State Design Pattern in GOF Design Patterns

State Design Pattern. One of the Gang of Four Design Patterns. Also known as State Design Pattern.

1. State Pattern Software Design and Architecture Presented by Khadija Noor BSEF20A512 Tania BSEF20A508

2. Agenda What is State Pattern? Relation to Finite State Machines Structure Pros and Cons Applicability and Use cases Pseudocode Notable Usage

3. What is the State Design Pattern? The State Design Pattern is a behavioral design pattern that allows an object to change its behavior when its internal state changes. This pattern is particularly useful when an object’s behavior depends on its state, and the state can change during the object’s lifecycle. The object will appear to change its class. It is also known as Object for States .

4. Relation to the Finite State Machine The State pattern is closely related to the concept of a  Finite-State Machine . The main idea is that, at any given moment, there’s a  finite number of states   in which a program can be in. Within any unique state, the program behaves differently, and the program can be switched from one state to another instantaneously. However, depending on the current state, the program may or may not switch to certain other states. These switching rules, called  transitions , are also finite and predetermined.

5. Motivation Imagine that we have a Document class. A document can be in one of three states: Draft, Moderation, and Published. The publish method of the document works a little bit differently in each state: In Draft, it moves the document to moderation. In Moderation, it makes the document public, but only if the current user is an administrator. In Published, it doesn’t do anything at all.

6. State Machine based Solution State machines are usually implemented with lots of conditional statements (if or switch) that select the appropriate behavior depending on the current state of the object Issue in this Approach: Most methods will contain monstrous conditionals that pick the proper behavior of a method according to the current state. Code like this is very difficult to maintain because any change to the transition logic may require changing state conditionals in every method. The problem tends to get bigger as a project evolves.

7. State Pattern Based Solution The State pattern suggests that you create new classes for all possible states of an object and extract all state-specific behaviors into these classes. Instead of implementing all behaviors on its own, the original object, called context, stores a reference to one of the state objects that represents its current state and delegates all the state-related work to that object.

8. Structure of the State Pattern

9. Pros and Cons

10. Applicability and its Use Cases

11. Applicability

12. Traffic Light Transitions

13. Gaming Transitions

14. Pseudocode

15. Context Class

16. State Interface

17. Concrete States

18. Ready State

19. Playing State

20. Use of State Design Pattern in TCP/IP Ralph Johnson and Jonathan Zweig characterize the State pattern and its application to TCP connection protocols

21. References Design Patterns: Elements of Reusable Object-Oriented Software Ralph E. Johnson and Jonathan Zweig. Delegation in C++.  Journal of Object-Oriented Programming , 4(11):22–35, November 1991. https://refactoring.guru/design-patterns/state https://www.geeksforgeeks.org/state-design-pattern/ https://www.ashishvishwakarma.com/GoF-Design-Patterns-by-Example/State-Pattern/ https://medium.com/well-red/state-machines-for-everyone-part-1-introduction-b7ac9aaf482e https://networkinterview.com/11-states-of-tcp-transition-explained/