Question

I'm working in a subclass of NSViewController where in viewDidLoad I fetch my entities from CoreData as below:

let delegate = AppDelegate.init()
let context = delegate.persistentContainer.viewContext
let fetch = PicPathEntity.entityFetchRequest()

var tempEntities: [PicPathEntity] = []
context.performAndWait {
    tempEntities = try! fetch.execute()
}
entities = tempEntities

Then during the itemForRepresentedObjectAt function, it appears that the count of entities is the same, but the property values have become nil/empty.

I did some searching and found similar problems with no clear answers (at least not for Swift)

I can't seem to find information on how to properly fetch and retain the data in my entities variable.

Answer 1

在阅读了对象与上下文之间的关系后，我想出了解决方法。

原来我要做的就是将上下文设为ViewController的属性，而不是viewDidLoad的局部变量。

var entities: [PicPathEntity] = []
var context: NSManagedObjectContext!

override func viewDidLoad() {
    super.viewDidLoad()

    let delegate = AppDelegate.init()
    context = delegate.persistentContainer.viewContext
    let fetch = PicPathEntity.entityFetchRequest()
    context.performAndWait {
        entities = try! fetch.execute()
    }
}

由于对此答案有帮助，我现在将代码更新为：

var entities: [PicPathEntity] = []
var context: NSManagedObjectContext!

override func viewDidLoad() {
    super.viewDidLoad()

    let delegate = NSApplication.shared.delegate as! AppDelegate
    context = delegate.persistentContainer.viewContext
    let fetch = PicPathEntity.entityFetchRequest()
    context.perform {
        self.entities = try! fetch.execute()
        self.picFinderEntityCollectionView.reloadData()
    }

    configureCollectionView()
}

NSManagedObject collection properties become nil after a while

1 个答案: