如何为此 SwiftUI CoreData 模型正确插入排序描述符？

Posted 2023-02-24

【中文标题】如何为此 SwiftUI CoreData 模型正确插入排序描述符？

【英文标题】：How do I correctly insert a sort descriptor for this SwiftUI CoreData model?

【发布时间】：2021-02-06 19:39:21

【问题描述】：

我正在尝试实现此处所述的 MVVM 架构：

https://medium.com/better-programming/swiftui-and-coredata-the-mvvm-way-ab9847cbff0f

但我在此文件中遇到运行时错误，无法确定在何处添加排序描述符：

import Foundation
import CoreData
import Combine
import UIKit

class ItemsStorage: NSObject, ObservableObject 
    var items = CurrentValueSubject<[Item], Never>([])
    private let itemFetchController: NSFetchedResultsController<Item>
    
    //SINGLETON INSTANCE
    static let shared: ItemsStorage = ItemsStorage()  ///Run time error: Thread 1: "An instance of NSFetchedResultsController requires a fetch request with sort descriptors"
    
    private override init() 
        
        /// This line doesn't solve the issue
        let sort = NSSortDescriptor(key: "name_", ascending: true)
        
        itemFetchController = NSFetchedResultsController(
        
        fetchRequest: Item.fetchRequest(),
        managedObjectContext: PersistenceController.shared.container.viewContext,
        sectionNameKeyPath: nil, cacheName: nil
        )
        
        /// This line doesn't solve the issue
        itemFetchController.fetchRequest.sortDescriptors = [sort]
        
        super.init()
        
        itemFetchController.delegate = self
        
        do 
            try itemFetchController.performFetch()
            items.value = itemFetchController.fetchedObjects ?? []
         catch 
            NSLog("Error: could not fetch objects")
        
    
    
    func add()
        
    
    
    func update(withID id: UUID) 
        
    
    
    func delete(id: UUID) 
        
    


extension ItemsStorage: NSFetchedResultsControllerDelegate  
    public func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) 
        guard let items = controller.fetchedObjects as? [Item] else  return 
        NSLog("Context has changed, reloading items ...")
        self.items.value = items
    

【参考方案1】：

在初始化获取结果控制器之前，您需要为 fetchRequest 设置排序描述符：

    let sort = NSSortDescriptor(key: "name_", ascending: true)
    let itemFetchRequest = Item.fetchRequest()
    itemFetchRequest.sortDescriptors = [sort]
    
    itemFetchController = NSFetchedResultsController(
    
    fetchRequest: itemFetchRequest,
    managedObjectContext: PersistenceController.shared.container.viewContext,
    sectionNameKeyPath: nil, cacheName: nil
    )

【讨论】：

谢谢。但是，我在“let itemFetchRequest = Item.fetchRequest()”这行出现错误。它说“模糊使用 'fetchRequest()'” 但在我的 Item+Helper.swift 文件中，我确实有定义 fetchRequest 的扩展名，如下所示：extension Item static func fetchRequest(_ predicate: NSPredicate) -> NSFetchRequest<Item> let request = NSFetchRequest<Item>(entityName: "Item") return request 我的问题还有什么？

好的，多看了一下，发现这个：***.com/questions/56838434/… 这似乎解决了后一个问题。现在我有了这个：let sort = NSSortDescriptor(key: "name_", ascending: true) let itemFetchRequest: NSFetchRequest<Item> = Item.fetchRequest() itemFetchRequest.sortDescriptors = [sort] itemFetchController = NSFetchedResultsController ( ...