Question

在Swift中，我有一个名为managedObjectContext: NSManagedObjectContext的计算属性。当我尝试初始化它时：

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!;
}

我收到错误消息：Cannot assign to "managedObjectContext" in self。

有人可以帮忙解决这个问题吗？

提前致谢。

Answer 1

当您尝试将值分配给let常量时，也会收到此编译错误。在这种情况下，将变量的类型更改为var应该可以解决它。

Answer 2

懒惰的存储属性（请参阅Swift编程语言手册，第313页）更适合您在此处尝试的操作。

lazy var managedObjectContext = createManagedObjectContext()

在您第一次（并且只是第一次）尝试访问该属性之前，不会调用

createManagedObjectContext()。

Answer 3

您的问题是您正在使用计算属性，就好像它是标准属性一样，您实际上是在尝试递归调用setter。你不需要在这里使用计算属性，只需要一个标准属性 -

var managedObjectContext: NSManagedObjectContext?

计算属性是根据其他属性在需要时计算其值的属性 - 例如＆＃34; center＆＃34; Swift编程语言中的属性 -

var center: Point {
    get {
        let centerX = origin.x + (size.width / 2)
        let centerY = origin.y + (size.height / 2)
        return Point(x: centerX, y: centerY)
    }
    set(newCenter) {
        origin.x = newCenter.x - (size.width / 2)
        origin.y = newCenter.y - (size.height / 2)
    }
 }

摘自：Apple Inc.“The Swift Programming Language。”iBooks。 https://itunes.apple.com/au/book/swift-programming-language/id881256329?mt=11

中心由原点和大小决定，设置中心会影响原点

Answer 4

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()
        }
    }
}

斯威夫特：不能在自己中分配给“”

4 个答案: