如何在 Swift 中将可转换属性保存到 Core Data

Posted 2023-03-05

【中文标题】如何在 Swift 中将可转换属性保存到 Core Data

【英文标题】：How to save transformable attribute to Core Data in Swift

【发布时间】：2020-08-13 21:08:04

【问题描述】：

我正在构建一个从 TMDB 获取电影数据的 SwiftUI 应用。当用户将电影添加到他们的观看列表时，我试图将数据保存为 Core Data 中的可转换属性。

问题是我无法保存数据。

这是我的核心数据模型

我已经为 TransformableMovie 实体创建了 NSManagedObject NSManagedObject 子类：

TransformableMovie+CoreDataClass.swift

import Foundation
import CoreData

@objc(TransformableMovie)
public class TransformableMovie: NSManagedObject 

TransformableMovie+CoreDataProperties.swift

import Foundation
import CoreData


extension TransformableMovie 

    @nonobjc public class func fetchRequest() -> NSFetchRequest<TransformableMovie> 
        return NSFetchRequest<TransformableMovie>(entityName: "TransformableMovie")
    

    @NSManaged public var movie: MovieClass?

这是我创建的类：

MovieClass.swift

import Foundation

public class MovieClass: NSObject, NSCoding 
    
    var movie: Movie
    
    init(movie: Movie) 
        self.movie = movie
        
    
    required public init?(coder aDecoder: NSCoder) 
        self.movie = aDecoder.decodeObject(forKey: "movie") as! Movie

    
    
    public func encode(with aCoder: NSCoder) 
        aCoder.encode(movie, forKey: "movie")
        
    

这是我试图保存数据的视图（在 navigationBarItems 按钮中）

MovieDetailView.swift

import SwiftUI
import CoreData

struct MovieDetailView: View 
    
    @ObservedObject private var detailVM = MovieDetailViewModel()
    
    // Core data
    @Environment(\.managedObjectContext) var managedObjectContext
    
    var movie: Movie
    
    @State private var showingAlert = false
    
    init(movie: Movie) 
        self.movie = movie
        detailVM.getMovieDetails(id: movie.id)
        
        // Stop Scrollview bounce
        UIScrollView.appearance().bounces = false
    
    
    var body: some View 
        
        VStack 
            
            ...

        .navigationBarTitle(movie.title)
        
        .navigationBarItems(trailing:
                                Button(action: 
                                    
                                    // Save to core data
                                    let movieToBeSaved = TransformableMovie(context: self.managedObjectContext)
                                    
                                    movieToBeSaved.movie?.movie = detailVM.fetchedMovie!
     
                                    self.showingAlert = true
                                    
                                    do 
                                        try self.managedObjectContext.save()
                                     catch 
                                        // handle the Core Data error
                                    
                                ) 
                                    Image(systemName: "plus")
                                        .renderingMode(.original)
                                .alert(isPresented: $showingAlert) 
                                    Alert(title: Text("Saved"), message: Text("Movie added to your watch list"), dismissButton: .default(Text("Ok")))
                                )
        
    

任何帮助将不胜感激。

【参考方案1】：

我认为问题在于这一行：

movieToBeSaved.movie?.movie = detailVM.fetchedMovie!

movie? 属性为 nil，因此赋值永远不会起作用。从您提供的代码中很难看出，但我认为您需要先创建一个 ModelClass 对象，然后分配它：

let movieModel = ModelClass(movie: detailVM.fetchedMovie)
movieToBeSaved.movie = movieModel