yii2/docs/guide/structure-application-components.md

Application Components

Applications are service locators. They host a set of the so-called application components that provide different services for processing requests. For example, the urlManager component is responsible for routing Web requests to appropriate controllers; the db component provides DB-related services; and so on.

Each application component has an ID that uniquely identifies itself among other application components in the same application. You can access an application component through the expression:

\Yii::$app->componentID

For example, you can use \Yii::$app->db to get the yii\db\Connection, and \Yii::$app->cache to get the yii\caching\Cache registered with the application.

An application component is created the first time it is accessed through the above expression. Any further accesses will return the same component instance.

Application components can be any objects. You can register them by configuring the yii\base\Application::components property in application configurations. For example,

[
    'components' => [
        // register "cache" component using a class name
        'cache' => 'yii\caching\ApcCache',

        // register "db" component using a configuration array
        'db' => [
            'class' => 'yii\db\Connection',
            'dsn' => 'mysql:host=localhost;dbname=demo',
            'username' => 'root',
            'password' => '',
        ],

        // register "search" component using an anonymous function
        'search' => function () {
            return new app\components\SolrService;
        },
    ],
]

Info: While you can register as many application components as you want, you should do this judiciously. Application components are like global variables. Using too many application components can potentially make your code harder to test and maintain. In many cases, you can simply create a local component and use it when needed.

Bootstrapping Components

As mentioned above, an application component will only be instantiated when it is being accessed the first time. If it is not accessed at all during a request, it will not be instantiated. Sometimes, however, you may want to instantiate an application component for every request, even if it is not explicitly accessed. To do so, you may list its ID in the yii\base\Application::bootstrap property of the application.

You can also use Closures to bootstrap customized components. Returning an instantiated component is not required. A Closure can also be used simply for running code after yii\base\Application instantiation.

For example, the following application configuration makes sure the log component is always loaded:

[
    'bootstrap' => [
        'log',
        function($app){
            return new ComponentX();
        },
        function($app){
            // some code
           return;
        }
    ],
    'components' => [
        'log' => [
            // configuration for "log" component
        ],
    ],
]

Core Application Components

Yii defines a set of core application components with fixed IDs and default configurations. For example, the yii\web\Application::request component is used to collect information about a user request and resolve it into a route; the yii\base\Application::db component represents a database connection through which you can perform database queries. It is with help of these core application components that Yii applications are able to handle user requests.

Below is the list of the predefined core application components. You may configure and customize them like you do with normal application components. When you are configuring a core application component, if you do not specify its class, the default one will be used.

• yii\web\AssetManager: manages asset bundles and asset publishing. Please refer to the Assets section for more details.
• yii\db\Connection: represents a database connection through which you can perform DB queries. Note that when you configure this component, you must specify the component class as well as other required component properties, such as yii\db\Connection::dsn. Please refer to the Database Access Objects section for more details.
• yii\base\Application::errorHandler: handles PHP errors and exceptions. Please refer to the Handling Errors section for more details.
• yii\i18n\Formatter: formats data when they are displayed to end users. For example, a number may be displayed with thousand separator, a date may be formatted in long format. Please refer to the Data Formatting section for more details.
• yii\i18n\I18N: supports message translation and formatting. Please refer to the Internationalization section for more details.
• yii\log\Dispatcher: manages log targets. Please refer to the Logging section for more details.
• yii\swiftmailer\Mailer: supports mail composing and sending. Please refer to the Mailing section for more details.
• yii\base\Application::response: represents the response being sent to end users. Please refer to the Responses section for more details.
• yii\base\Application::request: represents the request received from end users. Please refer to the Requests section for more details.
• yii\web\Session: represents the session information. This component is only available in yii\web\Application. Please refer to the Sessions and Cookies section for more details.
• yii\web\UrlManager: supports URL parsing and creation. Please refer to the Routing and URL Creation section for more details.
• yii\web\User: represents the user authentication information. This component is only available in yii\web\Application. Please refer to the Authentication section for more details.
• yii\web\View: supports view rendering. Please refer to the Views section for more details.