Fine-grained @observable interactions vs coarse-grained decoding

fine-grained-observable.swift

// With Observable

@Observable
class Book {
    var title: String
    var author: String
    var inStock: Bool

    init(title: String, author: String, inStock: Bool) {
        self.title = title
        self.author = author
        self.inStock = inStock
    }
}

struct Parent: View {
    @State var book: Book

    var body: some View {
        MyView(book: book)
            .onAppear {
                // Will not cause an update to MyView
                book.author = "Me"
            }
    }
}

struct MyView: View {
    let book: Book

    var body: some View {
        // Only updates when book.title changes
        // Doesn't update when book.author or book.inStock change
        Text(book.title)
    }
}


// With @State

class Book {
    var title: String
    var author: String
    var inStock: Bool

    init(title: String, author: String, inStock: Bool) {
        self.title = title
        self.author = author
        self.inStock = inStock
    }
}

struct Parent: View {
    @State var book: Book

    var body: some View {
        MyView(book: book)
            .onAppear {
                // Will cause an update to MyView
                book.author = "Me"
            }
    }
}

struct MyView: View {
    let book: Book

    var body: some View {
        // Updates any time `book` or any of its properties changes
        Text(book.title)
    }
}

// With @Observable and Decodable

@Observable
class Book: Decodable {
    var title: String
    var author: String
    var inStock: Bool

    init(title: String, author: String, inStock: Bool) {
        self.title = title
        self.author = author
        self.inStock = inStock
    }
}

struct Parent: View {
    @State var book: Book

    var body: some View {
        MyView(book: book)
            .task {
                // You're only ever replacing the entire book
                book = await fetchBook()
            }
    }

    func fetchBook() async -> Book {...}
}

struct MyView: View {
    let book: Book

    var body: some View {
        // Updates any time the whole book is fetched
        Text(book.title)
    }
}