.com
    
    

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35. 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
    

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36. $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'
    ));
    

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37. How do we actually use the container?
    

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

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

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

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41. Tools to help
    ●
    container:debug command
    ●
    JMSDebuggingBundle
    

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42. 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
    #--
    

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43. 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
    

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

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50. 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
    

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

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