yii2/docs/guide/concept-di-container.md

Dependency Injection Container

Both service locator and dependency injection are popular design patterns that allow building software in a loosely-coupled fashion. Yii uses service locator and dependency injection extensively, even though you may not be aware of them. In this tutorial, we will explore their implementation and support to help you write code more consciously. We also highly recommend you to read Martin's article to get a deeper understanding of service locator and dependency injection.

A dependency injection (DI) container is an object that knows how to instantiate and configure objects and all their dependent objects. Martin'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 class yii\di\Container. It supports the following kinds of dependency injection:

• Constructor injection;
• Setter injection;
• PHP callable injection.

Registering Dependencies

You can use yii\di\Container::set() to register dependencies. The registration requires a dependency name as well as a dependency definition. The name can be a class name, an interface name, or an alias name; and the definition can be a class name, a configuration array, or a PHP callable.

$container = new \yii\di\Container;

// register a class name as is. This can be skipped.
$container->set('yii\db\Connection');

// register an interface
// When a class depends on the interface, the corresponding class
// will be instantiated as the dependent object
$container->set('yii\mail\MailInterface', 'yii\swiftmailer\Mailer');

// register an alias name. You can use $container->get('foo')
// to create an instance of Connection
$container->set('foo', 'yii\db\Connection');

// register a class with configuration. The configuration
// will be applied when the class is instantiated by get()
$container->set('yii\db\Connection', [
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);

// register an alias name with class configuration
// In this case, a "class" element is required to specify the class
$container->set('db', [
    'class' => 'yii\db\Connection',
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);

// register a PHP callable
// The callable will be executed when $container->get('db') is called
$container->set('db', function ($container, $params, $config) {
    return new \yii\db\Connection($config);
});

Tip: If a dependency name is the same as the corresponding dependency definition, you do not need to register it with the DI container.

A dependency registered via set() will generate an instance each time the dependency is needed. You can use yii\di\Container::setSingleton() to register a dependency that only generates a single instance:

$container->setSingleton('yii\db\Connection', [
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);

Resolving Dependencies

Once you have registered dependencies, you can use the DI container to create new objects, and the container will automatically resolve dependencies by instantiating them and injecting them into the newly created objects. The dependency resolution is recursive, meaning that if a dependency has other dependencies, those dependencies will also be resolved automatically.

You use yii\di\Container::get() to create new objects. The method takes a class name or a dependency name (class name, interface name or alias name) that you previously registered via set() or setSingleton(). You may optionally provide a list of class constructor parameters and a list of name-value pairs to configure the newly created object. For example,

// equivalent to: $map = new \app\components\GoogleMap($apiKey);
$map = $container->get('app\components\GoogleMap', [$apiKey]);

// "db" is a previously registered alias name
$db = $container->get('db');

Behind the scene, the DI container does much more work than just creating a new object. The container will inspect the class constructor to find out dependent class or interface names and then automatically resolve those dependencies recursively.

The following code shows a more sophisticated example. The UserLister class depends on an object implementing the UserFinderInterface interface; the UserFinder class implements this interface and depends on a Connection object. All these dependencies are declared through type hinting of the class constructor parameters. With property dependency registration, the DI container is able to resolve these dependencies automatically and creates a new UserLister instance with a simple call of get('userLister').

namespace app\models;

use yii\base\Object;
use yii\db\Connection;
use yii\di\Container;

interface UserFinderInterface
{
    function findUser();
}

class UserFinder extends Object implements UserFinderInterface
{
    public $db;

    public function __construct(Connection $db, $config = [])
    {
        $this->db = $db;
        parent::__construct($config);
    }

    public function findUser()
    {
    }
}

class UserLister extends Object
{
    public $finder;

    public function __construct(UserFinderInterface $finder, $config = [])
    {
        $this->finder = $finder;
        parent::__construct($config);
    }
}

$container = new Container;
$container->set('yii\db\Connection', [
    'dsn' => '...',
]);
$container->set('app\models\UserFinderInterface', [
    'class' => 'app\models\UserFinder',
]);
$container->set('userLister', 'app\models\UserLister');

$lister = $container->get('userLister');

// which is equivalent to:

$db = new \yii\db\Connection(['dsn' => '...']);
$finder = new UserFinder($db);
$lister = new UserLister($finder);

Practical Usage

Yii creates a DI container when you include the yii.php file in your application's entry script. The DI container is accessible via Yii::$container. When you call Yii::createObject(), the method will actually call the container's yii\di\Container::get() method to create a new object. As aforementioned, the DI container will automatically resolve the dependencies (if any) and inject them into the newly created object. Because Yii uses Yii::createObject() in most of its core code to create new objects, this means you can customize the objects globally by dealing with Yii::$container.

For example, you can customize globally the default number of pagination buttons of yii\widgets\LinkPager:

\Yii::$container->set('yii\widgets\LinkPager', ['maxButtonCount' => 5]);

Now if you use the widget in a view with the following code, the maxButtonCount property will be initialized as 5 instead of 10 as defined in the class.

echo \yii\widgets\LinkPager::widget();

You can still override the value set via DI container:

echo \yii\widgets\LinkPager::widget(['maxButtonCount' => 20]);

Another example is to take advantage of the automatic constructor injection of the DI container. Assume your controller class depends on some other objects, such as a hotel booking service. You can declare the dependency through a constructor parameter and let the DI container to resolve it for you.

namespace app\controllers;

use yii\web\Controller;
use app\components\BookingInterface;

class HotelController extends Controller
{
    protected $bookingService;

    public function __construct($id, $module, BookingInterface $bookingService, $config = [])
    {
        $this->bookingService = $bookingService;
        parent::__construct($id, $module, $config);
    }
}

If you access this controller from browser, you will see an error complaining the BookingInterface cannot be instantiated. This is because you need to tell the DI container how to deal with this dependency:

\Yii::$container->set('app\components\BookingInterface', 'app\components\BookingService');

Now if you access the controller again, an instance of app\components\BookingService will be created and injected as the 3rd parameter to the controller's constructor.

When to Register Dependencies

Because dependencies are needed when new objects are being created, their registration should be done as early as possible. The followings are the recommended practices:

• If you are the developer of an application, you can register dependencies in your application's entry script or in a script that is included by the entry script.
• If you are the developer of a redistributable extension, you can register dependencies in the bootstrap class of the extension.