【发布时间】：2019-05-28 23:56:52
【问题描述】：
我已经为此苦苦挣扎了好几天。我会很感激任何帮助。
我有一个位置NSManagedObject 和一个图像NSManagedObject，它们具有一对多的关系，即一个位置有许多图像。
我有 2 个屏幕，在第一个屏幕中，用户在视图上下文中添加位置，并且它们被添加和检索而没有问题。
现在，在第二个屏幕中，我想根据在第一个屏幕中选择的位置检索图像，然后在 Collection View 中显示图像。图像首先从 flickr 检索，然后保存在数据库中。
我想在背景上下文中保存和检索图像，这给我带来了很多问题。
- 当我尝试保存从 flickr 检索到的每张图像时，我收到一条警告，指出存在悬空对象并且无法建立关系：
这是我的保存代码：
func saveImagesToDb () {
//Store the image in the DB along with its location on the background thread
if (doesImageExist()){
dataController.backgroundContext.perform {
for downloadedImage in self.downloadedImages {
print ("saving to context")
let imageOnMainContext = Image (context: self.dataController.viewContext)
let imageManagedObjectId = imageOnMainContext.objectID
let imageOnBackgroundContext = self.dataController.backgroundContext.object(with: imageManagedObjectId) as! Image
let locationObjectId = self.imagesLocation.objectID
let locationOnBackgroundContext = self.dataController.backgroundContext.object(with: locationObjectId) as! Location
let imageData = NSData (data: downloadedImage.jpegData(compressionQuality: 0.5)!)
imageOnBackgroundContext.image = imageData as Data
imageOnBackgroundContext.location = locationOnBackgroundContext
try? self.dataController.backgroundContext.save ()
}
}
}
}
正如您在上面的代码中所见，我正在根据从视图上下文中检索到的 ID 在后台上下文中构建 NSManagedObject。每次调用saveImagesToDb 我都会收到警告，那么问题是什么？
- 尽管有上面的警告，当我通过 FetchedResultsController（在后台上下文中工作）检索数据时。集合视图有时可以很好地查看图像，有时我会收到此错误：
Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid update: invalid number of items in section 0. The number of items contained in an existing section after the update (4) must be equal to the number of items contained in that section before the update (1), plus or minus the number of items inserted or deleted from that section (1 inserted, 0 deleted) and plus or minus the number of items moved into or out of that section (0 moved in, 0 moved out).'
这里有一些与设置 FetchedResultsController 和根据上下文或 FetchedResultsController 中的更改更新 Collection View 相关的代码 sn-ps。
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
guard let imagesCount = fetchedResultsController.fetchedObjects?.count else {return 0}
return imagesCount
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
print ("cell data")
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "photoCell", for: indexPath) as! ImageCell
//cell.placeImage.image = UIImage (named: "placeholder")
let imageObject = fetchedResultsController.object(at: indexPath)
let imageData = imageObject.image
let uiImage = UIImage (data: imageData!)
cell.placeImage.image = uiImage
return cell
}
func setUpFetchedResultsController () {
print ("setting up controller")
//Build a request for the Image ManagedObject
let fetchRequest : NSFetchRequest <Image> = Image.fetchRequest()
//Fetch the images only related to the images location
let locationObjectId = self.imagesLocation.objectID
let locationOnBackgroundContext = self.dataController.backgroundContext.object(with: locationObjectId) as! Location
let predicate = NSPredicate (format: "location == %@", locationOnBackgroundContext)
fetchRequest.predicate = predicate
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "location", ascending: true)]
fetchedResultsController = NSFetchedResultsController (fetchRequest: fetchRequest, managedObjectContext: dataController.backgroundContext, sectionNameKeyPath: nil, cacheName: "\(latLongString) images")
fetchedResultsController.delegate = self
do {
try fetchedResultsController.performFetch ()
} catch {
fatalError("couldn't retrive images for the selected location")
}
}
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
print ("object info changed in fecthed controller")
switch type {
case .insert:
print ("insert")
DispatchQueue.main.async {
print ("calling section items")
self.collectionView!.numberOfItems(inSection: 0)
self.collectionView.insertItems(at: [newIndexPath!])
}
break
case .delete:
print ("delete")
DispatchQueue.main.async {
self.collectionView!.numberOfItems(inSection: 0)
self.collectionView.deleteItems(at: [indexPath!])
}
break
case .update:
print ("update")
DispatchQueue.main.async {
self.collectionView!.numberOfItems(inSection: 0)
self.collectionView.reloadItems(at: [indexPath!])
}
break
case .move:
print ("move")
DispatchQueue.main.async {
self.collectionView!.numberOfItems(inSection: 0)
self.collectionView.moveItem(at: indexPath!, to: newIndexPath!)
}
}
}
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange sectionInfo: NSFetchedResultsSectionInfo, atSectionIndex sectionIndex: Int, for type: NSFetchedResultsChangeType) {
print ("section info changed in fecthed controller")
let indexSet = IndexSet(integer: sectionIndex)
switch type {
case .insert:
self.collectionView!.numberOfItems(inSection: 0)
collectionView.insertSections(indexSet)
break
case .delete:
self.collectionView!.numberOfItems(inSection: 0)
collectionView.deleteSections(indexSet)
case .update, .move:
fatalError("Invalid change type in controller(_:didChange:atSectionIndex:for:). Only .insert or .delete should be possible.")
}
}
func addSaveNotificationObserver() {
removeSaveNotificationObserver()
print ("context onbserver notified")
saveObserverToken = NotificationCenter.default.addObserver(forName: .NSManagedObjectContextObjectsDidChange, object: dataController?.backgroundContext, queue: nil, using: handleSaveNotification(notification:))
}
func removeSaveNotificationObserver() {
if let token = saveObserverToken {
NotificationCenter.default.removeObserver(token)
}
}
func handleSaveNotification(notification:Notification) {
DispatchQueue.main.async {
self.collectionView!.numberOfItems(inSection: 0)
self.collectionView.reloadData()
}
}
我做错了什么？我会很感激任何帮助。
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标签： ios swift multithreading core-data data-persistence