observer

type Observable interface {
	data.Identifiable

	AddObserver(o Observer) error     // register an Observer to be kept in touch
	NotifyObservers() error           // notify observers a change occurred on content (using Observer.SignalChange)
	RemoveObserver(name string) error // deregister an Observer that will not be notified further
}

type Observer interface {
	data.Identifiable

	SignalChange(id string)  // is called by Observable to signal an Observer a change occurred
	MarkAsFreed(id string)   // is called by Observable to signal an Observer the content will not be used any more (decreasing the counter of uses)
	MarkAsDeleted(id string) // used to mark the Observable as deleted (allowing to remove the entry from the Observer internals)
}