onAdminSave should provide field's information

I've created a custom field intended for admin-side. The value POSTed by the widget is a `JSON.stringify()` in a hidden field, but I want to process it (mostly `json_decode()`).

For this, I hook into `onAdminSave()`.
Eg:
```php
public function onAdminSave(Event $event)
{
if (isset($event['object']->header()->form['field_selection'])) {
$event['object']->header()->form['field_selection'] = json_decode($event['object']->header()->form['field_selection'], true);
}
}
```

But this processing is bound on hardcoding the field **name**. I'd like to be able to process all instance of my-field whatever their name is.