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Service Locator
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===============
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> Note: This chapter needs cleanup.
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Both service locator and dependency injection are popular design patterns that allow building software
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in a loosely-coupled fashion. Yii uses service locator and dependency injection extensively,
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even though you may not be aware of them. In this tutorial, we will explore their implementation
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and support to help you write code more consciously. We also highly recommend you to read
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[Martin's article](http://martinfowler.com/articles/injection.html) to get a deeper understanding of
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service locator and dependency injection.
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A service locator is an object that knows how to provide all sorts of services (or components) that an application
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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
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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::$app`. The services it provides are called *application components*, such as `request`, `response`,
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`urlManager`. You may configure these components or replace them with your own implementations easily
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through functionality provided the service locator.
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Besides the application object, each module object is also a service locator.
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To use a service locator, the first step is to register components. A component can be registered
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via [[yii\di\ServiceLocator::set()]]. The following code shows different ways of registering components:
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```php
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$locator = new \yii\di\ServiceLocator;
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// register "cache" using a class name that can be used to create a component
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$locator->set('cache', 'yii\caching\ApcCache');
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// register "db" using a configuration array that can be used to create a component
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$locator->set('db', [
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'class' => 'yii\db\Connection',
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'dsn' => 'mysql:host=localhost;dbname=demo',
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'username' => 'root',
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'password' => '',
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]);
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// register "db" using an anonymous function that builds a component
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$locator->set('search', function () {
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return new app\components\SolrService;
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});
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```
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Once a component is registered, you can access it using its ID in one of the following two ways:
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```php
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$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.
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When you access a component for the first time, [[yii\di\ServiceLocator]] will use the component registration
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information to create a new instance of the component and return it. Later if the component is accessed again,
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the service locator will return the same instance.
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You may use [[yii\di\ServiceLocator::has()]] to check if a component ID has already been registered.
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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 configured using configuration arrays, a method named
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[[yii\di\ServiceLocator::setComponents()]] is provided to allow registering components in configuration arrays.
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The method is a setter which defines a writable property `components` that can be configured.
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The following code shows a configuration array that can be used to configure an application and register
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the "db", "cache" and "search" components:
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```php
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return [
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// ...
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'components' => [
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'db' => [
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'class' => 'yii\db\Connection',
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'dsn' => 'mysql:host=localhost;dbname=demo',
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'username' => 'root',
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'password' => '',
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],
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'cache' => 'yii\caching\ApcCache',
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'search' => function () {
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return new app\components\SolrService;
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},
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],
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];
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```
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