Object Oriented Programming in Swift

Ch 2: Polymorphism

Sai Li @ Yowoo Tech. 2016/12/23

References

• Agile Software Development: Principles, Patterns, and Practice ( : )by Robert C. Martin

• Fundamental Object Oriented Programming by Hsuan-Tien Lin

• by Teddy Chen

• Essential Object-Oriented Programmingby Josh Ko

OOP

• Encapsulation • Inheritance • Polymorphism

Polymorphism

One Thing, Many shapes….

Strawberry

Strawberry

Polymorphism in Swift

1. Class Inheritance 2. Protocol Interface

Fruit

Orange Strawberry

Playable

CD Piano

Polymorphism

Polymorphism means that the sender of a stimulus does not need to know the receiving instance’s class. The receiving instance can belong to an arbitrary class.

S.O.L.I.D

S.O.L.I.D

• SRP: Single Responsibility Principle

• OCP: Open Closed Principle• LSP: Liskov Substitution Principle • ISP: Interface Segregation Principle • DIP: Dependency Inversion Principle

Open Closed Principle

– Ivar Jacobson

“All systems change during their life cycles.

This must be born in mind when devloping systems expected to last longer than the first version.”

Open Closed Principle

Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification

- Bertrand Meyer (1988)

• Open for extension

• Closed for modification

Triangle?

• No ripple effect

• further changes of that kind are achieved by adding new code, not by changing old code that already works.

• Elastic

<Design Pattern > GoF

“Program to an interface,

not an implementation.”

What you can do rather than who you are.

Black Cat & White Cat

How ?

Barbara Liskov(1939 - )

• UCB, Stanford • 1st female CS PhD • CLU • von Neumann Award • Turing Award

Liskov Substitution Principle

Let Φ(x) be a property provable about objects x of type T.

Then Φ(y) should be true for objects y of type S where S is a subtype of T

Liskov Substitution Principle

Subtypes must be substitutable for their base types.

Liskov Substitution Principle

Subtypes must be substitutable for their base types.

Let Φ(x) be a property provable about objects x of type T. Then Φ(y) should be true for objects y of type S where S

is a subtype of T

( )

LSP Violation

p.s: Circle Ellipse Problem

LSP Violation

LSP Violation

Why?

Design By Contract

Class Invariant

Postcondition

Subtype func precondition can’t be stronger than base type

Subtype func postcondition can't be weaker than base type

Recap

• Polymorphism • Open Closed Principle • Liskov Substitution Principle

To be continued …

References

• https://zh.wikipedia.org/wiki/File:Barbara_Liskov.PNG

• http://www.relatably.com/m/img/wtf-memes/grandma-finds-the-internet-meme-generator-wtf-b12499.jpg

• http://i.imgur.com/BTpuqPX.jpg • wiki: https://en.wikipedia.org/wiki/Circle-

ellipse_problem