Changing computed property in swift

Question

I'm having NSFetchedResultsController as lazy computed property. Based on a variable, sort descriptor is created. Here's my code:

private var sortOption: Options = .none
fileprivate lazy var inspirationsResults: NSFetchedResultsController<Inspiration> = {
    // Create Fetch Request
    let fetchRequest: NSFetchRequest<Inspiration> = Inspiration.fetchRequest()

    // Configure Fetch Request
    //let optn = self.sortOption.rawValue
    switch (self.sortOption) {
    case .sortAscending:
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "inspirationName", ascending: true)]
    case .sortDescending:
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "inspirationName", ascending: false)]
    case .sortByAdding:
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "timeStamp", ascending: false)]
    case .sortByUpdated:
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "timeStamp", ascending: false)]
    case .showFilter:
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "timeStamp", ascending: false)]
    default:
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "timeStamp", ascending: false)]
    }
    fetchRequest.sortDescriptors = [NSSortDescriptor(key: "timeStamp", ascending: false)]
    fetchRequest.fetchBatchSize = 10
    // Create Fetched Results Controller
    let fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: CoreDataManager.shared.getContext(), sectionNameKeyPath: nil, cacheName: nil)

    // Configure Fetched Results Controller
    fetchedResultsController.delegate = self

    return fetchedResultsController
}()

When sortOption variable's value is changed, I want to recompute "inspirationsResults" variable and change the sort descriptor accordingly. How to achieve it?

Answer 1

Put the code to compute the sort descriptor into the didSet observer of the sortOption property:

private var sortOption: Options = .none {
    didSet {
        let sortDescriptor : NSSortDescriptor
        switch sortOption {
            case .sortAscending:
                sortDescriptor = NSSortDescriptor(key: "inspirationName", ascending: true)
            case .sortDescending:
                sortDescriptor = NSSortDescriptor(key: "inspirationName", ascending: false)
            default:
                sortDescriptor = NSSortDescriptor(key: "timeStamp", ascending: false)
         }
         fetchedResultsController.fetchRequest.sortDescriptors = [sortDescriptor]
         do {
            try fetchedResultsController.performFetch()
            tableView.reloadData()
         } catch {
            print(error)
         }
    }
}

The default case covers all explicit cases which sort by timeStamp - false.
After changing the sort descriptor you need to perform a new fetch and reload the table view.

And in the method to initialize the results controller simply write:

self.sortOption = .none