iOS App Architecture

// ViewModel
class TaskListViewModel {

    // MARK: - Dependencies
    private let taskRepository: TaskRepositoryProtocol

    // MARK: - Published Properties
    @Published var tasks: [TaskViewModel] = []
    @Published var isLoading = false
    @Published var error: String? = nil

    // MARK: - Initialization
    init(taskRepository: TaskRepositoryProtocol) {
        self.taskRepository = taskRepository
    }

    // MARK: - Public Methods
    func fetchTasks() {
        isLoading = true
        error = nil

        taskRepository.fetchTasks { [weak self] result in
            guard let self = self else { return }
            self.isLoading = false

            switch result {
            case .success(let tasks):
                self.tasks = tasks.map { TaskViewModel(task: $0) }
            case .failure(let error):
                self.error = error.localizedDescription
            }
        }
    }

    func toggleTaskCompletion(at index: Int) {
        guard index >= 0 && index < tasks.count else { return }

        var task = tasks[index]
        task.isCompleted.toggle()
        tasks[index] = task

        taskRepository.updateTask(id: task.id, isCompleted: task.isCompleted) { [weak self] result in
            if case .failure(let error) = result {
                self?.error = error.localizedDescription
            }
        }
    }
}

// View Model for individual task items
struct TaskViewModel: Identifiable {
    let id: String
    let title: String
    let dueDate: Date
    var isCompleted: Bool

    init(task: Task) {
        self.id = task.id
        self.title = task.title
        self.dueDate = task.dueDate
        self.isCompleted = task.isCompleted
    }

    var dueDateFormatted: String {
        let formatter = DateFormatter()
        formatter.dateStyle = .medium
        formatter.timeStyle = .short
        return formatter.string(from: dueDate)
    }
}