swift - 核心数据，初始化 managedObjectContext

Question

我正在尝试学习如何使用 Core Date 为我正在开发的应用程序存储图像。我创建了一个实体并导入了核心数据。到目前为止，在我看过的所有教程中，他们都创建了一个包含以下内容的 var。

let moc = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext

出于某种原因，当我将该行放入我的项目时，出现错误:

Value of type AppDelegate has no member managedObjectContext

Answer 1

你看的教程好像都是在创建项目的时候勾选“Use Core Data”复选框:

如果选中此复选框，您将在 AppDelegate 中声明一个名为 managedObjectContext 的属性。

但由于您没有选中该复选框，因此不会为您生成。

下面是将生成的额外代码:

// 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.TimerReminder" in the application's documents Application Support directory.
    let urls = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
    return urls[urls.count-1]
}()

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("XXX", withExtension: "momd")!
    return NSManagedObjectModel(contentsOfURL: modelURL)!
}()

lazy var persistentStoreCoordinator: NSPersistentStoreCoordinator = {
    // The persistent store coordinator for the application. This implementation creates and returns 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
    let coordinator = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
    let url = self.applicationDocumentsDirectory.URLByAppendingPathComponent("SingleViewCoreData.sqlite")
    var failureReason = "There was an error creating or loading the application's saved data."
    do {
        try coordinator.addPersistentStoreWithType(NSSQLiteStoreType, configuration: nil, URL: url, options: nil)
    } catch {
        // Report any error we got.
        var dict = [String: AnyObject]()
        dict[NSLocalizedDescriptionKey] = "Failed to initialize the application's saved data"
        dict[NSLocalizedFailureReasonErrorKey] = failureReason

        dict[NSUnderlyingErrorKey] = error as NSError
        let wrappedError = NSError(domain: "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 \(wrappedError), \(wrappedError.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
    var managedObjectContext = NSManagedObjectContext(concurrencyType: .MainQueueConcurrencyType)
    managedObjectContext.persistentStoreCoordinator = coordinator
    return managedObjectContext
}()

// MARK: - Core Data Saving support

func saveContext () {
    if managedObjectContext.hasChanges {
        do {
            try managedObjectContext.save()
        } catch {
            // 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.
            let nserror = error as NSError
            NSLog("Unresolved error \(nserror), \(nserror.userInfo)")
            abort()
        }
    }
}

只需复制上面的代码并添加到您的应用委托(delegate)文件中。请注意，您应该替换此行中的 "XXX":

let modelURL = NSBundle.mainBundle()
    .URLForResource("XXX", withExtension: "momd")!

使用您的项目名称。

此外，复选框还使 Xcode 在 applicationWillTerminate 方法中生成对 saveContext 的调用:

func applicationWillTerminate(application: UIApplication) {
    // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
    // Saves changes in the application's managed object context before the application terminates.
    self.saveContext()
}