Dependency Injection
for Mere Humans

SunshinePHP 2019

Ian Littman / @iansltx

follow along at https://ian.im/disun19

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

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 a testing 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;

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

Pros

  • Makes dependencies explicit
  • Can't 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(
    Twig $view,
    AuthorizationServer $auth,
    LogRepository $errorRepo,
    AuthRequestRepository $authRequestRepo,
    ClientRepository $clientRepo,
    UserRepository $userRepo,
    ResourceServer $resAuth,
    AccessTokenRepository $accessTokenRepo,
    UserValidatorInterface $validator
) {/* a whole bunch of property assigns */}

Antipattern #1: __construct($options)

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

Setter injection

class ClassThatUsesLoggingButDoesNotRequireIt {
    private $logger = null;

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

Yolo Property injection

class ClassThatMightWantAThingCalledALogger {
    public $logger = null;
}

 

class SlightlySaferClassThatWantsALogger {
    public Logger $logger = null;
} // PHP 7.4+

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 **/
    }
}

Don't Implement This Null Logger...

...because there's one built into PSR/log

Traits + Setter Injection

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

 

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

 

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

Parameter Injection

public function __invoke(
    \Slim\Http\Request $request,
    \Slim\Http\Response $response,
    array $args = [])
{
    /** do something, return a Response **/
}

Parameter Injection

public function orderAction(
    \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.

Should I use "interface" in my interface names?

class Twilio implements SmsProvider {}
class NexmoSMS implements SmsProviderInterface {}

Should I use "interface" in my interface names?

class Twilio implements SmsProvider {}
class NexmoSMS implements SmsProviderInterface {}

 

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

Abstractions should not be leaky

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

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

Abstractions should not be leaky

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

class ElasticUserRepo 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);
    }
}

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

PSR-11 (fka Container-Interop)

namespace Psr\Container;

interface ContainerInterface {
    public function get($id);
    public function has($id);
}
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

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

ANTIPATTERN #2: Service Location

Twittee: A container in a 140 tweet

class Container {
 protected $s=array();
 function __set($k, $c) { $this->s[$k]=$c; }
 function __get($k) { return $this->s[$k]($this); }
}
    

Using Twittee

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

$c = new Container;
$c->logger = function() { return new Logger; };
$c->myService = function($c) {
    return new NeedsALogger($c->logger);
};

var_dump($c->myService); // includes the Logger

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

class Container implements Psr\Container\ContainerInterface {
 protected $s = [];
 function __set($k, $c) { $this->s[$k]=$c; }
 function __get($k) { return $this->s[$k]($this); }
 function get($k) { return $this->s[$k]($this); }
 function has($k) { return isset($s[$k]); }
}

Dependency Injected Fizzbuzz

class Fizz { public function __toString() { return 'Fizz'; } }
class Buzz { public function __toString() { return 'Buzz'; } }
class FizzBuzz {
    protected $fizz; protected $buzz;
    public function __construct(Fizz $fizz, Buzz $buzz) {
        $this->fizz = $fizz; $this->buzz = $buzz;
    }
    public function __invoke(int $i) {
        if (!($i % 15)) return $this->fizz . $this->buzz;
        if (!($i % 3)) return (string) $this->fizz;
        if (!($i % 5)) return (string) $this->buzz;
        return (string) $i;
    }
}

Dependency Injected Fizzbuzz

$c = new Container;
$c->fizz = function() { return new Fizz; };
$c->buzz = function() { return new Buzz; };
$c->fizzBuzz = function($c) {
    return new FizzBuzz($c->fizz, $c->buzz);
};


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

...but don't use Twittee++

  • Every major framework has one
  • Standalone ones for use elsewhere
    • Pimple (used in Slim)
    • League\Container
    • Aura.Di
    • Disco

...but don't use Twittee++

  • Every major framework has one
  • Standalone ones for use elsewhere
    • Pimple (used in Slim)
    • League\Container
    • Aura.Di
    • Disco
$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 add 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

Let's refactor a Slim 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 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 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 route

class GetUsersAction {
    protected $userRepository;

    public function __construct(UserRepository $repo) {
        $this->userRepository = $repo;
    }

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

Let's refactor a Slim route

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

$app->get('/', 'getUsers');
$c = new League\Container\Container;
$c->share('LoggerInterface', 'Logger')
$c->add('myClass', 'NeedsALogger')
  ->addArgument('LoggerInterface');

 

var_dump($c->get('myClass')); // NeedsALogger
  • 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.
  • Check out jenssegers/lean to use with Slim

// NeedsALogger __construct(LoggerInterface $log)

$c = new League\Container\Container;
$c->share('LoggerInterface', 'Logger')
$c->delegate(
    new League\Container\ReflectionContainer
);
$c->get('NeedsALogger'); // NeedsALogger w\Logger
$c = new League\Container\Container;
$c->share('LoggerInterface', 'Logger')
$c->add('myClass', 'MightWantALogger')
  ->addMethodCall('setLogger',
                   ['LoggerInterface']);
// 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']);

Thanks! Questions?

Dependency Injection for Mere Humans - SunshinePHP 2019

By Ian Littman

Dependency Injection for Mere Humans - SunshinePHP 2019

What’s the difference between service location and dependency injection? Why is this dependency injection thing such a big deal anyway, and how do you use that tool correctly? I’ll answer these questions and more, including real-world examples of refactoring an application toward the more explicit, testable, closer-to-SOLID applications.

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