Behavioral : Observer Design Pattern

The Observer pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

The object which is being watched is called the subject.The objects which are watching the state changes are called observers or listeners.

in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems. The Observer pattern is also a key part in the familiar Model View Controller (MVC) architectural pattern.

The "View" part of Model-View-Controller.