I'm trying to convert some data, then save it back to Core Data. Sometimes this works fine without an issue, but occasionally I'll get an error
Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)
It seems to occur when saving the core data context. I'm having trouble trying to debug it as it doesn't happen on the same object each time and can't reliably recreate the error
Full view code can be found https://pastebin.com/d974V5Si but main functions below
var body: some View {
VStack {
// Visual code here
}
.onAppear() {
DispatchQueue.global(qos: .background).async {
while (getHowManyProjectsToUpdate() > 0) {
leftToUpdate = getHowManyProjectsToUpdate()
updateLocal()
}
if getHowManyProjectsToUpdate() == 0 {
while (getNumberOfFilesInDocumentsDirectory() > 0) {
deleteImagesFromDocumentsDirectory()
}
if getNumberOfFilesInDocumentsDirectory() == 0 {
DispatchQueue.main.asyncAfter(deadline: .now()) {
withAnimation {
self.isActive = true
}
}
}
}
}
}
}
update local function
func updateLocal() {
autoreleasepool {
let fetchRequest: NSFetchRequest<Project> = Project.fetchRequest()
fetchRequest.predicate = NSPredicate(format: "converted = %d", false)
fetchRequest.fetchLimit = 1
fetchRequest.sortDescriptors = [NSSortDescriptor(keyPath: \Project.name, ascending: true), NSSortDescriptor(keyPath: \Project.name, ascending: true)]
do {
let projects = try viewContext.fetch(fetchRequest)
for project in projects {
currentPicNumber = 0
currentProjectName = project.name ?? "Error loading project"
if let projectMain = project.mainPicture {
currentProjectImage = getUIImage(picture: projectMain)
}
if let pictures = project.pictures {
projectPicNumber = pictures.count
// Get main image
if let projectMain = project.mainPicture {
if let imgThumbData = convertImageThumb(picture: projectMain) {
project.mainPictureData = imgThumbData
}
}
while (getTotalImagesToConvertForProject(project: project ) > 0) {
convertImageBatch(project: project)
}
project.converted = true
saveContext()
viewContext.refreshAllObjects()
}
}
} catch {
print("Fetch Failed")
}
}
}
convertImageBatch function
func convertImageBatch(project: Project) {
autoreleasepool {
let fetchRequestPic: NSFetchRequest<Picture> = Picture.fetchRequest()
let projectPredicate = NSPredicate(format: "project = %@", project)
let dataPredicate = NSPredicate(format: "pictureData == NULL")
fetchRequestPic.predicate = NSCompoundPredicate(andPredicateWithSubpredicates: [projectPredicate, dataPredicate])
fetchRequestPic.fetchLimit = 5
fetchRequestPic.sortDescriptors = [NSSortDescriptor(keyPath: \Picture.dateTaken, ascending: true)]
do {
let pictures = try viewContext.fetch(fetchRequestPic)
for picture in pictures {
currentPicNumber = currentPicNumber + 1
if let imgData = convertImage(picture: picture), let imgThumbData = convertImageThumb(picture: picture) {
// Save Converted
picture.pictureData = imgData
picture.pictureThumbnailData = imgThumbData
// Save Image
saveContext()
viewContext.refreshAllObjects()
} else {
viewContext.delete(picture)
saveContext()
viewContext.refreshAllObjects()
}
}
} catch {
print("Fetch Failed")
}
}
}
And finally saving
func saveContext() {
do {
try viewContext.save()
} catch {
let nsError = error as NSError
fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
}
}
As @deeje pointed out, using viewContext in a background queue, as shown in your following code snippet, does not follow the Core Data multi-threading programming pattern that requires the access to a Core Data context (NSManagedObjectContext) and its objects be from the context’s queue.
DispatchQueue.global(qos: .background).async {
while (getHowManyProjectsToUpdate() > 0) {
leftToUpdate = getHowManyProjectsToUpdate()
updateLocal()
}
...
}
func updateLocal() {
...
viewContext.refreshAllObjects()
}
Violating the programming pattern can trigger random crashes in Core Data, which probably explains the issue you described.
With Xcode, you can use -com.apple.CoreData.ConcurrencyDebug 1 as a launch argument to check if your code has the concurrency issue. Note that the argument has the “-“ prefix. To do that:
- Open your project with Xcode.
- Click the target in the middle of Xcode’s title bar to show the drop list, and then click “Edit Schema...” to show the schema editing dialog.
- Add the argument to the “Arguments Passed on Launch” list, as shown in the attached screenshot.
- Run your app and try to reproduce the issue.
If your app hits a concurrency violation, the debugger will halt at the following symbol:
+[NSManagedObjectContext __Multithreading_Violation_AllThatIsLeftToUsIsHonor__].
Please give it a try and let me know if this helps.
In general, you can fix this kind of issue by following this pattern:
-
Always use a queue-based context, meaning that alway use init(concurrencyType:) to create your
NSManagedObjectContextinstance. -
Wrap the code accessing the context and its objects with perform(_:) or performAndWait(_:).
-
If you need to pass Core Data objects across different contexts, use NSManagedObjectID, rather than
NSManagedObject.
To better understand the Core Data concurrency topic, I recommend you to go through the following technical resources:
Best,
——
Ziqiao Chen
Worldwide Developer Relations.
