LandmarkDetail.swift

import Combine
import SwiftUI

struct IndexedCollection<Base: RandomAccessCollection>: RandomAccessCollection {
    typealias Index = Base.Index
    typealias Element = (index: Index, element: Base.Element)

    let base: Base

    var startIndex: Index { base.startIndex }

    var endIndex: Index { base.endIndex }

    func index(after i: Index) -> Index {
        base.index(after: i)
    }

    func index(before i: Index) -> Index {
        base.index(before: i)
    }

    func index(_ i: Index, offsetBy distance: Int) -> Index {
        base.index(i, offsetBy: distance)
    }

    subscript(position: Index) -> Element {
        (index: position, element: base[position])
    }
}

extension RandomAccessCollection {
    func indexed() -> IndexedCollection<Self> {
        IndexedCollection(base: self)
    }
}

struct Landmark: Identifiable {
    var id: Int
    var name: String
    var isFavorite: Bool = false
}

let landmarkData = [
    Landmark(id: 1, name: "A"),
    Landmark(id: 2, name: "B"),
    Landmark(id: 3, name: "C"),
    Landmark(id: 4, name: "D"),
]

final class UserData: ObservableObject  {
    @Published var showFavoritesOnly = false
    @Published var landmarks = landmarkData
}

struct LandmarkDetail: View {
    @Binding var landmark: Landmark

    var body: some View {
        Toggle(landmark.name, isOn: $landmark.isFavorite)
    }
}

struct LandmarkList: View {
    @Binding var landmarks: [Landmark]
    @Binding var showFavoritesOnly: Bool

    var filteredLandmarks: Binding<[Landmark]> {
        Binding<[Landmark]>(get: {
            self.landmarks.filter { !(self.showFavoritesOnly && !$0.isFavorite) }
        }, set: { newValue in
            self.landmarks = newValue
        })
    }

    func bindingLandmark(for landmarks: Binding<[Landmark]>, index: Int) -> Binding<Landmark> {
        Binding(get: {
            landmarks.value[index]
        }, set: { newValue in
            var tmp = landmarks.value
            tmp[index] = newValue
            landmarks.value = tmp
        })
    }

    var body: some View {
        List(filteredLandmarks.value.indexed(), id: \.1.id) { (index, landmark) in
            NavigationLink(
                landmark.name + (landmark.isFavorite ? "★" : ""),
                destination: LandmarkDetail(landmark: self.bindingLandmark(for: self.filteredLandmarks, index: index))
            )
        }
    }
}

struct ContentView: View {
    @EnvironmentObject var userData: UserData

    var body: some View {
        NavigationView {
            LandmarkList(landmarks: $userData.landmarks, showFavoritesOnly: $userData.showFavoritesOnly)
                .navigationBarTitle("Landmarks")
                .navigationBarItems(trailing: Toggle("showFavoritesOnly", isOn: $userData.showFavoritesOnly))
        }
    }
}

#if DEBUG
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}
#endif