concept-service-locator.md

Service Locator

A service locator is an object that knows how to provide all sorts of services (or components) that an application might need. Within a service locator, each component has only a single instance which is uniquely identified by an ID. You use the ID to retrieve a component from the service locator.

In Yii, a service locator is simply an instance of [[yii\di\ServiceLocator]] or its child class.

The most commonly used service locator in Yii is the application object which can be accessed through \Yii::$app. The services it provides are called application components, such as the request, response, urlManager components. You may configure these components or even replace them with your own implementations easily through functionality provided by the service locator.

Besides the application object, each module object is also a service locator.

To use a service locator, the first step is to register components. A component can be registered via [[yii\di\ServiceLocator::set()]]. The following code shows different ways of registering components:

use yii\di\ServiceLocator;
use yii\caching\FileCache;

$locator = new ServiceLocator;

// register "cache" using a class name that can be used to create a component
$locator->set('cache', 'yii\caching\ApcCache');

// register "db" using a configuration array that can be used to create a component
$locator->set('db', [
    'class' => 'yii\db\Connection',
    'dsn' => 'mysql:host=localhost;dbname=demo',
    'username' => 'root',
    'password' => '',
]);

// register "db" using an anonymous function that builds a component
$locator->set('search', function () {
    return new app\components\SolrService;
});

// register "pageCache" using a component
$locator->set('pageCache', new FileCache);

Once a component is registered, you can access it using its ID in one of the following two ways:

$cache = $locator->get('cache');
// or alternatively
$cache = $locator->cache;

As shown above, [[yii\di\ServiceLocator]] allows you to access a component like a property using the component ID. When you access a component for the first time, [[yii\di\ServiceLocator]] will use the component registration information to create a new instance of the component and return it. Later if the component is accessed again, the service locator will return the same instance.

You may use [[yii\di\ServiceLocator::has()]] to check if a component ID has already been registered. If you call [[yii\di\ServiceLocator::get()]] with an invalid ID, an exception will be thrown.

Because service locators are often being created with configurations, a writable property named [[yii\di\ServiceLocator::setComponents()|components]] is provided so that you can configure it and register multiple components at once. The following code shows a configuration array that can be used to configure an application and register the "db", "cache" and "search" components:

return [
    // ...
    'components' => [
        'db' => [
            'class' => 'yii\db\Connection',
            'dsn' => 'mysql:host=localhost;dbname=demo',
            'username' => 'root',
            'password' => '',
        ],
        'cache' => 'yii\caching\ApcCache',
        'search' => function () {
            return new app\components\SolrService;
        },
    ],
];