Lecture 3: MVVM and the Swift type system | Summary and Q&A

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May 24, 2021
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Stanford
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Lecture 3: MVVM and the Swift type system

TL;DR

Learn how to apply the MVVM (Model-View-ViewModel) architecture to a memory game app, separating the user interface code from the logic and data of the game.

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Questions & Answers

Q: Why is the MVVM architecture important in SwiftUI?

In SwiftUI, the MVVM architecture is crucial as it provides a clear separation of concerns and facilitates the reactive updates of the user interface based on changes in the data model.

Q: How is the Model different from the View in MVVM?

The Model represents the data and logic of the application, while the View is responsible for presenting the data to the user. The Model is UI independent, while the View reflects the current state of the Model.

Q: How does the ViewModel facilitate communication between the Model and View?

The ViewModel binds the View to the Model, ensuring that changes in the Model trigger updates in the View. It also interprets the data from the Model and handles user intents, keeping the View code as simple as possible.

Q: Why is SwiftUI considered declarative programming?

SwiftUI follows a declarative programming approach, where the View is described in a body var that reflects the current state of the Model. This allows for easier code reading and writing, as well as more reliable and provable code.

Summary

In this video, the instructor discusses the MVVM design pattern and the Swift type system. The MVVM design pattern separates the user interface code (View) from the logic and data (Model) of the application. The ViewModel acts as the intermediary between the View and the Model, facilitating communication and reacting to changes in the Model. The instructor also explains the differences between structs and classes in Swift, with structs being value types and classes being reference types. Additionally, the instructor introduces generics in Swift, which allow for the creation of "don't care" types, such as arrays that can hold elements of any type.

Questions & Answers

Q: What is the MVVM design pattern and how does it differ from MVC?

The MVVM design pattern stands for Model-View-ViewModel. It separates the user interface code (View) from the logic and data (Model) of the application. MVVM is different from MVC (Model-View-Controller), which is how apps were built before SwiftUI. The key difference is that MVVM separates the View and the Model more clearly, with the ViewModel acting as the intermediary between them.

Q: What is the purpose of the Model in MVVM?

The Model in MVVM captures all the data and logic that describes "what" the application does. In the context of a card matching game, the Model includes the card data and the decision-making logic for matching cards. The Model is UI-independent and serves as the single source of truth about the game's data. This means that the View is always a reflection of the current state of the Model.

Q: How does the View reflect the state of the Model in MVVM?

The View in MVVM is stateless and only reflects the current state of the Model. The View's body var is responsible for constructing the UI based on the Model's state. It is always looking at the current state of the Model and returning a UI that reflects that state. The system asks for the value of the body var at any time, and the only way the View can change on-screen is by redrawing the whole thing.

Q: What is the difference between imperative and declarative coding?

Imperative coding involves calling functions to piece the UI together over time and modifying the UI as the user interacts with it. In this approach, developers need to anticipate every possible path through the code that could affect the UI state. Declarative coding, on the other hand, involves declaring what the View's UI looks like in its body var. The View is constructed all at once, and changes in the Model trigger a rebuild of the affected parts of the View. Declarative coding is more reliable and easier to write and read compared to imperative coding.

Q: How does the ViewModel facilitate communication between the View and the Model in MVVM?

The ViewModel's job is to bind the View to the Model so that changes in the Model cause the View to react and be rebuilt. The ViewModel tracks changes in the Model and publishes these change announcements to the Views that have subscribed to it. When a publishing event occurs, the affected Views are rebuilt by accessing their body vars, which reflect the new state of the Model. The ViewModel also serves as an interpreter between the Model and the View, providing the data and functionality that the View needs.

Q: How does Swift handle the differences between structs and classes?

Structs and classes have different behaviors in Swift. Structs are value types, meaning that each time they are passed or assigned, a copy is made. Structs have copy-on-write optimization, where copies are made only when modifications are made to the struct. Classes, on the other hand, are reference types, meaning that pointers to the class objects are passed and assigned. Swift uses automatic reference counting to keep track of how many pointers exist to a class object and clean up the memory when no pointers reference it.

Q: Why are structs preferred over classes in Swift?

Structs are preferred over classes in Swift because they follow a more functional programming approach, which leads to code that is easier to reason about and ensure correctness. Structs are immutable by default and do not have the potential pitfalls associated with shared mutable state that can arise from using classes. However, there are still cases where classes are useful, such as when there is a need for sharing data among multiple objects.

Q: What are "don't care" types in Swift?

"Don't care" types, also known as generics, allow for the creation of types in Swift that don't care about the specific type they contain. An example of this is the Array type, which can hold elements of any type. Generics enable code reusability and type safety by allowing functions and types to be written in a way that is agnostic to the actual types involved. The use of generics makes it possible to write flexible and reusable code that can handle various types.

Q: How are "don't care" types specified in Swift?

"Don't care" types are specified using angle brackets (<>) followed by the name of the "don't care" type. This is done when declaring a struct or function that wants to be generic. By specifying a generic type parameter, such as Element in the case of an Array, the type can be used without knowing or caring about the specific type it will contain. When using the generic type, the specific type must be provided, such as specifying Array<Int>.

Q: What features of Swift combine generics and protocols?

Swift combines generics and protocols to create powerful features such as creating generic protocols and associated types. This allows for defining protocols that can be implemented by multiple types, while still being specific about the associated types involved. Swift's generic protocols and associated types enable flexible and reusable code that can work with different types while maintaining type safety and ensuring that the code can be written in a way that is agnostic to the specific types involved.

Takeaways

The MVVM design pattern is a powerful way to organize code in SwiftUI, separating the user interface (View) from the logic and data (Model) of the application. The ViewModel acts as the intermediary between the View and the Model, facilitating communication and reacting to changes in the Model. Structs and classes have different behaviors in Swift, with structs being value types and classes being reference types. Structs are preferred over classes in most cases due to the benefits of immutability and functional programming. "Don't care" types, also known as generics, allow for the creation of types that don't care about the specific type they contain. Generics and protocols can be combined in Swift to create flexible and reusable code that can work with different types while maintaining type safety.

Summary & Key Takeaways

  • The MVVM architecture organizes code and separates the user interface code (View) from the logic and data (Model).

  • The Model captures all the data and logic of the application, while the View represents how the app is presented to the user.

  • The ViewModel acts as an intermediary between the Model and View, interpreting the data and facilitating communication.

  • The View is updated reactively based on changes in the Model, and the ViewModel handles user intents and updates the Model accordingly.

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