Simple Main/Detail Setup in SwiftUI where you can edit passed object in Detail view

gistfile1.txt

//
//  ContentView.swift
//  ActivityTracker
//
//  Created by TJ Barber on 12/10/20.
//

import SwiftUI

struct Activity: Identifiable, Codable, Hashable {
    let id: UUID = UUID()
    let name: String
    let description: String
    var timesCompleted: Int
    
    func hash(into hasher: inout Hasher) {
        hasher.combine(id)
    }
}

class ActivityStore: ObservableObject {
    private let keyPath = "activityStore"
    
    @Published var items = [Activity]() {
        didSet {
            let encoder = JSONEncoder()
            if let encoded = try? encoder.encode(items) {
                UserDefaults.standard.setValue(encoded, forKeyPath: keyPath)
            }
        }
    }
    
    init() {
        let decoder = JSONDecoder()
        
        if let data = UserDefaults.standard.data(forKey: keyPath),
           let decoded = try? decoder.decode([Activity].self, from: data) {
            self.items = decoded
        }
    }
}

struct ContentView: View {
    @ObservedObject var activityStore = ActivityStore()
    @State private var showingAddActivity = false
    
    var body: some View {
        NavigationView {
            List {
                ForEach(Array(zip(activityStore.items.indices, activityStore.items)), id: \.1) { index, activity in
                    NavigationLink(destination: ActivityDetailView(activity: self.$activityStore.items[index])) {
                        HStack {
                            Text(activity.name)
                            Spacer()
                            Text("\(activity.timesCompleted)")
                        }
                    }
                }
            }
            .listStyle(PlainListStyle())
            .navigationBarTitle("ActivityTracker")
            .navigationBarItems(trailing: Button(action: {
                showingAddActivity = true
            }) {
                Image(systemName: "plus")
            })
            .sheet(isPresented: $showingAddActivity) {
                AddActivityView(activityStore: activityStore)
            }
        }
        
    }
}

struct AddActivityView: View {
    @Environment(\.presentationMode) var presentationMode
    @ObservedObject var activityStore: ActivityStore

    @State private var name: String = ""
    @State private var description: String = ""

    var body: some View {
        NavigationView {
            Form {
                TextField("Name", text: $name)
                TextField("Description", text: $description)
            }
            .navigationBarTitle("Add Activity")
            .navigationBarItems(trailing: Button("Save") {
                let activity = Activity(name: self.name, description: self.description, timesCompleted: 0)
                self.activityStore.items.append(activity)
                self.presentationMode.wrappedValue.dismiss()
            })
        }
    }
}

struct ActivityDetailView: View {
    @Binding var activity: Activity
    
    var body: some View {
        VStack {
            Text(activity.name)
                .font(.headline)
            
            Text(activity.description)
            
            Spacer()
            
            Button("I did this!") {
                activity.timesCompleted += 1
                print("\(activity.timesCompleted)")
            }
            .padding()
        }
    }
}