SwiftUI @FetchRequest 使应用程序崩溃并返回错误 [英] SwiftUI @FetchRequest crashes the app and returns error

问题描述

我正在尝试使用 Xcode 11 在我的 mac 应用程序中使用带有 SwiftUI 的核心数据.我在创建项目时检查了使用核心数据".我还创建了实体(称为 VisitedCases)并使用编辑器创建 NSManagedObject 子类文件.我还将 Codegen 设置为手动/无.下面是生成的 NSManagedObject 文件中的代码:

I am trying to use core data within my mac app with SwiftUI using Xcode 11. I have checked "Using Core Data" when creating the project. I also have created the entity (called VisitedCases) and used editor to create NSManagedObject Subclass files. I also have set the Codegen to Manual/none. Here is the code in the generated NSManagedObject files:

VisitedCases+CoreDataProperties.swift

extension VisitedCases {

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

    @NSManaged public var caseNumber: String

} 

VisitedCases+CoreDataClass.swift

@objc(VisitedCases)
public class VisitedCases: NSManagedObject {

}

我将 ContentView.swift 中的@Environment 变量和@FetchRequest 称为:

I called the @Environment variable and the @FetchRequest in ContentView.swift as:

struct ContentView: View {
    @Environment(\.managedObjectContext) var managedObjectContext

    @FetchRequest(entity: VisitedCases.entity(),
                  sortDescriptors: []
                  ) var orders: FetchedResults<VisitedCases>
//@State vars and the rest of the code
}

但是，当我运行时，应用程序一启动就崩溃，输出中出现以下错误:

However, when I run, the app crashed as soon as launch with the following errors in the output:

2020-02-23 18:36:16.889306+0330 ImageSelector[17874:149503] [error] error: No NSEntityDescriptions in any model claim the NSManagedObject subclass 'VisitedCases' so +entity is confused.  Have you loaded your NSManagedObjectModel yet ?
CoreData: error: No NSEntityDescriptions in any model claim the NSManagedObject subclass 'VisitedCases' so +entity is confused.  Have you loaded your NSManagedObjectModel yet ?
2020-02-23 18:36:16.889389+0330 ImageSelector[17874:149503] [error] error: +[VisitedCases entity] Failed to find a unique match for an NSEntityDescription to a managed object subclass
CoreData: error: +[VisitedCases entity] Failed to find a unique match for an NSEntityDescription to a managed object subclass
2020-02-23 18:36:16.921131+0330 ImageSelector[17874:149503] executeFetchRequest:error: A fetch request must have an entity.

我还有一个函数可以将字符串保存到存储中，并且似乎工作得很好:

I also have a function that saves a string to storage and seems to work just fine:

    func addCaseNumber (caseNo: String) {
        guard caseNo != "" else {return}
        let newCaseNumber = VisitedCases(context: self.managedObjectContext)
        newCaseNumber.caseNumber = caseNo
        do {
         try self.managedObjectContext.save()
         print("Case number saved.")
        } catch {
         print(error.localizedDescription)
         }
    }

我的代码有什么问题，我应该怎么做才能修复它?

What is wrong with my code and what should I do to fix it?

推荐答案

在@enviromentVAr中，有时需要自己设置环境.

In the @enviromentVAr, sometimes, you need to set the enviroment by yourself.

 let managedObjectContext: NSManagedObjectContext =  ((UIApplication.shared.delegate as? AppDelegate)?.persistentContainer.viewContext)!

 ContentView().environment(\.managedObjectContext, managedObjectContext)

那么 managedObjectContext 就可以工作了.

Then the managedObjectContext can work.

 @Environment(\.managedObjectContext) var managedObjectContext

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