When the object referred to by the extended weak references has no live references to it, it will be stored in a serialized format. Thus, the target of an
ExtendedWeakRefernence object must be serializable. You use a selector and an identifier to identify a group of one or more weak references. The selector should be a type known only to the user of the
ExtendedWeakReference object. Note that you can use the same selector with multiple IDs to create groups of
ExtendedWeakReference objects.