onDelete 导致 NSRangeException

Question

In this app, there is a main screen (WorkoutScreen) that displays the contents of a list one at a time as it iterates through the list (current workout in a list of many). In a popOver, a list that contains all of the workouts appears and has the ability to add, delete or move items in that list.

When I delete the bottom most item, there is no error. When I delete any other item in the list I get this NSRangeException error that crashes the app:

/*
    2022-04-24 15:41:21.874306-0400 Trellis         
    beta[9560:3067012] *** Terminating app due to     
    uncaught exception 'NSRangeException', reason: 
    '*** __boundsFail: index 3 beyond bounds [0 .. 
    2]'
    *** First throw call stack:
    (0x1809150fc 0x19914fd64 0x180a1e564 0x180a2588c 
    0x1808c0444 0x1852dcce4 0x1852e1400 0x185424670 
    0x185423df0 0x185428a40 0x18843e4a0 0x188510458 
    0x188fd83ec 0x10102f3bc 0x1010500a4 0x188494f4c 
    0x10102c664 0x10103e0d4 0x18841a944 0x10102be18 
    0x10103122c 0x18837b8ac 0x188363484 0x18834bb64 
    0x188371d20 0x1883b88e4 0x1b28fe910 0x1b28fe318 
    0x1b28fd160 0x18831e780 0x18832f3cc 0x1883f5e34 
    0x18834206c 0x188345f00 0x182eb0798 0x184613138 
    0x184605958 0x184619f80 0x184622874 0x1846050b0 
    0x183266cc0 0x1835015fc 0x183b7d5b0 0x183b7cba0 
    0x1809370d0 0x180947d90 0x180882098 0x1808878a4 
    0x18089b468 0x19c42638c 0x18323d088 0x182fbb958 
    0x1885547a4 0x188483928 0x1884650c0 0x10109a630 
    0x10109a700 0x1015b9aa4)
    libc++abi: terminating with uncaught exception 
    of type NSException
    dyld4 config: 
    DYLD_LIBRARY_PATH=/usr/lib/system/introspection 
    DYLD_INSERT_LIBRARIES=/Developer/usr/lib/libBacktrac 
 
eRecording.dylib:/Developer/usr/lib/libMainThreadChecker.dylib:/Developer/Library/Private 
    Frameworks/DTDDISupport.framework/libViewDebuggerSupport.dylib
    *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** 
    __boundsFail: index 3 beyond bounds [0 .. 2]'
    terminating with uncaught exception of type NSException
    (lldb)
    */

struct WorkoutScreen: View {

    @EnvironmentObject var workoutList: CoreDataViewModel //calls it from environment
    @StateObject var vm = CoreDataViewModel()  //access on the page
    @Environment(\.scenePhase) var scenePhase

    var body: some View{
        //displays the current item in the list
    }
}

When I add an item to the list I get the error: ''' CoreData: warning: Multiple NSEntityDescriptions claim the NSManagedObject subclass 'FruitEntity' so +entity is unable to disambiguate." '''

Moving Items without adding or deleting any prior gives me this error upon closing the pop over: ''' Error saving: Error Domain=NSCocoaErrorDomain Code=133020 "Could not merge changes." UserInfo={conflictList=( "NSMergeConflict (0x2804f1480) for NSManagedObject (0x28327d900) with objectID '0x9ede5774e26501a4... '''

Here is the core data and related functions:

class CoreDataViewModel: NSObject, ObservableObject {
    private let container: NSPersistentContainer
    private let context: NSManagedObjectContext
    // Whenever you put your Core Data fetch in a view model, you should use an NSFetchedResultsController.
    // This allows you to automatically update your @Published var when your Core Data store changes.
    // You must inherit from NSObject to use it.
    private let fetchResultsController: NSFetchedResultsController<FruitEntity>
        
    @Published var savedEntities: [FruitEntity] = []

    override init() {
        container = NSPersistentContainer(name: "FruitsContainer")
        container.loadPersistentStores { (description, error) in
            if let error = error {
                print("ERROR LOADING CORE DATA: \(error)")
            }
            else {
                print("Successfully loaded core data")
            }
        }
        context = container.viewContext
        let request = NSFetchRequest<FruitEntity>(entityName: "FruitEntity")
        let sort = NSSortDescriptor(keyPath: \FruitEntity.order, ascending: true)
        request.sortDescriptors = [sort]
        // This initializes the fetchResultsController
        fetchResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
           
        // Because you inherit from NSObject, you must call super.init() to properly init the parent class. The order of when
        // this is to be called has changed.
        super.init()
        // Because this is a delegate action, you must set the delegate. Since the view model will respond, we set the delegate to self.
        fetchResultsController.delegate = self
            
        fetchFruits()
    }

    func fetchFruits() {
        do {
            // Instead of calling container.viewContext.fetch(request) which is static, use fetchResultsController.performFetch()
                try fetchResultsController.performFetch()
            // Make sure the fetch result is not nil
                guard let fruitRequest = fetchResultsController.fetchedObjects else { return }
            savedEntities = fruitRequest
            // You do not need to let error. error is automatically captured in a do catch.
        } catch {
            print("Error fetching \(error)")
        }
     }

   func addFruit(text: String, nummSets: Int16, nummWeights: Int16, nummReps: Int16, secOrRepz: Bool, orderNumz: Int64, multilimbz:  Bool, countDownz: Int16, repTimez: Int16, restTimez: Int16, circuitz: Bool) {
        let newFruit = FruitEntity(context: container.viewContext)
        newFruit.name = text
        newFruit.numOFSets = nummSets
        newFruit.numOFWeight = nummWeights
        newFruit.numOFReps = nummReps
        newFruit.measure = secOrRepz
        newFruit.order = orderNumz
        newFruit.multiLimb = multilimbz
        newFruit.countDownSec = countDownz
        newFruit.timePerRep = repTimez
        newFruit.restTime = restTimez
        newFruit.circuit = circuitz
        
        saveData()
    }

    func deleteFunction(indexSet: IndexSet) {
        guard let index = indexSet.first else { return }
        let entity = savedEntities[index]
        container.viewContext.delete(entity)
        saveData()
    }

    func saveData() {
        do {
            try context.save()
            fetchFruits()
        } catch let error {
            print("Error saving: \(error)")
          }
        }
}

    // This is your delegate extension that handles the updating when your Core Data Store changes.
    extension CoreDataViewModel: NSFetchedResultsControllerDelegate {
    func controllerDidChangeContent(_ controller: 
        NSFetchedResultsController<NSFetchRequestResult>) {
        // Essentially, you are redoing just the fetch as the NSFetchedResultsController knows how to fetch from above
        guard let fruits = controller.fetchedObjects as? [FruitEntity] else { return }
        self.savedEntities = fruits
        }
    }

Here is the list struct:

struct WorkoutListPopUp: View {
    
    @ObservedObject var vm = CoreDataViewModel()
    @EnvironmentObject var listViewModel: ListViewModel
    @EnvironmentObject var workoutList: CoreDataViewModel
    
    //Too many @State var to list here
    
    var body: some View {
        
        Button (action: {
            //this triggers the bug>
            vm.addFruit(text: "Workout name", nummSets: Int16(addSets) ?? 3, nummWeights: Int16(addWeights) ?? 0, nummReps: Int16(addReps) ?? 8, secOrRepz: addSecOrReps, orderNumz: Int64((vm.savedEntities.count)), multilimbz: dualLimbs, countDownz: 10, repTimez: 3, restTimez: 60, circuitz: false)
            loadNums()
        }, label: {
            Image(systemName: "plus")
                .resizable()
                .aspectRatio(contentMode: .fit)
                .frame(width:20, height: 20)
                .foregroundColor(Color.pink.opacity(1.0))
                .padding(.top, 0)
        })
        List(){
            ForEach(vm.savedEntities) {entity in
                VStack{
                    EditWorkouts(entity: entity, prescribeMeasure: $prescribeMeasure, addReps: $addReps, measurePrescribed: $measurePrescribed, repTimePicker: $repTimePicker, repz: $repz, restPicker: $restPicker, setz: $setz, ready2Press: $ready2Press, workoutz: $workoutz, weightz: $weightz, setsRemaining: $setsRemaining, workoutNum: $workoutNum, workoutInstructions: $workoutInstructions, multiplelimbs: $multiplelimbs, showAllInfo: $showAllInfo)
                    
                    //are these onChanges needed if "EditWorkouts" file is saving?
                        .onChange(of: entity.name) { text in
                            vm.saveData()
                            loadNums()
                        }
                        .onChange(of: entity.numOFSets) { text in
                            vm.saveData()
                            loadNums()
                        }
                        .onChange(of: entity.numOFReps) { text in
                            vm.saveData()
                            loadNums()
                        }
                        .onChange(of: entity.numOFWeight) { text in
                            vm.saveData()
                            loadNums()
                        }
                        .onChange(of: entity.measure) { text in
                            vm.saveData()
                            loadNums()
                        }
                        .onChange(of: entity.order) { text in
                            vm.saveData()
                            loadNums()
                        }
                        .onChange(of: entity.circuit) { text in
                            vm.saveData()
                            loadNums()
                        }
                }
            }
            .onDelete(perform: vm.deleteFunction)
            .onMove(perform: moveItem)
        }
        
        
        
    }
    
    func loadNums(){
        if vm.savedEntities.count > 0 {
            workoutz = vm.savedEntities[workoutNum].name ?? "NO Name"
            setz = String(vm.savedEntities[workoutNum].numOFSets)
            weightz = String(vm.savedEntities[workoutNum].numOFWeight)
            repz = String(vm.savedEntities[workoutNum].numOFReps)
            multiplelimbs = vm.savedEntities[workoutNum].multiLimb
            prescribeMeasure = vm.savedEntities[workoutNum].measure
            if setsRemaining == 0 && ((workoutNum + 1) - (Int(vm.savedEntities.count)) == 0) {
                workoutInstructions = "Goal: \(repz) \(measurePrescribed)"
            }
            else {
                workoutInstructions = "Goal: \(repz) \(measurePrescribed)"
            }
        }
        else {
            workoutz = "Add a Workout  👉"
            workoutInstructions = " "
        }
    }
    
    func moveItem(indexSet: IndexSet, destination: Int) {
        let source = indexSet.first!
        if destination > source   {
            var startIndex = source + 1
            let endIndex = destination - 1
            var startOrder = vm.savedEntities[source].order
            while startIndex <= endIndex {
                vm.savedEntities[startIndex].order = startOrder
                startOrder = startOrder + 1
                startIndex = startIndex + 1
            }
            vm.savedEntities[source].order = startOrder
        }
        
        else if destination < source {
            var startIndex = destination
            let endIndex = source - 1
            var startOrder = vm.savedEntities[destination].order + 1
            let newOrder = vm.savedEntities[destination].order
            while startIndex <= endIndex {
                vm.savedEntities[startIndex].order = startOrder
                startOrder = startOrder + 1
                startIndex = startIndex + 1
            }
            vm.savedEntities[source].order = newOrder
        }
        vm.savedEntities[source].circuit = false
        vm.saveData()
        loadNums()
    }
}

This is the EditWorkouts file that the WorkoutPopUp file connects to:

struct EditWorkouts: View {
    @EnvironmentObject var workoutList: CoreDataViewModel 
    @StateObject var vm = CoreDataViewModel()  
    @EnvironmentObject var listViewModel: ListViewModel
    let entity: FruitEntity
    //too many @State vars to post

    var body: some View {
        
        VStack{
            HStack{
                //many lines of code for options that alter the respective workout on the list. All are followed by their version of:
               //.onChange(of:
               //vm.savedEntities[Int(entity.order)].multiLimb) { _ in
               //vm.saveData()
               //loadNums()"
               //}
               //-or-
               //.onChange(of:vm.savedEntities[Int(entity.order)].circuit) { _ in
               //entity.circuit = entity.circuit
               //vm.saveData()
               //}
            }
        }
    }
}

Picture of CoreData FruitEntity: Image

Thank you again for your time!!

Answer 1

There are a couple of issues with your code. I suspect one is the sole contributor to the crash, but the other may be contributing as well. First, the most likely culprit. If you use .onDelete() , you can't use id: \.self . The reason is pretty simple: the ForEach can get pretty confused as to which entity is which. .self is often not unique, and it really needs to be if you are deleting and rearranging things in the ForEach() , ie .onDelete() and .onMove() .

The solution is simple. Whatever you are using in the ForEach should conform to Identifiable . Core Data managed objects all conform to Identifiable , so the fix is easy; remove the `id: .self``:

struct ListView: View {
    
    @StateObject var vm = CoreDataViewModel()
    
    var body: some View {
        List {
            ForEach(vm.savedEntities) {entity in
                Text(entity.name ?? "")
            }
            .onDelete(perform: vm.deleteFunction)
        }
        // This just adds a button to create entities.
        .toolbar {
            ToolbarItem(placement: .navigationBarTrailing) {
                Button {
                    vm.addFruit()
                } label: {
                    Image(systemName: "plus")
                }
            }
        }
    }
}

That fix alone will most likely stop the crash. However, I also noticed that you were having issues with your updates in your view. That is because you did not implement an NSFetchedResultsController and NSFetchedResultsControllerDelegate which updates your array when your Core Data store changes. Your view model should look like this:

import SwiftUI
import CoreData

class CoreDataViewModel: NSObject, ObservableObject {
    private let container: NSPersistentContainer
    private let context: NSManagedObjectContext
    // Whenever you put your Core Data fetch in a view model, you should use an NSFetchedResultsController.
    // This allows you to automatically update your @Published var when your Core Data store changes.
    // You must inherit from NSObject to use it.
    private let fetchResultsController: NSFetchedResultsController<FruitEntity>
    
    @Published var savedEntities: [FruitEntity] = []
    
    override init() {
        container = NSPersistentContainer(name: "FruitsContainer")
        container.loadPersistentStores { (description, error) in
            if let error = error {
                print("ERROR LOADING CORE DATA: \(error)")
            }
            else {
                print("Successfully loaded core data")
            }
        }
        context = container.viewContext
        let request = NSFetchRequest<FruitEntity>(entityName: "FruitEntity")
        let sort = NSSortDescriptor(keyPath: \FruitEntity.order, ascending: true)
        request.sortDescriptors = [sort]
        // This initializes the fetchResultsController
        fetchResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
        
        // Because you inherit from NSObject, you must call super.init() to properly init the parent class. The order of when
        // this is to be called has changed.
        super.init()
        // Because this is a delegate action, you must set the delegate. Since the view model will respond, we set the delegate to self.
        fetchResultsController.delegate = self
        
        // Renamed function to conform to naming conventions. You should use an active verb like fetch to start the name.
        fetchFruits()
    }
    
    func fetchFruits() {
        
        do {
            // Instead of calling container.viewContext.fetch(request) which is static, use fetchResultsController.performFetch()
            try fetchResultsController.performFetch()
            // Make sure the fetch result is not nil
            guard let fruitRequest = fetchResultsController.fetchedObjects else { return }
            savedEntities = fruitRequest
            // You do not need to let error. error is automatically captured in a do catch.
        } catch {
            print("Error fetching \(error)")
        }
    }
    
    // This is just to be able to add some data to test.
    func addFruit() {
        var dateFormatter: DateFormatter {
            let df = DateFormatter()
            df.dateStyle = .short
            return df
        }
        let fruit = FruitEntity(context: context)
        fruit.name = dateFormatter.string(from: Date())
        fruit.measure = false
        fruit.numOfReps = 0
        fruit.numOfSets = 0
        fruit.numOfWeight = 0
        fruit.order = 0
        saveData()
    }
    
    func deleteFunction(indexSet: IndexSet) {
        guard let index = indexSet.first else { return }
        let entity = savedEntities[index]
        container.viewContext.delete(entity)
        saveData()
    }
    
    func saveData() {
        do {
            try context.save()
        } catch let error {
            print("Error saving: \(error)")
        }
    }
}

// This is your delegate extension that handles the updating when your Core Data Store changes.
extension CoreDataViewModel: NSFetchedResultsControllerDelegate {
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        // Essentially, you are redoing just the fetch as the NSFetchedResultsController knows how to fetch from above
        guard let fruits = controller.fetchedObjects as? [FruitEntity] else { return }
        self.savedEntities = fruits
    }
}

You will notice refreshID no longer exists in the view. It updates without it. Also, please note that by incorporating the data store init into your view model, you can't expand it to have other entities with other views. Each will have a different context and they will crash the app. You are better off having a controller class that creates a singleton for the Core Data store, such as what Apple gives you in the default set up.

In the end, I think you issue was a combination of using id: .self which is known to crash with .onDelete() AND the fact that you were using refreshID not NSFetchedResultsController to update the List .