Dependency injection and container

Dependency injection

There are two ways of re-using things in OOP: inheritance and composition.

Inheritance is simple:

class Cache
{
    public function getCachedValue($key)
    {
        // ..
    }
}

class CachedWidget extends Cache
{
    public function render(): string
    {
        $output = $this->getCachedValue('cachedWidget');
        if ($output !== null) {
            return $output;
        }
        // ...        
    }
}

The issue here is that these two are becoming unnecessarily coupled or inter-dependent making them more fragile.

Another way to handle this is composition:

interface CacheInterface
{
    public function getCachedValue($key);
}

final class Cache implements CacheInterface
{
    public function getCachedValue($key)
    {
        // ..
    }
}

final class CachedWidget
{
    private CacheInterface $cache;

    public function __construct(CacheInterface $cache)
    {
        $this->cache = $cache;
    }
    
    public function render(): string
    {
        $output = $this->cache->getCachedValue('cachedWidget');
        if ($output !== null) {
            return $output;
        }
        // ...        
    }
}

We’ve avoided unnecessary inheritance and used interface to reduce coupling. You can replace cache implementation without changing CachedWidget so it’s becoming more stable.

The CacheInterface here is a dependency: an object another object depends on. The process of putting an instance of dependency into an object (CachedWidget) is called dependency injection. There are many ways to perform it:

• Constructor injection. Best for mandatory dependencies.
• Method injection. Best for optional dependencies.
• Property injection. Better to be avoided in PHP except maybe data transfer objects.

DI container

Injecting basic dependencies is simple and easy. You’re choosing a place where you don’t care about dependencies, which is usually an action handler, which you aren’t going to unit-test ever, create instances of dependencies needed and pass these to dependent classes.

It works well when there aren’t many dependencies overall and when there are no nested dependencies. When there are many and each dependency has dependencies itself, instantiating the whole hierarchy becomes a tedious process, which requires lots of code and may lead to hard to debug mistakes.

Additionally, lots of dependencies, such as certain third party API wrapper, are the same for any class using it. So it makes sense to:

• Define how to instantiate such API wrapper once.
• Instantiate it when required and only once per request.

That’s what dependency containers are for.

A dependency injection (DI) container is an object that knows how to instantiate and configure objects and all their dependent objects. Martin Fowler’s article has well explained why DI container is useful. Here we will mainly explain the usage of the DI container provided by Yii.

Yii provides the DI container feature through the yiisoft/di package and yiisoft/injector package.

Configuring container

Because to create a new object you need its dependencies, you should register them as early as possible. You can do it in the application configuration, config/web.php. For the following service:

class MyService implements MyServiceInterface
{
    public function __construct(int $amount)
    {
    }

    public function setDiscount(int $discount): void
    {
    
    }
}

configuration could be:

return [
    MyServiceInterface::class => [
        'class' => MyService::class,
        '__construct()' => [42],
        'setDiscount()' => [10],
    ],
];

That’s equal to the following:

$myService = new MyService(42);
$myService->setDiscount(10);

There are extra methods of declaring dependencies:

return [
    // declare a class for an interface, resolve dependencies automatically
    EngineInterface::class => EngineMarkOne::class,

    // array definition (same as above)
    'full_definition' => [
        'class' => EngineMarkOne::class,
        '__construct()' => [42], 
        '$propertyName' => 'value',
        'setX()' => [42],
    ],

    // closure
    'closure' => static function(ContainerInterface $container) {
        return new MyClass($container->get('db'));
    },

    // static call
    'static_call' => [MyFactory::class, 'create'],

    // instance of an object
    'object' => new MyClass(),
];

Injecting dependencies

Directly referencing container in a class is a bad idea since the code becomes non-generic, coupled to container interface and, what’s worse, dependencies are becoming hidden. Because of that, Yii inverts the control by automatically injecting objects from a container in some constructors and methods based on method argument types.

This is primarily done in constructor and handing method of action handlers:

use \Yiisoft\Cache\CacheInterface;

class MyController
{
    private CacheInterface $cache;

    public function __construct(CacheInterface $cache) {
        $this->cache = $cache;    
    }

    public function actionDashboard(RevenueReport $report)
    {
        $reportData = $this->cache->getOrSet('revenue_report', function() use ($report) {
            return $report->getData();               
        });

        return $this->render('dashboard', [
           'reportData' => $reportData,
        ]);
    }
}

Since it’s yiisoft/injector that instantiates and calls action handler, it checks the constructor and method argument types, get dependencies of these types from a container and pass them as arguments. That’s usually called auto-wiring. It happens for sub-dependencies as well, that’s if you don’t give dependency explicitly, container would check if it has such a dependency first. It’s enough to declare a dependency you need, and it would be got from a container automatically.

References

• Inversion of Control Containers and the Dependency Injection pattern by Martin Fowler