从CoreData获取并强制转换

Question

When fetching data from CoreData the results is an array of AnyObjects . When I cast this array to the related class, I can use the data with no problems.

My question is, I want to do something when the objects are being initialized but I don't know where and when the objects are getting initialised after it has been fetched from CoreData.

A sample of how I get the data from CoreData:

let request = NSFetchRequest(entityName: "Buildings")
let results = (try? context.executeFetchRequest(request)) as? [Buildings] ?? []

with my class as:

class Buildings: NSManagedObject {

    @NSManaged var name: String

    convenience init(context: NSManagedObjectContext, name: String) {
        let description = NSEntityDescription.entityForName("Buildings", inManagedObjectContext: context)!
        self.init(entity: description, insertIntoManagedObjectContext: context)

        self.name = name
    }
}

Answer 1

You need to overwrite awakeFromFetch() method in your Buildings class like

func awakeFromFetch() {
    super.awakeFromFetch()
    // do your initialization
}

This method is called on every fetch of the object from the persistent store.

Answer 2

What you are talking about is faulting, basically Core Data tries to keep its memory footprint as low as possible and one of the strategies it uses to accomplish this is faulting. When you fetch the records for your entity, Core Data executed the fetch request, but it didn't fully initialize the managed objects representing the fetched records.

The moment you access an attribute or relationship of a managed object, the fault is fired, which means that Core Data changes the fault into a realized managed object.