When a subject needs to notify observers about something interesting happening, it broadcasts a notification to the observers (which can include specific data related to the topic of the notification).
When we no longer wish for a particular observer to be notified of changes by the subject they are registered with, the subject can remove them from the list of observers.
concept: GoF book --Design Patterns: Elements of Reusable Object-Oriented Software
"One or more observers are interested in the state of a subject and register their interest with the subject by attaching themselves. When something changes in our subject that the observer may be interested in, a notify message is sent which calls the update method in each observer. When the observer is no longer interested in the subject's state, they can simply detach themselves."
** 模范案例