Hi, new forum member here, although I'm on Day 47 of the program. I've got the habit tracker working to a point. I can add a habit, see the detail view, and increase the number of times it is completed, but I don't see the count increase when I tap the button. I have to leave the detail view and then open it again to see the increased count. I assume it must have something to do with how I'm managing state, but I guess I don't fully understand all the concepts yet. Any help would be appreciated.

UPDATE: I used a LazyVGrid instead of this complicated custom grid layout and it just worked.

ContentView:

import SwiftUI

struct HabitItem: Identifiable, Codable, Equatable {
    var id = UUID()
    let name: String
    let description: String
    var completed: Int
}

@Observable
class Habits {
    var items = [HabitItem]() {
        didSet {
            if let encoded = try? JSONEncoder().encode(items) {
                UserDefaults.standard.set(encoded, forKey: "Items")
            }
        }
    }

    init() {
        if let savedItems = UserDefaults.standard.data(forKey: "Items") {
            if let decodedItems = try? JSONDecoder().decode([HabitItem].self, from: savedItems) {
                items = decodedItems
                return
            }
        }

        items = []
    }
}

struct ContentView: View {
    @State private var habits = Habits()
    @State private var showingAddHabit = false

    var body: some View {
        NavigationStack {
            List {
                ForEach(habits.items) { item in
                    NavigationLink {
                        HabitView(habits: habits, habit: item)
                    } label: {
                        HStack {
                            VStack(alignment: .leading) {
                                Text(item.name)
                                    .font(.headline)
                            }
                        }
                    }
                }
            }
            .navigationTitle("Habit Tracker")
            Button("Add a Habit to Track") {
                showingAddHabit = true
            }
            .buttonStyle(.borderedProminent)
            .controlSize(.large)
        }
        .sheet(isPresented: $showingAddHabit) {
            AddHabitView(habits: habits)
        }
    }
}

#Preview {
    ContentView()
}

And HabitView

import SwiftUI

struct HabitView: View {
    var habits = Habits()
    let habit: HabitItem
    @State var numCells: (Int, Int)

    init(habits: Habits = Habits(), habit: HabitItem) {
        self.habits = habits
        self.habit = habit
        _numCells = State(initialValue: habit.completed.quotientAndRemainder(dividingBy: 20))
    }

    var body: some View {
        Text(habit.name)
            .font(.largeTitle)
        Text(habit.description)
        ScrollView {
            Grid {
                ForEach(0..<numCells.0 + 1, id: \.self) {number in
                    GridRow {
                        if number + 1 == numCells.0 + 1 {
                            ForEach(0..<numCells.1) {cell in
                                Rectangle()
                                    .frame(width: 10, height: 10)
                                    .foregroundColor(/*@START_MENU_TOKEN@*/.blue/*@END_MENU_TOKEN@*/)
                            }
                        } else {
                            ForEach(0..<20) {cell in
                                Rectangle()
                                    .frame(width: 10, height: 10)
                                    .foregroundColor(/*@START_MENU_TOKEN@*/.blue/*@END_MENU_TOKEN@*/)
                            }
                        }
                    }
                }
            }
        }
        Button("Did It") {
            var tempHabit = habit
            tempHabit.completed += 1
            if let index = habits.items.firstIndex(of: habit) {
              habits.items[index] = tempHabit  // index is not optional any more
            }
        }
        .buttonStyle(.borderedProminent)
    }
}

#Preview {
    HabitView(habit: HabitItem(id: UUID(), name: "Name", description: "Foo", completed: 257))
}