Stanford CS193p - Lecture 2 Notes

Introduction

• Lecture 2 of Stanford CS193p (Spring 2020)
• Topics covered:
  • MVVM (Model-View-ViewModel) design pattern
  • Swift type system

MVVM Overview

• MVVM is a design paradigm for organizing code in SwiftUI.

  • Necessary for SwiftUI development.
  • Different from MVC (Model-View-Controller used in UIKit).

• Model:

  • The backend of the app, UI independent.
  • Encapsulates data and logic (e.g., card data and matching logic).
  • Always considered the truth for the app's data.

• View:

  • Reflects the Model.
  • Stateless: Does not hold state, only displays the current state of the Model.
  • Declarative: Changes only when the Model changes.

• ViewModel:

  • Binds the View to the Model.
  • Interprets Model data for the View.
  • Notifies the View of changes in the Model.

Key Concepts of MVVM

1. Data Flow:

   • Data flows from the Model to the View.
   • The View displays the current state of the Model.

2. Reactive Programming:

   • The View updates automatically when the Model changes.
   • ViewModel notices changes and publishes updates.

3. User Intent:

   • The View communicates user actions to the ViewModel, which modifies the Model accordingly.
   • Example: User choosing a card in a memory game.

Swift Type System

• Important types in Swift:
  • Struct: Value type, gets a free initializer.
  • Class: Reference type, needs an explicit initializer.
  • Protocol: A blueprint of methods, properties, and other requirements.
  • Generics: Types that don't strictly define what the data type must be.
  • Functions: Can be treated as types.

Struct vs Class

• Struct:

  • Value type, copied when passed around.
  • No inheritance.
  • Free initializer for all properties.

• Class:

  • Reference type, passed by reference.
  • Supports single inheritance.
  • Requires explicit initializers for properties.

Generics and Functions

• Generics allow functions to accept any data type.
• Functions can be passed as parameters to other functions, supporting functional programming principles.

Practical Application in demo

• Implementing MVVM for a memory game application using SwiftUI.
• Card model and ViewModel examples.
• Use of @State and ObservableObject to handle state changes in SwiftUI.

Conclusion

• Homework Assignment: Reproduce the demo from the lecture.
  • Shuffle cards.
  • Implement a random number of card pairs.
• Encourage students to ask questions on Piazza.