return [
    // ...
    'modules' => [
        'notifications' => [
            'class' => 'machour\yii2\notifications\NotificationsModule',
            // Point this to your own Notification class
            // See the "Declaring your notifications" section below
            'notificationClass' => 'app\models\Notification',
            // This callable should return your logged in user Id
            'userId' => function() {
                return \Yii::$app->user->id;
            }
        ],
        // your other modules ..
    ],
    // ...
]

namespace backend\components;

use Yii;
use common\models\Meeting;
use common\models\Message;
use machour\yii2\notifications\models\Notification as BaseNotification;

class Notification extends BaseNotification
{

    /**
     * A new message notification
     */
    const KEY_NEW_MESSAGE = 'new_message';
    /**
     * A meeting reminder notification
     */
    const KEY_MEETING_REMINDER = 'meeting_reminder';
    /**
     * No disk space left !
     */
    const KEY_NO_DISK_SPACE = 'no_disk_space';

    /**
     * @var array Holds all usable notifications
     */
    public static $keys = [
        self::KEY_NEW_MESSAGE,
        self::KEY_MEETING_REMINDER,
        self::KEY_NO_DISK_SPACE,
    ];

    /**
     * @inheritdoc
     */
    public function getTitle()
    {
        switch ($this->key) {
            case self::KEY_MEETING_REMINDER:
                return Yii::t('app', 'Meeting reminder');

            case self::KEY_NEW_MESSAGE:
                return Yii::t('app', 'You got a new message');
                
            case self::KEY_NO_DISK_SPACE:
                return Yii::t('app', 'No disk space left');
        }
    }

    /**
     * @inheritdoc
     */
    public function getDescription()
    {
        switch ($this->key) {
            case self::KEY_MEETING_REMINDER:
                $meeting = Meeting::findOne($this->key_id);
                return Yii::t('app', 'You are meeting with {customer}', ['customer' => $meeting->customer->name]);

            case self::KEY_NEW_MESSAGE:
                $message = Message::findOne($this->key_id);
                return Yii::t('app', '{customer} sent you a message', ['customer' => $meeting->customer->name]);

            case self::KEY_NO_DISK_SPACE:
                // We don't have a key_id here
                return 'Please buy more space immediately';
        }
    }

    /**
     * @inheritdoc
     */
    public function getRoute()
    {
        switch ($this->key) {
            case self::KEY_MEETING_REMINDER:
                return ['meeting', 'id' => $this->key_id];

            case self::KEY_NEW_MESSAGE:
                return ['message/read', 'id' => $this->key_id];

            case self::KEY_NO_DISK_SPACE:
                return 'https://aws.amazon.com/';
        };
    }

}

// $message was just created by the logged in user, and sent to $recipient_id
Notification::notify(Notification::KEY_NEW_MESSAGE, $recipient_id, $message->id);

// You may also use the following static methods to set the notification type:
Notification::warning(Notification::KEY_NEW_MESSAGE, $recipient_id, $message->id);
Notification::success(Notification::ORDER_PLACED, $admin_id, $order->id);
Notification::error(Notification::KEY_NO_DISK_SPACE, $admin_id);

use machour\yii2\notifications\widgets\NotificationsWidget;

NotificationsWidget::widget([
    'theme' => NotificationsWidget::THEME_GROWL,
     // If the notifications count changes, the $('.notifications-count') element
     // will be updated with the current count
    'counters' => ['.notifications-count'],
    'clientOptions' => [
        'size' => 'large',
    ],
]);