TextFieldでoptionalを扱う場合について

ContetView.swift

struct ModelView: View {
    @ObservedObject var model: Model
    @State var timeStr: String = ""
    
    @State var isEditing = false
    
    @State var timer = Timer.publish(every: 0.1, on: .current, in: .common).autoconnect()

    var body: some View {
        HStack {
            Text(model.name)
            TextField("", text: $timeStr) { editing in
                isEditing = editing
            }
        }
        .onAppear() {
            stopTimer()
            reloadTime()
        }
        .onChange(of: model) { newValue in
            startTimer()
        }
        .onChange(of: isEditing, perform: { editing in
            if editing == false,
               let t = Double(timeStr) {
                debugPrint("save \(model.name)")
                model.time = t
            }
        })
        .onReceive(timer) { timer in
            stopTimer()
            reloadTime()
        }

    }
    
    private func reloadTime() {
        if let t = model.time {
            timeStr = String(t)
        } else {
            timeStr = ""
        }
    }
    
    private func startTimer() {
        timer = self.timer.upstream.autoconnect()
    }
    
    private func stopTimer() {
        timer.upstream.connect().cancel()
    }
}


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, Equatable {
    init(name: String, time: Double?) {
        self.name = name
        self.time = time
    }
    var name: String
    var time: Double?
    
    static func == (lhs: Model, rhs: Model) -> Bool {
        return lhs.name == rhs.name && lhs.time == rhs.time
    }
    
}

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