Hi everyone. I recently finished the project on Day 47, but I'm still failing to understand how SwiftUI treats data between views. Some explanation would be appreciated.

In my ActivityDetail view, I was trying to update the timesCompleted property of the Activity struct directly, by incrementing the count with a button and then setting the new value directly with this function. The code checked out, but it wouldn't let me change the timesCompleted property because it stated that "activity" was a constant (even though I declared it with var).

Essentially, I tried to modify one property in the struct that the class contained and it didn't work.

func updateTimes(count: Int, name: String) {
        for activity in activities {
            if activity.activityName == name {
                activity.timesCompleted = count
            }
        }
    }

Eventually I found that I needed to swap the Activity item in the Activities array completely, using this function:

    func updateActivity() {
        if let position = activities.activities.firstIndex(where: {$0.id == activity.id}) {
            activities.activities[position] = activity
        }
    }

My question is: why isn't it possible to modify only the timesCompleted property? Why does Swift require a new instance of Activity to replace the old one?

Full code here of code that didn't work (error message cannot assign to property: 'activity' is a 'let' constant:

struct Activity: Identifiable, Codable {
    let id = UUID()
    let activityName: String
    let activityDetails: String
    let measureUnit: String
    var timesCompleted: Int

    mutating func incrementTimes() {
        self.timesCompleted += 1
    }
}

class Activities: ObservableObject {
    @Published var activities = [Activity]() {
        didSet {
            let encoder = JSONEncoder()

            if let encoded = try? encoder.encode(activities) {
                UserDefaults.standard.set(encoded, forKey: "Activities")
            }
        }
    }

    init() {
        if let items = UserDefaults.standard.data(forKey: "Activities") {
            let decoder = JSONDecoder()

            if let decoded = try? decoder.decode([Activity].self, from: items) {
                self.activities = decoded
                return
            }
        }
    }

    func updateTimes(count: Int, name: String) {
        for activity in activities {
            if activity.activityName == name {
                activity.timesCompleted = count
            }
        }
    }

}

struct ContentView: View {

    @ObservedObject var activities = Activities()
    @State private var showingAddActivity = false

    var body: some View {
        NavigationView {
            List(activities.activities) { activity in
                NavigationLink(destination: ActivityDetails(activity: activity, activities: self.activities)) {
                    VStack(alignment: .leading) {
                        Text(activity.activityName)
                        Text("Times Completed: \(String(activity.timesCompleted))")
                    }

                }

            }
                //.onDelete(perform: self.removeItems)
                .navigationBarTitle("Be Better")
                .navigationBarItems(leading: EditButton(), trailing: Button(action: {
                    self.showingAddActivity = true
                }) {
                    Image(systemName: "plus")
                })
                .sheet(isPresented: $showingAddActivity) {
                    AddActivity(activities: self.activities)
            }
        }
    }
    func removeItems(at offsets: IndexSet) {
        activities.activities.remove(atOffsets: offsets)
    }
}

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

struct ActivityDetails: View {

    @Environment(\.presentationMode) var presentationMode
    @State var activity: Activity
    @ObservedObject var activities: Activities

    var body: some View {
        NavigationView {
            VStack {
                Text(activity.activityName)
                    .font(.title)
                    .padding()
                HStack {
                    Button(action: {
                        self.activity.incrementTimes()
                    }) {
                        Image(systemName: "arrow.left")
                    }
                    Text(String(activity.timesCompleted))
                    Button(action: {
                        self.activity.incrementTimes()
                        self.activities.updateTimes(count: self.activity.timesCompleted, name: self.activity.activityName)
                    }) {
                        Image(systemName: "arrow.right")
                    }
                }
                .padding()
                Text(activity.activityDetails)
                .padding()
                Spacer()
            }

            navigationBarTitle("Activity information")
            .navigationBarTitle("Add a new activity")
            .navigationBarItems(leading: Button("Back") {
                self.presentationMode.wrappedValue.dismiss()
            })

        }
    }

Thank you very much for the explanation! I understand now how my original code wouldn't have worked.