Service Locator

Note: This chapter needs cleanup.

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 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 request, response, urlManager. You may configure these components or replace them with your own implementations easily through functionality provided 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:

$locator = new \yii\di\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;
});

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 configured using configuration arrays, a method named yii\di\ServiceLocator::setComponents() is provided to allow registering components in configuration arrays. The method is a setter which defines a writable property components that can be configured. 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;
        },
    ],
];

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Service Locator

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