在Swift中,我有一个名为managedObjectContext: NSManagedObjectContext
的计算属性。当我尝试像这样初始化它时:
var managedObjectContext: NSManagedObjectContext? {
get {
createManagedObjectContext()
return self.managedObjectContext
}
set(newManagedObjectContext) {
self.manageObjectContext = newManagedObjectContext
}
}
func createManagedObjectContext() -> NSManagedObjectContext {
if let coordinator = persistentStorageCoordinator() as NSPersistentStoreCoordinator!
{
self.managedObjectContext = NSManagedObjectContext()
self.managedObjectContext!.persistentStoreCoordinator = coordinator
NSNotificationCenter.defaultCenter().addObserver(self, selector: "handleBackgroundMOCDidSaveNotification:", name: NSManagedObjectContextDidSaveNotification, object: self.managedObjectContext)
}
return self.managedObjectContext!;
}
我收到一个错误:Cannot assign to "managedObjectContext" in self
。
有人能帮我解决这个问题吗?
提前谢谢。
发布于 2015-03-01 17:04:59
当您尝试为let
常量赋值时,也会收到此编译错误。在本例中,将变量的类型改为var
应该可以解决这个问题。
发布于 2014-07-08 00:14:11
惰性存储属性(参见Swift Programming Language一书,第313页)更适合您在这里尝试做的事情。
lazy var managedObjectContext = createManagedObjectContext()
直到您第一次尝试访问该属性(且仅在第一次尝试访问)时,才会调用createManagedObjectContext()
。
发布于 2014-09-01 17:22:13
添加!!=)
// MARK: - Core Data stack
lazy var applicationDocumentsDirectory: NSURL = {
// The directory the application uses to store the Core Data store file. This code uses a directory named "com.YOURNAME_OTHERS.YOURAPP" in the application's documents Application Support directory.
let urls = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
return urls[urls.count-1] as NSURL
}()
lazy var managedObjectModel: NSManagedObjectModel = {
// The managed object model for the application. This property is not optional. It is a fatal error for the application not to be able to find and load its model.
let modelURL = NSBundle.mainBundle().URLForResource("YOURAPP", withExtension: "momd")
return NSManagedObjectModel(contentsOfURL: modelURL)
}()
lazy var persistentStoreCoordinator: NSPersistentStoreCoordinator? = {
// The persistent store coordinator for the application. This implementation creates and return a coordinator, having added the store for the application to it. This property is optional since there are legitimate error conditions that could cause the creation of the store to fail.
// Create the coordinator and store
var coordinator: NSPersistentStoreCoordinator? = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
let url = self.applicationDocumentsDirectory.URLByAppendingPathComponent("YOURAPP.sqlite")
var error: NSError? = nil
var failureReason = "There was an error creating or loading the application's saved data."
if coordinator!.addPersistentStoreWithType(NSSQLiteStoreType, configuration: nil, URL: url, options: nil, error: &error) == nil {
coordinator = nil
// Report any error we got.
let dict = NSMutableDictionary()
dict[NSLocalizedDescriptionKey] = "Failed to initialize the application's saved data"
dict[NSLocalizedFailureReasonErrorKey] = failureReason
dict[NSUnderlyingErrorKey] = error
error = NSError.errorWithDomain("YOUR_ERROR_DOMAIN", code: 9999, userInfo: dict)
// Replace this with code to handle the error appropriately.
// abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
NSLog("Unresolved error \(error), \(error!.userInfo)")
abort()
}
return coordinator
}()
lazy var managedObjectContext: NSManagedObjectContext? = {
// Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.) This property is optional since there are legitimate error conditions that could cause the creation of the context to fail.
let coordinator = self.persistentStoreCoordinator
if coordinator == nil {
return nil
}
var managedObjectContext = NSManagedObjectContext()
managedObjectContext.persistentStoreCoordinator = coordinator
return managedObjectContext
}()
// MARK: - Core Data Saving support
func saveContext () {
if let moc = self.managedObjectContext {
var error: NSError? = nil
if moc.hasChanges && !moc.save(&error) {
// Replace this implementation with code to handle the error appropriately.
// abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
NSLog("Unresolved error \(error), \(error!.userInfo)")
//abort()
}
}
}
https://stackoverflow.com/questions/24603064
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