Why can't I write to an #ObservedObject? - swiftui

Question

I have a struct called Activity which has an id (UUID), name (String), description (String) and timesCompleted (Int).
I also have a class called Activities that contains an array of Activity structs called activityList. Activities is marked with ObservableObject.
I have activities declared as a #StateObject in my ContentView and I pass it to my ActivityDetailView where it is declared as an #ObservedObject.
However I can only partially write to activities.activityList in the child view. I can append, but I can't overwrite, update or remove an element from the array. No error is thrown but the view immediately crashes and the app returns to the main ContentView.
How do you update/write to an #ObservedObject? As you can see from the comments in my updateTimesCompleted() function I've tried all kinds of things to update/overwrite an existing element. All crash silently and return to ContentView. Append does not fail, but isn't the behavior I want, I want to update/overwrite an array element, not append a new copy.
Activity Struct:
struct Activity : Codable, Identifiable, Equatable {
var id = UUID()
var name: String
var description: String
var timesCompleted: Int
}
Activities Class:
class Activities: ObservableObject {
#Published var activityList = [Activity]() {
didSet {
if let encoded = try? JSONEncoder().encode(activityList) {
UserDefaults.standard.set(encoded, forKey: "activityList")
}
}
}
init() {
if let savedList = UserDefaults.standard.data(forKey: "activityList") {
if let decodedList = try? JSONDecoder().decode([Activity].self, from: savedList) {
activityList = decodedList
return
}
}
activityList = []
}
init(activityList: [Activity]) {
self.activityList = activityList
}
subscript(index: Int) -> Activity {
get {
assert(index < activityList.count, "Index out of range")
return activityList[index]
}
set {
assert(index < activityList.count, "Index out of range")
activityList[index] = newValue
}
}
}
ContentView:
struct ContentView: View {
#StateObject var activities = Activities()
#State private var showingAddActivity = false
var body: some View {
NavigationView {
List {
ForEach(activities.activityList) { activity in
NavigationLink {
ActivityDetailView(activity: activity, activities: activities)
} label: {
Text(activity.name)
}
}
}
.navigationTitle("Habits")
.toolbar {
Button {
showingAddActivity = true
let _ = print("add activity")
}
label: {
Image(systemName: "plus")
}
}
}
.sheet(isPresented: $showingAddActivity) {
AddActivityView(activities: activities)
}
}
}
ActivityDetailView:
struct ActivityDetailView: View {
#State private var timesCompleted = 0
let activity: Activity
#ObservedObject var activities: Activities
var body: some View {
NavigationView {
Form {
Text("Activity: \(activity.name)")
Text("Description: \(activity.description)")
Stepper {
Text("Times Completed: \(timesCompleted)")
} onIncrement: {
timesCompleted += 1
updateTimesCompleted()
} onDecrement: {
if timesCompleted > 0 {
timesCompleted -= 1
updateTimesCompleted()
}
}
}
.navigationTitle("Activity Details")
}
}
func updateTimesCompleted() {
let newActivity = Activity(name: activity.name, description: activity.description, timesCompleted: timesCompleted)
let _ = print("count: \(activities.activityList.count)")
let index = activities.activityList.firstIndex(of: activity)
let _ = print(index ?? -666)
if let index = index {
activities.activityList[index] = Activity(name: activity.name, description: activity.description, timesCompleted: timesCompleted)
//activities.activityList.swapAt(index, activities.activityList.count - 1)
//activities.activityList[index].incrementTimesCompleted()
//activities.activityList.append(newActivity)
//activities.activityList.remove(at: index)
//activities.activityList.removeAll()
//activities.activityList.append(newActivity)
}
}
}

Answer 1

You could try this approach, where the activity is passed to the ActivityDetailView
as a binding.
In addition, #ObservedObject var activities: Activities is used directly in AddActivityView to add an Activity to the list.
struct Activity : Codable, Identifiable, Equatable {
let id = UUID() // <-- here
var name: String
var description: String
var timesCompleted: Int
enum CodingKeys: String, CodingKey { // <-- here
case name,description,timesCompleted
}
}
class Activities: ObservableObject {
#Published var activityList = [Activity]() {
didSet {
if let encoded = try? JSONEncoder().encode(activityList) {
UserDefaults.standard.set(encoded, forKey: "activityList")
}
}
}
init() {
if let savedList = UserDefaults.standard.data(forKey: "activityList") {
if let decodedList = try? JSONDecoder().decode([Activity].self, from: savedList) {
activityList = decodedList
return
}
}
activityList = []
}
init(activityList: [Activity]) {
self.activityList = activityList
}
subscript(index: Int) -> Activity {
get {
assert(index < activityList.count, "Index out of range")
return activityList[index]
}
set {
assert(index < activityList.count, "Index out of range")
activityList[index] = newValue
}
}
}
struct ContentView: View {
#StateObject var activities = Activities()
#State private var showingAddActivity = false
var body: some View {
NavigationView {
List {
ForEach($activities.activityList) { $activity in // <-- here
NavigationLink {
ActivityDetailView(activity: $activity) // <-- here
} label: {
Text(activity.name)
}
}
}
.navigationTitle("Habits")
.toolbar {
Button {
showingAddActivity = true
}
label: {
Image(systemName: "plus")
}
}
}
.sheet(isPresented: $showingAddActivity) {
AddActivityView(activities: activities)
}
.onAppear {
// for testing
if activities.activityList.isEmpty {
activities.activityList.append(Activity(name: "activity-1", description: "activity-1", timesCompleted: 1))
activities.activityList.append(Activity(name: "activity-2", description: "activity-2", timesCompleted: 2))
activities.activityList.append(Activity(name: "activity-3", description: "activity-3", timesCompleted: 3))
}
}
}
}
// -- here for testing
struct AddActivityView: View {
#ObservedObject var activities: Activities
var body: some View {
Text("AddActivityView")
Button("add activity") {
activities.activityList.append(Activity(name: "workingDog", description: "workingDog", timesCompleted: 5))
}
}
}
struct ActivityDetailView: View {
#Binding var activity: Activity // <-- here
var body: some View {
Form {
Text("Activity: \(activity.name)")
Text("Description: \(activity.description)")
Stepper {
Text("Times Completed: \(activity.timesCompleted)")
} onIncrement: {
activity.timesCompleted += 1 // <-- here
} onDecrement: {
if activity.timesCompleted > 0 {
activity.timesCompleted -= 1 // <-- here
}
}
}
}
}