SwiftUI: Checkmarks disappear when changing from one view to another using NavigationLink – Ios swift

As already mentioned in the comment, I don’r see where you read back from UserDefaults, so whatever gets stored there, you don’t read it. But even if so, each Toggle is using the same key, so you are overwriting the value.

Instead of using the @State var isChecked, which is used just locally, I’d create another struct item which gets the title from the json and which contains a boolean that gets initialized with false.

From what I understood, I assume a solution could look like the following code. Just a few things:

1. I am not sure how your json looks like, so I am not loading from a json, I add ItemSections Objects with a title and a random number of items (actually just titles again) with a function.
2. Instead of a print with the number of checked toggles, I added a text output on the UI. It shows you on first page the number of all checked toggles.
3. Instead of using UserDefaults I used @AppStorage.
   To make that work you have to make Array conform to RawRepresentable you achieve that with the following code/extension (just add it once somewhere in your project)
4. Maybe you should thing about a ViewModel (e.g. ItemSectionViewModel), to load the data from the json and provide it to the views as an @ObservableObject.

The code for the views:

//
//  CheckItem.swift
//  CheckItem
//
//  Created by Sebastian on 24.08.22.
//

import SwiftUI

struct ContentView: View {
    
    var body: some View {
        VStack() {
            CheckItemView()
        }
    }
}

struct CheckItemView: View {
    
    let testStringForTestData: String = "Check Item Title"
    
    @AppStorage("itemSections") var itemSections: [ItemSection] = []
    
    func addCheckItem(title: String, numberOfItems: Int) {
        var itemArray: [Item] = []
        for i in 0...numberOfItems {
            itemArray.append(Item(title: "item (i)"))
        }
        itemSections.append(ItemSection(title: title, items: itemArray))
    }
    
    func getSelectedItemsCount() -> Int{
        var i: Int = 0
        for itemSection in itemSections {
            let filteredItems = itemSection.items.filter { item in
                return item.isOn
            }
            i = i + filteredItems.count
        }
        return i
    }
    
    var body: some View {
        NavigationView{
            VStack() {
                List(){
                    ForEach(itemSections.indices, id: .self){ id in
                        NavigationLink(destination: ItemSectionDetailedView(items: $itemSections[id].items)) {
                            Text(itemSections[id].title)
                        }
                        .padding()
                    }
                }
                Text("Number of checked items:  (self.getSelectedItemsCount())")
                    .padding()
                
                Button(action: {
                    self.addCheckItem(title: testStringForTestData, numberOfItems: Int.random(in: 0..<4))
                }) {
                    Text("Add Item")
                }
                .padding()
            }
        }
    }
}

struct ItemSectionDetailedView: View {
    
    @Binding var items: [Item]
    
    var body: some View {
        ScrollView() {
            ForEach(items.indices, id: .self){ id in
                Toggle(items[id].title, isOn: $items[id].isOn)
                    .padding()
            }
        }
    }
}

struct ItemSection: Identifiable, Hashable, Codable  {
    var id: String = UUID().uuidString
    var title: String
    var items: [Item]
}

struct Item: Identifiable, Hashable, Codable  {
    var id: String = UUID().uuidString
    var title: String
    var isOn: Bool = false
}

Here the adjustment to work with @AppStorage:

extension Array: RawRepresentable where Element: Codable {
    public init?(rawValue: String) {
        guard let data = rawValue.data(using: .utf8),
              let result = try? JSONDecoder().decode([Element].self, from: data)
        else {
            return nil
        }
        self = result
    }
    
    public var rawValue: String {
        guard let data = try? JSONEncoder().encode(self),
              let result = String(data: data, encoding: .utf8)
        else {
            return "[]"
        }
        return result
    }
}