NSPersistentDocument FetchRequest warp 属性 在 macOS Document App SwiftUI 项目上崩溃

Question

1. 使用 Xcode macOS Document App 模板创建项目，选中 Use Core Data 复选框。
2. 将图书实体添加到 Document.xcdatamodeld
3. 将 FetchRequest 扭曲 属性 添加到 ContentView，

@FetchRequest(entity: Book.entity(), sortDescriptors: []) var books: FetchedResults<Book>

4. 生成并运行、崩溃！

来自控制台的崩溃日志是

2020-07-03 23:12:23.597880+0800 DocMacDemo[15236:4376209] [error] error: No NSEntityDescriptions in any model claim the NSManagedObject subclass 'DocMacDemo.Book' so +entity is confused.  Have you loaded your NSManagedObjectModel yet ?
CoreData: error: No NSEntityDescriptions in any model claim the NSManagedObject subclass 'DocMacDemo.Book' so +entity is confused.  Have you loaded your NSManagedObjectModel yet ?
2020-07-03 23:12:23.598287+0800 DocMacDemo[15236:4376209] [error] error: +[DocMacDemo.Book entity] Failed to find a unique match for an NSEntityDescription to a managed object subclass
CoreData: error: +[DocMacDemo.Book entity] Failed to find a unique match for an NSEntityDescription to a managed object subclass
2020-07-03 23:12:23.644491+0800 DocMacDemo[15236:4376209] executeFetchRequest:error: A fetch request must have an entity.
2020-07-03 23:12:23.653769+0800 DocMacDemo[15236:4376209] [error] error: The fetch request's entity 0x600003500420 'Book' appears to be from a different NSManagedObjectModel than this context's
CoreData: error: The fetch request's entity 0x600003500420 'Book' appears to be from a different NSManagedObjectModel than this context's
(lldb)

几天来我一直在寻找 NSPersistentDocument SwiftUI 示例，但找不到。 这里有一些相似或相关的问题。可惜这个问题没有解决。

• Using @fetchRequest(entity: ) for SwiftUI macOS app crashes
• https://developer.apple.com/forums/thread/124656?answerId=417869022#417869022
• https://developer.apple.com/forums/thread/132624

编辑： 上传本期项目到Github, https://github.com/donly/DocMacDemo.

Answer 1

这是因为新文档为空。与任何基于文档的应用程序一样，您必须为新文档准备一些默认的初始数据

这是可能的解决方案。使用 Xcode 11.4 / iOS 13.4

测试

在Document.swift

class Document: NSPersistentDocument {

    // .. other code here

    override func makeWindowControllers() {

        // in case of new document create new empty book in context
        // that will be shown in opened document window
        let isNew = self.fileURL == nil
        if isNew {
            _ = Book(context: self.managedObjectContext!)       // << here !!
        }

        let contentView = ContentView().environment(\.managedObjectContext, self.managedObjectContext!)

        // ... other code here

Answer 2

好吧，Asperi 的回答很好，但是在对象模型中留下了一本可能不需要的书。另一种方法是将上下文保存在 Asperi 建议的同一位置：

    if let context = managedObjectContext {
        try? context.save()
    }

那么 FetchRequest 也不会有错误信息。