Shout out to @jcarouth and his presentation back in 2015
Dependencies are the external objects, resources, or functions an object uses to accomplish its responsibility.
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);
}
}
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);
}
}
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);
}
}
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);
}
}
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.
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.
class ClassThatNeedsLogging { private $logger; public function __construct(Logger $logger) { $this->logger = $logger; } }
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 */}
"I know, I'll just throw everything in an $options array!"
class ClassThatUsesLoggingButDoesNotRequireIt { private $logger = null; public function setLogger(Logger $logger) { $this->logger = $logger; } }
class ClassThatMightWantAThingCalledALogger { public $logger = null; }
class SlightlySaferClassThatWantsALogger { public Logger $logger = null; } // PHP 7.4+
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'); }
}
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'); }
}
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 **/ } }
e.g. Psr\Log\{LoggerAwareInterface, LoggerAwareTrait}
Sorry, you can't have an interface implement a trait directly.
public function __invoke( \Slim\Http\Request $request, \Slim\Http\Response $response, array $args = []) { /** do something, return a Response **/ }
public function orderAction( \Illuminate\Http\Request $request, \App\Services\OrderService $orderService) { /** do something, return a Response **/ }
* Some frameworks take care of this for you for controllers/commands
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.
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.
class Twilio implements SmsProvider {}
class NexmoSMS implements SmsProviderInterface {}
class Twilio implements SmsProvider {}
class NexmoSMS implements SmsProviderInterface {}
Doesn't matter either way. Pick a convention and stick with it.
class Camry implements HasGasPedal { public function pressAccelerator(); } // namespace Toyota\Vehicles class Model3 implements HasGasPedal { public function pressAccelerator(); } // namespace Tesla\Vehicles
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 **/ }
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);
}
}
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);
}
}
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.
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);
}
}
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);
}
}
class Container { protected $s=array(); function __set($k, $c) { $this->s[$k]=$c; } function __get($k) { return $this->s[$k]($this); } }
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
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]); } }
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;
}
}
$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";
}
$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
$app = new Slim\App();
$app->get('/', function(Request $req, Response $res) {
$userRepository = new UserRepository(new PDO(/* */));
$users = $userRepository->listAll();
return $res->withJson($users);
});
$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); });
// 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);
});
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);
}
};
};
$c['getUsers'] = function($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
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']);