Observer-Design-Pattern.md

Observer-Design-Pattern

Problem Statement :

Sometimes when one or more objects need to observe something which gets updated frequently, then observer pattern can be used.

Observer Design Pattern

The Observer Pattern defines a one to many dependency between objects so that one object changes state, all of its dependents are notified and updated automatically. It consists of 2 major component :

• Observable (Something that changes its state)
• Observers (Something that needs to get updated whenever Observable changes its state)

Points to Note :

• Both Observer and Observable are Interfaces
• Concrete implementation of Observer needs to have 'is-a' relationship with Observer
• Concrete implementation of Observer should have 'has-a' relationship with concrete implementation of Observable which should be defined during instantiation using Dependency injection
• In case of one observer having multiple observable, that observer should specify those (concrete implementation of) observables in the dependency injection (using constructor) and the update() method should be parameterised as : update(IObservable observable) and the to know which concrete implementation of Observable had notified the observer, use instanceof condition.

For the implememtation, check out the following repo : Observer Design Pattern.