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 exists as only a single instance, 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 from a 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`, and
`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 with it. A component can be registered
via [[yii\di\ServiceLocator::set()]]. The following code shows different ways of registering components:

```php
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 "search" 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 has been registered, you can access it using its ID, in one of the two following ways:

```php
$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](concept-configurations.md),
a writable property named [[yii\di\ServiceLocator::setComponents()|components]] is provided. This allows you to configure and register multiple components at once. The following code shows a configuration array
that can be used to configure an application, while also registering the "db", "cache" and "search" components:

```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;
        },
    ],
];
```
