如何将现有对象保存到 Core Data

Question

我一直在做一个项目并且有一个我正在使用的现有对象。我试图找到一种简单的方法来保存、检索和删除 Core Data 中这些对象的列表。

这是我的对象

import Foundation

class Book : Codable {

    var coverIndex:Int?
    var authorName:[String]?
    var title:String?
    var editionCount:Int?
    var firstPublishYear:Int?
    var key: String?
    var publishPlace:[String]?
    var publisher:[String]?

    public enum BookResponseCodingKeys: String, CodingKey {
        case coverIndex = "cover_i"
        case authorName = "author_name"
        case editionCount = "edition_count"
        case firstPublishYear = "first_publish_year"
        case key = "key"
        case title = "title"
        case publishPlace = "publish_place"
        case publisher = "publisher"
    }

    public required init(from decoder: Decoder) throws {

        let container = try decoder.container(keyedBy: BookResponseCodingKeys.self)

        self.coverIndex = try container.decodeIfPresent(Int.self, forKey: .coverIndex)
        self.authorName = try container.decodeIfPresent([String].self, forKey: .authorName)
        self.editionCount = try container.decodeIfPresent(Int.self, forKey: .editionCount)
        self.firstPublishYear = try container.decodeIfPresent(Int.self, forKey: .firstPublishYear)
        self.key = try container.decodeIfPresent(String.self, forKey: .key)
        self.title = try container.decodeIfPresent(String.self, forKey: .title)
        self.publishPlace = try container.decodeIfPresent([String].self, forKey: .publishPlace)
        self.publisher = try container.decodeIfPresent([String].self, forKey: .publisher)
    }
}

将其保存在 Core Data 中（或将其映射到 Core Data 模型）最直接的方法是什么。

Answer 1

在您xcdatamodeld定义一个实体，例如User：

添加具有 Transformable 类型的属性。将其命名为“书籍”。

接下来，将 Transformable 属性的类设置为 Book 数组。在Custom Class下面的字段中。

使用以下代码从当前上下文中获取当前数组。这些方法应该放在某种 DataManager 类中（应该是单例）：

import CoreData

open class DataManager: NSObject {
    
    public static let sharedInstance = DataManager()
    
    private override init() {}

    // Helper func for getting the current context.
    private func getContext() -> NSManagedObjectContext? {
        guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else { return nil }
        return appDelegate.persistentContainer.viewContext
    }

    func retrieveUser() -> NSManagedObject? {
        guard let managedContext = getContext() else { return nil }
        let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "User")
        
        do {
            let result = try managedContext.fetch(fetchRequest) as! [NSManagedObject]
            if result.count > 0 {
                // Assuming there will only ever be one User in the app.
                return result[0]
            } else {
                return nil
            }
        } catch let error as NSError {
            print("Retrieving user failed. \(error): \(error.userInfo)")
           return nil
        }
    }

    func saveBook(_ book: Book) {
        print(NSStringFromClass(type(of: book)))
        guard let managedContext = getContext() else { return }
        guard let user = retrieveUser() else { return }
        
        var books: [Book] = []
        if let pastBooks = user.value(forKey: "books") as? [Book] {
            books += pastBooks
        }
        books.append(book)
        user.setValue(books, forKey: "books")
        
        do {
            print("Saving session...")
            try managedContext.save()
        } catch let error as NSError {
            print("Failed to save session data! \(error): \(error.userInfo)")
        }
    }

}

您还需要一种方法来创建用户（并且可能删除，假设我们要遵循 CRUD）。首先，您需要获取对用户实体的引用来创建一个。此定义应位于 DataManager 类的顶部。

extension DataManager {
    private lazy var userEntity: NSEntityDescription = {
        let managedContext = getContext()
        return NSEntityDescription.entity(forEntityName: "User", in: managedContext!)!
    }()
}

然后实现这个函数来创建一个。

extension DataManager {
    /// Creates a new user with fresh starting data.
    func createUser() {
        guard let managedContext = getContext() else { return }
        let user = NSManagedObject(entity: userEntity, insertInto: managedContext)
        
        do {
            try managedContext.save()
        } catch let error as NSError {
            print("Failed to save new user! \(error): \(error.userInfo)")
        }
    }
}

现在只需调用：

DataManager.sharedInstance.createUser()

创建一个新用户。然后，将书籍附加到用户的存储中：

DataManager.sharedInstance.saveBook(book)