I wanted to add the counter on how many times the activity was completed. But the button I created in ActivityView does not work because of the error "cannot use mutating member on immutable value: self is immutable"

Why is my Activity immutable? How should I increment the counter?

Activity

import Foundation

struct Activity: Codable, Identifiable {
    var id = UUID()  // swift will create ids automatically
    let title: String
    let description: String
    var completedCount = 0

    mutating func completed() {
        self.completedCount += 1
    }
}

Activities

import Foundation

class Activities: ObservableObject {
    @Published var items = [Activity]() {
        // Whenever you set this property it will save the data to disk in UserDefaults
        didSet {
            let encoder = JSONEncoder()
            if let encoded = try? encoder.encode(items) {
                UserDefaults.standard.set(encoded, forKey: "activities")
            }
        }
    }

    init() {
        // Try to read saved data at init-time, if there is no data then create an empty array
        if let savedActivities = UserDefaults.standard.data(forKey: "activities") {
            if let decoded = try? JSONDecoder().decode([Activity].self, from: savedActivities) {
                items = decoded
                return
            }
        }
        items = [Activity]()
    }

    static var previewData: Activities {
        let sample = Activities()
        sample.items.append(Activity(title: "Outdoor Cycling", description: "Cycling outside in the nature."))
        return sample
    }
}

ActivityView

import SwiftUI

struct ActivityView: View {
    var activity: Activity

    var body: some View {
        GeometryReader { geometry in
            ScrollView {
                VStack(alignment: .leading) {
                    Text("This activity has been completed: \(activity.completedCount) times")
                        .padding(.top, 15)
                    Button {
                        activity.completed()    // HERE is the error
                    } label: {
                        Image(systemName: "plus")
                    }
                    Text(activity.description)
                        .padding(.top, 10)
                }
                .padding([.horizontal])
            }
        }
        .navigationTitle(activity.title)
    }
}

struct ActivityView_Previews: PreviewProvider {
    static var previews: some View {
        ActivityView(activity: Activities.previewData.items.popLast()!)
    }
}

ContentView

import SwiftUI

struct ContentView: View {
    @StateObject var activities = Activities()
    @State private var showingAddActivity = false

    @State private var dummies = ["Outdoor Cycle", "Walk", "Run"]

    var body: some View {
        NavigationView {
            List {
                Section {
                    ForEach(activities.items, id: \.id) { activity in
                        NavigationLink {
                            ActivityView(activity: activity)
                        } label: {
                            Text(activity.title)
                        }
                    }
                    .onDelete(perform: removeActivityItem(at:))
                } header: {
                    Text("My Activities")
                }
            }
            .navigationTitle("Time Tracker")
            .toolbar {
                Button {
                    showingAddActivity = true
                } label: {
                    Image(systemName: "plus")
                }
            }
            .sheet(isPresented: $showingAddActivity) {
                AddActivityView(activities: activities)
            }
        }
    }

    func removeActivityItem(at offsets: IndexSet) {
        activities.items.remove(atOffsets: offsets)
    }
}

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

You are trying to mutate the View itself, however Views are immutable. If you want to change anything in a View it will have to be marked as a @State.

Here is a brief note from Paul.

Normally with an @State the declaration would be

@State private var activity: Activity

However, since you are calling the ActivityView from ContentView and passing a parameter the compiler will complain that the initializer is inaccessible due to the private protection, so you should try this instead.

@State var activity: Activity

Hi, thanks for the pointer.

The Button woks now, of course the state is not preserved after I leave the detail view. Now I feel like I have to move away from passing a struct to the detail view, since I want to change it and preserve the change.

So instead of

...
ForEach(activities.items, id: \.id) { activity in
                        NavigationLink {
                            ActivityView(activity: activity)
...

I shall find out the index of the item the user touches (to go to the ActivityView()) and handing it over to ActivityView alongside the class instance activities. Then I could modify the count and have my class instance remember it. Hmmm...How would I get that index from ForEach?

Thank you for participating in my thinking process