Why do I lose Data here?(SwiftUI) - swift

Question

I have two pages in my app TodayPage and CalendarList page.
I use EnvironmentObject wrapper to pass data between these two pages.
When TodayPage appears on onAppear modifier I call a function to generate days of calendar for me till now everything works fine when I add text to the list of TodayPage then go to the calendarList page and come back again to TodayPage all of the text that I addd to list are gone.I find out I can avoid lost of data by adding simple if to onAppear but I'm not sure this solution is right.
I have to upload lots of code ,Thanks for your help
( DataModel ) :
import SwiftUI
import Foundation
import Combine
struct Day : Identifiable {
var id = UUID()
var name : String
var date : String
var month : String
var List : [Text1?]
}
struct Text1 : Identifiable , Hashable{
var id = UUID()
var name: String
var color: Color
}
class AppState : ObservableObject {
#Published var dataLoaded = false
#Published var allDays : [Day] = [.init(name : "",date: "",month: "",List : [])]
func getDays(number: Int) -> [Day] {
let today = Date()
let formatter = DateFormatter()
return (0..<number).map { index -> Day in
let date = Calendar.current.date(byAdding: .day, value: index, to: today) ?? Date()
return Day(name: date.dayOfWeek(withFormatter: formatter) ?? "", date: "\(Calendar.current.component(.day, from: date))", month: date.nameOfMonth(withFormatter: formatter) ?? "", List: [])
}
}
}
extension Date {
func dayOfWeek(withFormatter dateFormatter: DateFormatter) -> String? {
dateFormatter.dateFormat = "EEEE"
return dateFormatter.string(from: self).capitalized
}
func nameOfMonth(withFormatter dateFormatter: DateFormatter) -> String? {
dateFormatter.dateFormat = "LLLL"
return dateFormatter.string(from: self).capitalized
}
}
class AddListViewViewModel : ObservableObject {
#Published var textItemsToAdd : [Text1] = [.init(name: "", color: .clear)] //start with one empty item
func saveToAppState(appState: AppState) {
appState.allDays[0].List.append(contentsOf: textItemsToAdd.filter {
!$0.name.isEmpty })
}
func bindingForId(id: UUID) -> Binding<String> {
.init { () -> String in
self.textItemsToAdd.first(where: { $0.id == id })?.name ?? ""
} set: { (newValue) in
self.textItemsToAdd = self.textItemsToAdd.map {
guard $0.id == id else {
return $0
}
return .init(id: id, name: newValue, color: .clear)
}
}
}
}
List view :
struct ListView: View {
#State private var showAddListView = false
#EnvironmentObject var appState : AppState
#Binding var dayList : [Text1?]
var title : String
var body: some View {
NavigationView {
VStack {
ZStack {
List(dayList, id : \.self){ text in
Text(text?.name ?? "")
}
if showAddListView {
AddListView(showAddListView: $showAddListView)
.offset(y:-100)
}
}
}
.navigationTitle(title)
.navigationBarItems(trailing:
Button(action: {showAddListView = true}) {
Image(systemName: "plus")
.font(.title2)
}
)
}
}
}
pop up menu View(for adding text into the list)
struct AddListView: View {
#Binding var showAddListView : Bool
#EnvironmentObject var appState : AppState
#StateObject private var viewModel = AddListViewViewModel()
var body: some View {
ZStack {
Title(addItem: { viewModel.textItemsToAdd.append(.init(name: "", color: .clear)) })
VStack {
ScrollView {
ForEach(viewModel.textItemsToAdd, id: \.id) { item in //note this is id: \.id and not \.self
PreAddTextField(textInTextField: viewModel.bindingForId(id: item.id))
}
}
}
.padding()
.offset(y: 40)
Buttons(showAddListView: $showAddListView, save: {
viewModel.saveToAppState(appState: appState)
})
}
.frame(width: 300, height: 200)
.background(Color.white)
.shadow(color: Color.black.opacity(0.3), radius: 10, x: 0, y: 10)
}
}
struct PreAddTextField: View {
#Binding var textInTextField : String
var body: some View {
VStack {
TextField("Enter text", text: $textInTextField)
}
}
}
struct Buttons: View {
#Binding var showAddListView : Bool
var save : () -> Void
var body: some View {
VStack {
HStack(spacing:100) {
Button(action: {
showAddListView = false}) {
Text("Cancel")
}
Button(action: {
showAddListView = false
save()
}) {
Text("Add")
}
}
}
.offset(y: 70)
}
}
struct Title: View {
var addItem : () -> Void
var body: some View {
VStack {
HStack {
Text("Add Text to list")
.font(.title2)
Spacer()
Button(action: {
addItem()
}) {
Image(systemName: "plus")
.font(.title2)
}
}
.padding()
Spacer()
}
}
}
TodayPage View :
struct TodayPage: View {
#EnvironmentObject var appState : AppState
var body: some View {
ListView(dayList: $appState.allDays[0].List, title: "Today")
.onAppear {
// To avoid data lost , we can use simple if below but I'm not sure it's a right solution
// if appState.dataLoaded == false {
appState.allDays = appState.getDays(number: 365)
// appState.dataLoaded = true
// }
}
}
}
CalendarListPage :
struct CalendarList: View {
#EnvironmentObject var appState : AppState
var body: some View {
NavigationView {
List {
ForEach(appState.allDays.indices, id:\.self) { index in
NavigationLink(destination: ListView(appState: _appState, dayList: $appState.allDays[index].List, title: appState.allDays[index].name).navigationBarTitleDisplayMode(.inline) ) {
HStack(alignment: .top) {
RoundedRectangle(cornerRadius: 23)
.frame(width: 74, height: 74)
.foregroundColor(Color.blue)
.overlay(
VStack {
Text(appState.allDays[index].date)
.font(.system(size: 35, weight: .regular))
.foregroundColor(.white)
Text(appState.allDays[index].month)
.foregroundColor(.white)
}
)
.padding(.trailing ,4)
VStack(alignment: .leading, spacing: 5) {
Text(appState.allDays[index].name)
.font(.system(size: 20, weight: .semibold))
}
}
.padding(.vertical ,6)
}
}
}
.navigationTitle("Calendar")
}.onAppear {
}
.navigationViewStyle(StackNavigationViewStyle())
}
}
and finally TabBar :
struct TabBar: View {
var body: some View {
let appState = AppState()
TabView {
TodayPage().tabItem {
Image(systemName: "info.circle")
Text("Today")
}
CalendarList().tabItem {
Image(systemName: "square.fill.text.grid.1x2")
Text("Calendar")
}
}
.environmentObject(appState)
}
}

Answer 1

Right now, because your let appState is inside the body of TabBar, it gets recreated every time TabBar is rendered. Instead, store it as a #StateObject (or #ObservedObject if you are pre iOS 14):
struct TabBar: View {
#StateObject var appState = AppState()
var body: some View {
TabView {
TodayPage().tabItem {
Image(systemName: "info.circle")
Text("Today")
}
CalendarList().tabItem {
Image(systemName: "square.fill.text.grid.1x2")
Text("Calendar")
}
}
.onAppear {
appState.allDays = appState.getDays(number: 365)
}
.environmentObject(appState)
}
}
Then, remove your other onAppear on TodayPage