Observer - Harsh4999/Design-Patterns GitHub Wiki

Observer

USE

Using observer we can notify multiple objects about state change
The object which are intrested in knowing about state change are called as Observer's
Its all called as publisher-subscriber design patter (pub-sub model)
Single object will publish its state and multiple subscriber
Object communication without tightly coupling

Subject: Interface for registering observers. Supports multiple observers it is also called as observable
Concrete Subject: Implementation of subject, sends notification to observers when state changes
Observers: Interface for objects that want notification when state changes
Concrete Observers: Implementation of observers, maintains referce to concrete subject

Implement Listners/Observers
Implement Subject/Observables
In subject implementation we take care of registration and deregistration, notification and state info methods
Observers will have an reference to Subject which they will use for notification service
Look closely and avoid a circular update loop which may lead to infinity
Observer can listen for changes in multiple subjects at a time. It becomes quite easy to find origincator for notification.
Performance cost may be high depending on notification list size
Eg: Util class Observer interface and Observable. HttpSessionListener & addListner in servlet context