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 and property injection;
- PHP callable injection.
The DI container supports constructor injection with the help of type hints for constructor parameters. The type hints tell the container which classes or interfaces are dependent when it is used to create a new object. The container will try to get the instances of the dependent classes or interfaces and then inject them into the new object through the constructor. For example,
class Foo
{
public function __construct(Bar $bar)
{
}
}
$foo = $container->get('Foo');
// which is equivalent to the following:
$bar = new Bar;
$foo = new Foo($bar);Setter and property injection is supported through configurations. When registering a dependency or when creating a new object, you can provide a configuration which will be used by the container to inject the dependencies through the corresponding setters or properties. For example,
use yii\base\Object;
class Foo extends Object
{
public $bar;
private $_qux;
public function getQux()
{
return $this->_qux;
}
public function setQux(Qux $qux)
{
$this->_qux = $qux;
}
}
$container->get('Foo', [], [
'bar' => $container->get('Bar'),
'qux' => $container->get('Qux'),
]);In this case, the container will use a registered PHP callable to build new instances of a class. The callable is responsible to resolve the dependencies and inject them appropriately to the newly created objects. For example,
$container->set('Foo', function () {
return new Foo(new Bar);
});
$foo = $container->get('Foo');You can use [[yii\di\Container::set()]] to register dependencies. The registration requires a dependency name as well as a dependency definition. A dependency name can be a class name, an interface name, or an alias name; and a dependency 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 each time when $container->get('db') is called
$container->set('db', function ($container, $params, $config) {
return new \yii\db\Connection($config);
});
// register a component instance
// $container->get('pageCache') will return the same instance each time it is called
$container->set('pageCache', new FileCache);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',
]);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 can use [[yii\di\Container::get()]] to create new objects. The method takes a dependency name,
which can be a class name, an interface name or an alias name. The dependency name may or may
not be registered via set() or setSingleton(). You may optionally provide a list of class
constructor parameters and a configuration to configure the newly created object.
For example,
// "db" is a previously registered alias name
$db = $container->get('db');
// equivalent to: $engine = new \app\components\SearchEngine($apiKey, ['type' => 1]);
$engine = $container->get('app\components\SearchEngine', [$apiKey], ['type' => 1]);Behind the scene, the DI container does much more work than just creating a new object. The container will first 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);Yii creates a DI container when you include the Yii.php file in the entry script
of your application. The DI container is accessible via [[Yii::$container]]. When you call [[Yii::createObject()]],
the method will actually call the container's [[yii\di\Container::get()|get()]] method to create a new object.
As aforementioned, the DI container will automatically resolve the dependencies (if any) and inject them
into the newly created object. Because Yii uses [[Yii::createObject()]] in most of its core code to create
new objects, this means you can customize the objects globally by dealing with [[Yii::$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 the default value 10 as defined in the class.
echo \yii\widgets\LinkPager::widget();You can still override the value set via DI container, though:
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.
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 bootstrapping class of the extension.
Both dependency injection and service locator are popular design patterns that allow building software in a loosely-coupled and more testable fashion. We highly recommend you to read Martin's article to get a deeper understanding of dependency injection and service locator.
Yii implements its service locator on top of the dependency injection (DI) container. When a service locator is trying to create a new object instance, it will forward the call to the DI container. The latter will resolve the dependencies automatically as described above.