framework/base/Event.php
public static function on($class, $name, $handler, $data = null, $append = true)
    {
        $class = ltrim($class, '\\');

        if (strpos($class, '*') !== false || strpos($name, '*') !== false) {
            if ($append || empty(self::$_eventWildcards[$name][$class])) {
                self::$_eventWildcards[$name][$class][] = [$handler, $data];
            } else {
                array_unshift(self::$_eventWildcards[$name][$class], [$handler, $data]);
            }
            return;
        }

        if ($append || empty(self::$_events[$name][$class])) {
            self::$_events[$name][$class][] = [$handler, $data];
        } else {
            array_unshift(self::$_events[$name][$class], [$handler, $data]);
        }
    }
public static function on($class, $name, $handler, $data = null, $append = true)
    {
        $class = ltrim($class, '\\');

        if (strpos($class, '*') !== false || strpos($name, '*') !== false) {
            if ($append || empty(self::$_eventWildcards[$name][$class])) {
                self::$_eventWildcards[$name][$class][] = [$handler, $data];
            } else {
                array_unshift(self::$_eventWildcards[$name][$class], [$handler, $data]);
            }
            return;
        }

        if ($append || empty(self::$_events[$name][$class])) {
            self::$_events[$name][$class][] = [$handler, $data];
        } else {
            array_unshift(self::$_events[$name][$class], [$handler, $data]);
        }
    }
Attaches an event handler to a class-level event.

When a class-level event is triggered, event handlers attached
to that class and all parent classes will be invoked.

For example, the following code attaches an event handler to `ActiveRecord`'s
`afterInsert` event:

```php
Event::on(ActiveRecord::className(), ActiveRecord::EVENT_AFTER_INSERT, function ($event) {
Yii::trace(get_class($event->sender) . ' is inserted.');
});
```

The handler will be invoked for EVERY successful ActiveRecord insertion.

Since 2.0.14 you can specify either class name or event name as a wildcard pattern:

```php
Event::on('app\models\db\*', '*Insert', function ($event) {
Yii::trace(get_class($event->sender) . ' is inserted.');
});
```

For more details about how to declare an event handler, please refer to [[Component::on()]].

@param string $class the fully qualified class name to which the event handler needs to attach.
@param string $name the event name.
@param callable $handler the event handler.
@param mixed $data the data to be passed to the event handler when the event is triggered.
When the event handler is invoked, this data can be accessed via [[Event::data]].
@param bool $append whether to append new event handler to the end of the existing
handler list. If `false`, the new handler will be inserted at the beginning of the existing
handler list.
@see off()