this pattern is used when we want to notify the changes of internal state of a class to other registered class

• Publisher - class that have state that when its change other concreteobserver class will notify
• ConcreteSubscriber - classes that registered in Publisher and will notify the changes of its state
• ConcreteState - class that take action on request diffrently from another concretestate class

• Subscriber - contains update method

image by refactoring guru