无法在 swift 文件中创建上下文答案

【问题标题】：unable to create a context in swift file无法在 swift 文件中创建上下文
【发布时间】：2019-02-04 09:19:11
【问题描述】：

我需要从 CoreData 获取一些数据并且需要经常这样做，因此尝试为它创建实用程序类。当我尝试为它创建上下文时，它给了我错误，下面是代码。我添加了一个新的 .swift 文件并粘贴在下面的代码

import Foundation
import UIKit
import CoreData

class armyDataSource{

    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    let context = appDelegate.persistentContainer.viewContext


}

真的不知道我在这里做错了什么。

【问题讨论】：

错误是什么？

标签： ios swift core-data

【解决方案1】：

如果你想为核心数据管理器创建一个包装类，你可以在你的实现的 swift 文件中编写如下代码所示的类。

 import UIKit
 import CoreData
class CoreDataManager {


static let sharedManager = CoreDataManager()
private init() {} // Prevent clients from creating another instance.

lazy var persistentContainer: NSPersistentContainer = {
    /*
     The persistent container for the application. This implementation
     creates and returns a container, having loaded 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.
     */
    let container = NSPersistentContainer(name: "StackOF")
    container.loadPersistentStores(completionHandler: { (storeDescription, error) in
        if let error = error as NSError? {
            // Replace this implementation with code to handle the error appropriately.
            // fatalError() 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.

            /*
             Typical reasons for an error here include:
             * The parent directory does not exist, cannot be created, or disallows writing.
             * The persistent store is not accessible, due to permissions or data protection when the device is locked.
             * The device is out of space.
             * The store could not be migrated to the current model version.
             Check the error message to determine what the actual problem was.
             */
            fatalError("Unresolved error \(error), \(error.userInfo)")
        }
    })
    return container
}()

// MARK: - Core Data Saving support

func saveContext () {
    let context = persistentContainer.viewContext
    if context.hasChanges {
        do {
            try context.save()
        } catch {
            // Replace this implementation with code to handle the error appropriately.
            // fatalError() 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
            fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
        }
    }
}
}

【讨论】：

【解决方案2】：

您不能像在类中那样初始化这些属性。您需要在方法中进行此初始化，最好在 init 调用中进行。

不能在属性初始化器中使用实例成员“appDelegate”；属性初始化程序在“self”可用之前运行

所以这意味着你不能使用一个属性来初始化另一个属性，因为这一切都是在调用 init 之前完成的，并且 self 完全可用。

试试这个：

class armyDataSource {

    let appDelegate: UIApplicationDelegate
    let context: NSManagedObjectContext

    init() {
        appDelegate = UIApplication.shared.delegate as! AppDelegate
        context = appDelegate.persistentContainer.viewContext
    }
}

【讨论】：