Persisting stuff. In the MyModel.xcdatamodel make an Entity called MyThingEntity, an attribute index with type int32, an attribute data with type Data. Give the entity a constraint on the index.

ViewController.swift

import UIKit
import CoreData

public struct MyThing: Codable {
    var state: Bool? = nil
    var value: String? = nil
}

class ViewController: UIViewController, UITextViewDelegate {
    
    @IBOutlet weak var toggleSwitch: UISwitch!
    @IBOutlet weak var textView: UITextView!
    var context: NSManagedObjectContext!
    var thing1: MyThingEntity!
    var thing2: MyThingEntity!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        let container = NSPersistentContainer(name: "MyModel")
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
        context = container.viewContext
        let things = (try! context.fetch(NSFetchRequest<MyThingEntity>(entityName: "MyThingEntity")))
        if things.isEmpty {
            thing1 = MyThingEntity(context: context)
            thing1.index = 0
            thing1.data = try! JSONEncoder().encode(MyThing())
            thing2 = MyThingEntity(context: context)
            thing2.index = 1
            thing2.data = try! JSONEncoder().encode(MyThing())
        } else {
            thing1 = things.first
            thing2 = things.last
        }
        
        textView.delegate = self
        textView.text = (try! JSONDecoder().decode(MyThing.self, from: thing1.data!)).value
    }

    @IBAction func doAToggle(_ sender: Any) {
        var entity: MyThingEntity
        if toggleSwitch.isOn {
            entity = thing1
        } else {
            entity = thing2
        }
        textView.text = (try! JSONDecoder().decode(MyThing.self, from: entity.data!)).value
    }
    
    func textViewDidChange(_ textView: UITextView) {
        var entity: MyThingEntity
        if toggleSwitch.isOn {
            entity = thing1
        } else {
            entity = thing2
        }
        var thing = (try! JSONDecoder().decode(MyThing.self, from: entity.data!))
        thing.value = textView.text
        entity.data = try! JSONEncoder().encode(thing)
        try! context.save()
    }
}