Swiftui＆amp;单位测试失败：使用核心数据堆栈时可以两次添加同一商店

Question

将Xcode更新为13.4后，我的核心数据持久性堆栈开始在SwiftUI预览和Xcode单元测试中失败。

错误消息：

Unresolved error Error Domain=NSCocoaErrorDomain Code=134081 "(null)" UserInfo={NSUnderlyingException=Can't add the same store twice}, ["NSUnderlyingException": Can't add the same store twice ...

持久性堆栈：

struct PersistenceController {
    static let shared = PersistenceController()

    let container: NSPersistentContainer

    init(inMemory: Bool = false) {
        container = NSPersistentContainer(name: "Persistence")
        if inMemory {
            container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
        }
        let description = NSPersistentStoreDescription()

        description.shouldInferMappingModelAutomatically = true
        description.shouldMigrateStoreAutomatically = true

        container.persistentStoreDescriptions.append(description)
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
        container.viewContext.automaticallyMergesChangesFromParent = true
    }
}

Swiftui预览的用法：

struct MyScreen_Previews: PreviewProvider {
    static var previews: some View {
        MyScreen()
            .environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
    }
}

支持Swiftui预览的扩展名：

extension PersistenceController {
    static var preview: PersistenceController = {
        let result = PersistenceController(inMemory: true)
        // Pre-fill core data with required information for preview.
    }
}

Answer 1

代码中存在一个错误，当您附加到描述数组时，您现在有2个描述。

将其更改为：

let description = container.persistentStoreDescriptions.first!
description.shouldInferMappingModelAutomatically = true
description.shouldMigrateStoreAutomatically = true

// Load

Answer 2

我注意到，加载持久性存储错误产生后。此错误（不能两次添加同一商店），对于SwiftUI预览和单位测试，可以忽略。

要忽略此类型的错误，需要检查 XctestSessionIdentifier 和 Xcode_running_for_previews 在过程信息中。

if let error = error as NSError? {
    if ProcessInfo.processInfo.environment["XCTestSessionIdentifier"] == nil && 
       ProcessInfo.processInfo.environment["XCODE_RUNNING_FOR_PREVIEWS"] == nil {
        fatalError("Unresolved error \(error), \(error.userInfo)")
    }
}