I'm trying to reorder items in a list that's stored with CoreData using this answer. This is a macOS swiftUI App using MVVM layout.

Here's the ContentView

  @ObservedObject var vm : MyBookmarksViewModel

          ForEach(vm.myBookmarks) { myBookmark in
             Text(myBookmark.name)
              ...................
              }
             .onMove(perform: vm.reorderItems(from:to:))

I'm getting the 'Cannot assign to property: 'order' is a get-only property' error inside the reorderItems function below next to myBookmarks[ reserveIndex ].order

MyBookmarksViewModel

 @Published var myBookmarks = [MyBookmarkViewModel]()

 let fetchedResultsController: NSFetchedResultsController<MyBookmark>

 private var context: NSManagedObjectContext


    init(context: NSManagedObjectContext){
        self.context = context
        self.fetchedResultsController = NSFetchedResultsController(fetchRequest: MyBookmark.all  , managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
        super.init()
        fetchedResultsController.delegate = self    
    }

     func reorderItems( from source: IndexSet, to destination: Int)
    {
        // Make an array of items from fetched results
        // change the order of the items in the array
        myBookmarks.move(fromOffsets: source, toOffset: destination )

        // update the userOrder attribute in revisedItems to
        // persist the new order. This is done in reverse order
        // to minimize changes to the indices.
        for reserveIndex in stride( from: myBookmarks.count - 1,
                                    through: 0,
                                    by: -1 )
        {
            myBookmarks[ reserveIndex ].order = // this is where i get an error
                Int64(reserveIndex)
        }
    }

MyBookmarkViewModel

struct MyBookmarkViewModel: Identifiable, Hashable {

    init(myBM: MyBookmark) {
        self.myBM = myBM
        
    }
    private let myBM: MyBookmark
    
    var id: NSManagedObjectID {
        myBM.objectID
    }
    
    var name: String {
        myBM.name ?? ""
    }
       
   var order: Int64 {
        myBM.order 

    }
}

MyBookmark

@objc(MyBookmark)
public class MyBookmark: NSManagedObject, BaseModel {
    static var all: NSFetchRequest<MyBookmark> {
        let request: NSFetchRequest<MyBookmark> = MyBookmark.fetchRequest()
        let sort = NSSortDescriptor(keyPath: \MyBookmark.order, ascending: true)
        request.sortDescriptors = [sort]
        return request
    
    }   
}


extension MyBookmark {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<MyBookmark> {
        return NSFetchRequest<MyBookmark>(entityName: "MyBookmark")
    }

    @NSManaged public var name: String?
    @NSManaged public var order: Int64

}

extension MyBookmark : Identifiable {

}

CodePudding user response：

In MyBookmarkViewModel order is a computed read-only property.

If you want to set it you need a set {} branch

var order: Int64 {
    get { myBM.order } 
    set { myBM.order = newValue }
}