concept-service-locator.md 3.31 KB
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Service Locator
===============

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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.
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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.
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The most commonly used service locator in Yii is the *application* object which can be accessed through
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`\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
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through functionality provided by the service locator.
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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:

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

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

Once a component is registered, you can access it using its ID in one of the following two ways:
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```php
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$cache = $locator->get('cache');
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// or alternatively
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$cache = $locator->cache;
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```

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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.


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Because service locators are often being created with [configurations](concept-configurations.md),
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:
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```php
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;
        },
    ],
];
```