Child stores

modeprojection.swift

import SwiftUI
import Combine
import PlaygroundSupport

// MARK: Data Model
struct Book: Identifiable, Equatable {
    let id: Int
}

// Direct changes to DataModel should propagate
// to the view via "Books" ObservableObject
// but only for books change, not for all change
class DataModel: ObservableObject {
    @Published var audiobooks: [Book] = []

    @Published var books: [Book] = [] {
        didSet {
            print("data model did update: \(books)")
        }
    }
}

extension DataModel {
    var booksStore: Books {
        Books(self)
    }
}

// MARK: Books data projection for the view
// Books data projection of DataModel
class Books: ObservableObject {
    @Published var list: [Book]

    private var bag = Set<AnyCancellable>()
    init(_ dataModel: DataModel) {
        self.list = dataModel.books

        dataModel.$books
            .removeDuplicates()
            // The assign operator captures self in this case. 
            // Consider using sink with a [weak self] capture list,
            // or the assign to operator with weak ownership from CombineExt.
            .assign(to: \.list, on: self) 
            .store(in: &bag)

        $list
            .removeDuplicates()
            .assign(to: \.books, on: dataModel)
            .store(in: &bag)

         dataModel.objectWillChange
            .sink { _ in
                print("books store did update: \(self.list)")
            }
            .store(in: &bag)
    }
}

// MARK: View
struct ContentView: View {
    @EnvironmentObject var books: Books

    @ViewBuilder
    var body: some View {
        VStack {
            Button("add to observed object") {
                books.list.append(Book(id: books.list.count + 1))
            }
            Button("delete directly via model") {
                books.list = books.list.dropLast()
            }
            Divider()
            ForEach(books.list) { book in
                Text("\(book.id)")
            }
            Spacer()
        }
    }
}

let dataModel = DataModel()

let contentView = ContentView()
     .environmentObject(dataModel.booksStore)
PlaygroundPage.current.setLiveView(
    contentView
        .frame(width: 300, height: 300, alignment: .leading)
)