What is Registering Dependencies in Yii framework?

Yii framework - Interview Questions

You can use yii\di\Container::set() to register dependencies. The registration requires a dependency name as well as a dependency definition. A dependency name can be a class name, an interface name, or an alias name; and a dependency definition can be a class name, a configuration array, or a PHP callable.

$container = new \yii\di\Container;

// register a class name as is. This can be skipped.
$container->set('yii\db\Connection');

// register an interface
// When a class depends on the interface, the corresponding class
// will be instantiated as the dependent object
$container->set('yii\mail\MailInterface', 'yii\swiftmailer\Mailer');

// register an alias name. You can use $container->get('foo')
// to create an instance of Connection
$container->set('foo', 'yii\db\Connection');

// register an alias with `Instance::of`
$container->set('bar', Instance::of('foo'));

// register a class with configuration. The configuration
// will be applied when the class is instantiated by get()
$container->set('yii\db\Connection', [
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);

// register an alias name with class configuration
// In this case, a "class" or "__class" element is required to specify the class
$container->set('db', [
    'class' => 'yii\db\Connection',
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);

// register callable closure or array
// The callable will be executed each time when $container->get('db') is called
$container->set('db', function ($container, $params, $config) {
    return new \yii\db\Connection($config);
});
$container->set('db', ['app\db\DbFactory', 'create']);

// register a component instance
// $container->get('pageCache') will return the same instance each time it is called
$container->set('pageCache', new FileCache);

Tip: If a dependency name is the same as the corresponding dependency definition, you do not need to register it with the DI container.

A dependency registered via set() will generate an instance each time the dependency is needed. You can use yii\di\Container::setSingleton() to register a dependency that only generates a single instance :

$container->setSingleton('yii\db\Connection', [
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);