TextFieldでoptionalを扱う場合について

ContetView.swift

import SwiftUI

struct ModelView: View {
    @ObservedObject var model: Model
    
    @State var isEditing = false

    var body: some View {
        HStack {
            Text(model.name)
            TextField("", text: $model.timeStr) { editing in
                isEditing = editing
            }
        }
        .onChange(of: isEditing, perform: { editing in
            if editing == false {
               model.save()
            }
        })
    }
}

struct ContentView: View {
    @State var selectedModel: Model? = nil
    var body: some View {
        VStack {
            Button("A") {
                selectedModel = models[0]
            }
            Button("B") {
                selectedModel = models[1]
            }
            if let selectedModel {
                ModelView(model: selectedModel)
            } else {
                EmptyView()
            }
            Spacer()
        }
        .frame(width: 100, height: 100)
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Model.swift

import Foundation
import SwiftUI

class Model: ObservableObject {
    init(name: String, time: Double?) {
        self.name = name
        self.time = time
    }
    var name: String
    var time: Double?
    
    private var _timeStr: String? = nil
    var timeStr: String {
        get {
            if let _timeStr {
                return _timeStr
            } else {
                if let t = time {
                    _timeStr = String(t)
                } else {
                    _timeStr = ""
                }
            }
            return _timeStr!
        }
        set {
            _timeStr = newValue
        }
    }
    
    public func save() {
        if let _timeStr,
           let n = Double(_timeStr) {
            time = n
            debugPrint("save \(name)")
        }
        _timeStr = nil
    }
    
}

var models: [Model] = [
    Model(name: "A", time: 1.0),
    Model(name: "B", time: 2.0),
]