Dependency Injection for Mere Humans

PHP UK Conference 2023

Ian Littman / @ian@phpc.social / @iansltx 

follow along at https://ian.im/diuk23

code samples at https://ian.im/dicode

Alternate title:

opinions and implementations on
Decoupled software design

Shout out to @jcarouth and his presentation back in 2015

What We'll Cover

• Types of dependencies
• What dependency injection is and why it's a good thing
• Types of dependency injection, pros + cons
• Refactoring to a dependency-injected architecture
• Dependency Injection Containers
  • What they are
  • How to build one
  • How to configure/use one
• ...and a few antipatterns along the way

Highly coupled code is bad

highly coupled code is bad

but some coupling is unavoidable

Decoupled Code is good

• Easier to test
• Easier to debug
• Easier to reason about

What's a dependency, anyway?

Dependencies are the external objects, resources, or functions an object uses to accomplish its responsibility.

• Per-class level <- we'll be here
• Library level (handled by Composer)
• OS level (handled by e.g. apt, apk, yum)

Some common Dependencies

• Databases/Entity Managers (e.g. PDO, Doctrine)
• Caching (e.g. Redis/Memcached adapters)
• File storage (e.g. local FS, S3, Flysystem)
• Notifications (email, SMS, push)
• Over-the-network APIs (or SDKs that wrap them)
• Logging

Some common Dependencies

• The current web request
• Environment variables
• The current user
• Session/API scope information
• The current time
• Static method calls
• Sources of randomness

Spot the dependencies

class OrderProcessor {
    function __construct() {
        $this->orderRepository = new MysqlOrderRepository();
    }

    function completeOrder($order) {
        global $logger;
        $order->completedAt = new \DateTimeImmutable;
        $logger->log("Order {$order->id} is complete");
        $this->orderRepository->save($order);
        Mailer::sendOrderCompleteEmail($order);
    }
}

Spot the dependencies

class OrderProcessor {
    function __construct() {
        $this->orderRepository = new MysqlOrderRepository();
    }

    function completeOrder($order) {
        global $logger;
        $order->completedAt = new \DateTimeImmutable;
        $logger->log("Order {$order->id} is complete");
        $this->orderRepository->save($order);
        Mailer::sendOrderCompleteEmail($order);
    }
}

Spot the dependencies

class OrderProcessor {
    function __construct() {
        $this->orderRepository = new MysqlOrderRepository();
    }

    function completeOrder($order) {
        $logger = \App::make('logger');
        $order->completedAt = new \DateTimeImmutable;
        $logger->log("Order {$order->id} is complete");
        $this->orderRepository->save($order);
        Mailer::sendOrderCompleteEmail($order);
    }
}

Spot the dependencies

class OrderProcessor {
    function __construct() {
        $this->orderRepository = new MysqlOrderRepository();
    }

    function completeOrder($order) {
        $logger = Logger::getInstance();
        $order->completedAt = new \DateTimeImmutable;
        $logger->log("Order {$order->id} is complete");
        $this->orderRepository->save($order);
        Mailer::sendOrderCompleteEmail($order);
    }
}

Hard questions

Q: What does OrderProcessor depend on?

A: Read the entire class to find out!
 

Q: Where is Mailer used in my application?

A: Grep everything in your project for "Mailer::".

Q: How can I test completeOrder() without sending emails?

A: ¯\_(ツ)_/¯ *

* Yes, there are workarounds, but this isn't exactly a testing/mocking talk.

WHat's dependency injection, anyway?

Dependency injection is the practice of pushing (injecting) dependencies into an object, rather than having objects find their dependencies on their own.

This isn't the same as the Dependency Inversion Principle in SOLID. We'll get to that later.

Types of Dependency Injection

• Constructor
• Setter + Property
• Parameter

Constructor injection

class ClassThatNeedsLogging {
    private Logger $logger;

    public function __construct(Logger $logger) {
        $this->logger = $logger;
    }
}

Constructor injection

class ClassThatNeedsLogging {
    public function __construct(
        private Logger $logger
    ) {
        // constructor property promotion
        // available since PHP 8.0
    }
}

Pros

• Makes dependencies explicit
• Can't externally modify dependencies after instantiation
• Discourages violation of Single Responsibility Principle

Cons

• May have a lot of constructor parameters
• May never use most dependencies
• Could be the wrong point in the object life cycle

public function __construct(
    private Twig $view,
    private AuthorizationServer $auth,
    private LogRepository $errorRepo,
    private AuthRequestRepository $authRequestRepo,
    private ClientRepository $clientRepo,
    private UserRepository $userRepo,
    private ResourceServer $resAuth,
    private AccessTokenRepository $accessTokenRepo,
    private UserValidatorInterface $validator
) {}

Antipattern #1: __construct($options)

"I know, I'll just throw everything in an $options array!"

If your options are optional, make them optional parameters

and maybe use named parameters (PHP 8.0+) to set them.

Setter injection

class ClassThatUsesLoggingButDoesNotRequireIt {
    private ?LoggerInterface $logger = null;

    public function setLogger(Logger $logger) {
        $this->logger = $logger;
    }
}

Yolo Property injection

class ClassThatMightWantAThingCalledALogger {
    // can't do initialization as default
    public $logger = null;
}

class SlightlySaferClassThatWantsALogger {
    public ?Logger $logger = null;
} // as of 7.4

Can't use readonly here

Pros

• Lazier loading
• Works well with optional dependencies
• Flexible across class lifecycle
  • Don't need every dependency for every method call
  • Can change dependencies without re-instantiating the class

Cons

• Harder to quickly see which dependencies a class has
• Existence of dependencies in fully instantiated class not guaranteed
  • Null checks inside the code
  • Conditional injection outside the code

Which would you rather have?

public function fileTPSReport(Report $rpt) {
    /* do some stuff */
    if ($this->logger instanceof Logger) {
        $this->logger->log('Did a thing');
    }

    /* do more stuff */
    if ($this->logger instanceof Logger) {
        $this->logger->log('Did things');
    }

}

Which would you rather have?

public function fileTPSReport(Report $rpt) {
    /* do some stuff */
    if ($this->logger) {
        $this->logger->log('Did a thing');
    }

    /* do more stuff */
    if ($this->logger) {
        $this->logger->log('Did things');
    }

}

Which would you rather have?

public function fileTPSReport(Report $rpt) {
    /* do some stuff */
    $this->logger->log('Did a thing');

    /* do more stuff */
    $this->logger->log('Did things');

}

class NullLogger implements Logger {
    public function log($message) {
        /** noop **/
    }
}

One weird constructor defaulting trick

public function __construct(
    private LoggerInterface $logger
        = new NullLogger()
) {} // PHP >= 8.1

also, that class exists so don't reimplement it

Traits + Setter Injection

1. Add setter to trait
2. Mirror setter in an interface
3. Import trait into classes
4. Implement interface on classes
5. Configure DI container to setter-inject based on interface

e.g. Psr\Log\{LoggerAwareInterface, LoggerAwareTrait}

Sorry, you can't have an interface implement a trait directly.

Parameter Injection

public function __invoke(
    ServerRequestInterface $request,
    ResponseInterface $response,
    array $args = []
): ResponseInterface {
    /** do something,
        return a ResponseInterface **/
}

Parameter Injection

public function getCollection(
    \Illuminate\Http\Request $request,
    \App\Services\OrderService $orderService
) {
    /** do something, return a Response **/
}

Pros

• What you need when you need it

Cons

• Moves the problem of dependency management to the caller*

* Some frameworks take care of this for you for controllers/commands

Mix & Match as needed

Dependency Inversion Principle

High level modules should not depend on low level modules; both should depend on abstractions. Abstractions should not depend on details.  Details should depend upon abstractions.

Dependency Inversion Principle

High level modules should not depend on low level modules; both should depend on abstractions. Abstractions should not depend on details.  Details should depend upon abstractions.

Tl;dr: use, and expose, interfaces with just enough functionality to get the job done.

Pro tip: interfaces support multiple inheritance

Pro tip 2: PHP 8.1 lets you ask for intersection types

Should I use "interface" in my interface names?

class Vonage implements SmsProvider {}

class RedSMS implements SmsProviderInterface {}

Should I use "interface" in my interface names?

class Vonage implements SmsProvider {}

class RedSMS implements SmsProviderInterface {}

Doesn't matter either way. Pick a convention and stick with it.

Abstractions should not be leaky

class Camry implements HasAcceleratorPedal {
    public function pressAccelerator(): void;
} // namespace Toyota\Vehicles

class Model3 implements HasAcceleratorPedal {
    public function pressAccelerator(): void;
} // namespace Tesla\Vehicles

Abstractions should not be leaky

class Camry implements HasAcceleratorPedal {
    public function pressAccelerator(): Vroom;
} // namespace Toyota\Vehicles

class Model3 implements HasAcceleratorPedal {
    public function pressAccelerator(): Zoom;
} // namespace Tesla\Vehicles

If Vroom and Zoom implement Acceleration

we're good here

Abstractions should not be leaky

class MysqlUserRepo implements UserRepository {
    public function getById(int $id): ?User {}
}

class RedisUserRepo implements UserRepository {

    public function getById(int $id): ?User {}

}

class gRPCUserAPI implements UserRepository {

    public function getById(int $id): ?User {}

}

interface User { /** various signatures **/ }

Let's do some refactoring

class OrderProcessor {
    function __construct() {
        $this->orderRepository = new MysqlOrderRepository();
    }

    function completeOrder($order) {

        $logger = Logger:getInstance();
        $order->completedAt = new \DateTimeImmutable;
        $logger->log("Order {$order->id} is complete");
        $this->orderRepository->save($order);
        Mailer::sendOrderCompleteEmail($order);
    }
}

Let's do some refactoring

class OrderProcessor {
    function __construct(OrderRepository $orderRepo,
                         Logger $logger) {
        $this->orderRepository = $orderRepo;
        $this->logger = $logger;
    }

    function completeOrder($order) {
        $order->completedAt = new \DateTimeImmutable;
        $this->logger->log("Order {$order->id} is complete");
        $this->orderRepository->save($order);
        Mailer::sendOrderCompleteEmail($order);
    }
}

class OrderProcessor {
    function __construct(OrderRepository $orderRepo,
                         Logger $logger,
                         \DateTimeInterface $now,
                         Mailer $mailer) {
        $this->orderRepository = $orderRepo;
        $this->logger = $logger;
        $this->now = $now;
        $this->mailer = $mailer;
    }
    function completeOrder($order) {
        $order->completedAt = $this->now;
        $this->logger->log("Order {$order->id} is complete");
        $this->orderRepository->save($order);
        $this->mailer->sendOrderCompleteEmail($order);
    }
}

class OrderProcessor {
    function __construct(OrderRepository $orderRepo,
                         Logger $logger,
                         Mailer $mailer) {
        $this->orderRepository = $orderRepo;
        $this->logger = $logger;
        $this->mailer = $mailer;
    }
    function completeOrder($order, \DateTimeInterface $now) {
        $order->completedAt = $now;
        $this->logger->log("Order {$order->id} is complete");
        $this->orderRepository->save($order);
        $this->mailer->sendOrderCompleteEmail($order);
    }
}

class OrderProcessor {
    function __construct(
            private OrderRepository $orderRepo,
            private Logger $logger,
            private Mailer $mailer
    ) {
        // constructor property promotion, PHP 8.0+
    }
    function completeOrder($order, \DateTimeInterface $now) {
        $order->completedAt = $now;
        $this->logger->log("Order {$order->id} is complete");
        $this->orderRepository->save($order);
        $this->mailer->sendOrderCompleteEmail($order);
    }
}

What's a Dependency Injection Container?

A dependency injection container is an object used to manage the instantiation of other objects.

If you have one, it will be the place where the "new" keyword gets used more than anywhere else in your app, either via configuration code you write or under the hood.

What's a Dependency Injection Container NOT?

• Not the only place you can (or should) use the "new" keyword in your application.
  • Factories
  • Value objects
• Not required for dependency injection.
• Not to be used as a Service Locator

class OrderProcessor {
    function __construct(ContainerInterface $c) {
        $this->orderRepository = $c->get('OrderRepository');
        $this->logger = $c->get('Logger');
        $this->mailer = $c->get('Mailer');
    }
    function completeOrder($order, \DateTimeInterface $now) {
        $order->completedAt = $now;
        $this->logger->log("Order {$order->id} is complete");
        $this->orderRepository->save($order);
        $this->mailer->sendOrderCompleteEmail($order);
    }
}

Antipattern #2: Service Location

ANTIPATTERN #2: Service Location

Twittee: A container in a 140 tweet

Using Twittee

PSR-11 (fka Container-Interop)

Twittee++: A PSR-11 container in a 280 tweet

Dependency Injected Fizzbuzz

Dependency Injected Fizzbuzz

...but don't use Twittee++

• Every major framework has one
  • Symfony (DependencyInjection component)
  • Zend (ServiceManager)
  • Laravel (Illuminate\Container)
• Standalone ones for use elsewhere
  • Pimple
  • PHP-DI
  • League\Container
  • Aura.Di

...but don't use Twittee++

• Every major framework has one
  • Symfony (DependencyInjection component)
  • Zend (ServiceManager)
  • Laravel (Illuminate\Container)
• Standalone ones for use elsewhere
  • Pimple
  • PHP-DI
  • League\Container
  • Aura.Di

$c = new Pimple\Container;
$c[NeedsALogger::class] = function($c) {
    return new NeedsALogger($c['logger']);
};
$c['logger'] = function() {
    return new Logger;
};

var_dump($c[NeedsALogger::class]); // NeedsALogger

Pimple - Now PSR-11 Compliant

Pimple

• Use $c->factory(callable) if you don't want the same instance every time you ask for a dependency.
• Use $c->protect(callable) if you want to set a closure to your container.
• Use $c->raw(dep) if you want to get a closure you set without using protect()
• Not much magic
• Default container of Slim 3
• Can include in Slim 4 as it conforms to PSR-11

Let's refactor a Slim 3 route

$app = new Slim\App();
$app->get('/', function(Request $req, Response $res) {
    $userRepository = new UserRepository(new PDO(/* */));
    $users = $userRepository->listAll();
    return $res->withJson($users);
});

Let's refactor a Slim 3 route

$app = new Slim\App();
$c = $app->getContainer();
$c['db'] = function($c) { return new PDO(/* */); };

$app->get('/', function(Request $req, Response $res) use ($c) {
    $userRepository = new UserRepository($c['db']);
    $users = $userRepository->listAll();
    return $res->withJson($users);
});

Let's refactor a Slim 3 route

// snip
$c[UserRepository::class] = function($c) {
    return new UserRepository($c['db']);
};

$app->get('/', function(Request $req, Response $res) {
    $userRepository = $this->get(UserRepository::class);   
    $users = $userRepository->listAll();
    return $res->withJson($users);
});

Let's refactor a Slim 3 route

class GetUsersAction {
    public function __construct(
        private UserRepository $repo
    ) {}

    public function __invoke(Request $req, Response $res) {
            $users = $this->userRepository->listAll();
            return $res->withJson($users);
        }
    };
};

Let's refactor a Slim 3 route

$c['getUsers'] = function($c) {
    return new GetUsersAction($c[UserRepository::class]);
};

$app->get('/', 'getUsers');

PHP-DI

use function DI\create;

$c = new \DI\Container();
$c->set('LoggerInterface', create('Logger'));

// returns a NeedsALogger
var_dump($c->get('NeedsALogger'));

...wait, what?

• NeedsALogger has a param of type LoggerInterface in its constructor
• By default, PHP-DI comes with "autowiring"
• Autowiring uses PHP's built-in reflection to see parameter types
• We bound LoggerInterface to our Logger concrete implementation when configuring the container
• This gave PHP-DI enough info to build out the class

A Note on Autowiring

• PHP's built-in reflection system is rather slow
• Autowired containers generally cache dependency information to get around this, at least for production
• PHP-DI implements caching of autowiring information

PHP-DI - Cacheable Autowiring

use function DI\autowire;

$builder = new \DI\ContainerBuilder();
$builder->enableCompilation('/tmp');
$builder->addDefinitions([
    'LoggerInterface' => autowire('Logger'),
    'NeedsALogger' => autowire()
]);

$c = $builder->build();

var_dump($c->get('NeedsALogger')); // cached!

PHP-DI + FizzBuzz

require 'FizzBuzz.php';

$c = new \DI\Container();

foreach (range(1, 15) as $i) {
    echo $c->get('fizzBuzz')($i) . "\n";
}

Further reading for PHP-DI

• Annotations Attributes
• Lazy-loading via proxies
• Integration into Slim 4's skeleton app

$c = new League\Container\Container;
$c->share('LoggerInterface', 'Logger');

$c->add('myClass', 'NeedsALogger')
  ->addArgument('LoggerInterface');

var_dump($c->get('myClass')); // NeedsALogger

League Container

League container

• Use $c->share() if you do want the same instance every time you ask for a dependency.
• Use $c->add(callable) if you want to use a closure to build your dependency, a la Pimple.

// NeedsALogger __construct(LoggerInterface $log)

$c = new League\Container\Container;
$c->share('LoggerInterface', 'Logger');
$c->delegate(
    new League\Container\ReflectionContainer
);

// NeedsALogger w\Logger
var_dump($c->get('NeedsALogger'));

Autowiring With League Container

$c = new League\Container\Container;
$c->share('LoggerInterface', 'Logger')

$c->add('myClass', 'MightWantALogger')
  ->addMethodCall('setLogger',
                   ['LoggerInterface']);

Setter Injection With League Container

// MyClass, MyOtherClass implement
// LoggerAware, which has setLogger()

$c = new League\Container\Container;
$c->share('LoggerInterface', 'Logger')
$c->add('MyClass');
$c->add('MyOtherClass');

$c->inflector('LoggerAware')
  ->invokeMethod('setLogger',
                 ['LoggerInterface']);

League Container Inflectors

Thanks! Questions?

• https://ian.im/diuk23 - these slides
• https://ian.im/dicode - code samples
• https://phpc.social/@ian - me!
• https://twitter.com/iansltx - also me
• https://github.com/iansltx - my code!