ios - 核心数据。如何交换 NSPersistentStores 并通知 NSFetchedResultsController？

Question

我正在(通过 Dropbox)实现用户的Core Data 持久数据的备份和恢复。为了进行恢复，我从 Dropbox 中提取文件并将它们临时存储在 Documents 目录中。然后，我创建一个新的 NSPercientContainer 并用它来替换当前的持久存储(在 ApplicationSupport 目录中)，然后再删除 Documents 目录中不再需要的文件。

我现在使用 MasterDetail 模板，因此我有通常的时间戳实体和随之而来的 NSFetchedResultsController 。进行替换后，我切换回主 tableView，不幸的是看到了原始的实体集。当应用重新启动时，我按预期看到恢复的数据，但需要弄清楚如何使NSFetchedResultsController自动看到新恢复的数据。

这是我的恢复代码:

func restorePSFromBackup() {
    // Current persistent store is in the ApplicationSupport directory.
    let currentPSFolderUrl = FileManager.default.urls(for: .applicationSupportDirectory, in:.userDomainMask).first!
    // The current Core Data file (url).
    let currentPSUrl1 = currentPSFolderUrl.appendingPathComponent("DBCDTest.sqlite")
    // Backup persistent store with which to restore is in the Documents directory.
    let backUpFolderUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
    // The 3 backup Core Data files (urls).
    let backupUrl1 = backUpFolderUrl.appendingPathComponent("DBCDTest.sqlite")
    let backupUrl2 = backUpFolderUrl.appendingPathComponent("DBCDTest.sqlite-wal")
    let backupUrl3 = backUpFolderUrl.appendingPathComponent("DBCDTest.sqlite-shm")
    let sourceSqliteURLs = [backupUrl1, backupUrl2, backupUrl3]

    let container = NSPersistentContainer(name: "DBCDTest")

    do {
        // Replace current persistent store with the restore/backup persistent store.
        try container.persistentStoreCoordinator.replacePersistentStore(at: currentPSUrl1, destinationOptions: nil, withPersistentStoreFrom: backupUrl1, sourceOptions: nil, ofType: NSSQLiteStoreType)
        // Delete the restore/backup files from the Application directory.
        do {
            for index in 0..<sourceSqliteURLs.count {
                try FileManager.default.removeItem(at: sourceSqliteURLs[index])
            }
        } catch let error {
            print("Failed to delete sqlite files.")
            print(error.localizedDescription)
        }
    } catch let error {
        print("Failed to replace persistent store.")
        print(error.localizedDescription)
    }
}

我尝试使用依赖注入(inject)来引用在 AppDelegate 中创建的容器，并使用它来执行交换，但尝试替换持久存储失败，而不是创建一个新的 NSPercientContainer。这可能是某种 NSManagedObjectContext 问题吗？在访问新数据存储之前是否需要刷新？

Answer 1

我设法从许多类似问题的答案中汇总出一个答案，特别是 Tom Harrington 中的这个答案。 。简而言之，我需要做的是:

1. 创建一个新的 NSPercientContainer。
2. 使用 persistentStoreCoordinator 上的 replacePersistentStore 方法将当前持久存储替换为恢复/备份持久存储。
3. 销毁备份存储。
4. 删除与备份存储关联的文件。
5. 在 AppDelegate 中重建 Core Data 堆栈。
6. 保存、nil，然后重新初始化 MasterViewController 的 managementObjectContext 和 NSFetchedResultsController。

6号花了一些时间让我看到曙光。我最终的恢复方法是:

func restorePSFromBackup() {
    // Current persistent store is in the ApplicationSupport directory.
    let currentPSFolderUrl = FileManager.default.urls(for: .applicationSupportDirectory, in:.userDomainMask).first!
    // The current Core Data file (url).
    let currentPSUrl1 = currentPSFolderUrl.appendingPathComponent("DBCDTest.sqlite")
    // Backup persistent store with which to restore is in the Documents directory.
    let backUpFolderUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
    // The 3 backup Core Data files (urls).
    let backupUrl1 = backUpFolderUrl.appendingPathComponent("DBCDTest.sqlite")
    let backupUrl2 = backUpFolderUrl.appendingPathComponent("DBCDTest.sqlite-wal")
    let backupUrl3 = backUpFolderUrl.appendingPathComponent("DBCDTest.sqlite-shm")
    let sourceSqliteURLs = [backupUrl1, backupUrl2, backupUrl3]

    let container = NSPersistentContainer(name: "DBCDTest")

    do {
        // Replace current persistent store with the restore/backup persistent store.
        try container.persistentStoreCoordinator.replacePersistentStore(at: currentPSUrl1, destinationOptions: nil, withPersistentStoreFrom: backupUrl1, sourceOptions: nil, ofType: NSSQLiteStoreType)
        // Destroy the backup store.
        try container.persistentStoreCoordinator.destroyPersistentStore(at: backupUrl1, ofType: NSSQLiteStoreType, options: nil)
        // Delete the restore/backup files from the Application directory.
        do {
            for index in 0..<sourceSqliteURLs.count {
                try FileManager.default.removeItem(at: sourceSqliteURLs[index])
            }
        } catch let error {
            print("Failed to delete sqlite files.")
            print(error.localizedDescription)
        }
        // Rebuild the AppDelegate's Core Data stack.
        (UIApplication.shared.delegate as! AppDelegate).persistentContainer = NSPersistentContainer(name: "DBCDTest")
        (UIApplication.shared.delegate as! AppDelegate).persistentContainer.loadPersistentStores(completionHandler: { (storeDescription, error) in
            print(NSPersistentContainer.defaultDirectoryURL())
            if let error = error as NSError? {
                fatalError("Unresolved error \(error), \(error.userInfo)")
            } else {
                // Save, nil and then reinitialise the MasterViewController managedObjectContext and NSFetchedResultsController.
                do {
                    try self.masterViewController.managedObjectContext?.save()
                } catch let error {
                    print("Failed to save managedObjectContext.")
                    print(error.localizedDescription)
                }
                self.masterViewController.managedObjectContext = nil
                self.masterViewController.managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
                self.masterViewController._fetchedResultsController = nil
                let _ = self.masterViewController.fetchedResultsController
            }
        })
    } catch let error {
        print("Failed to replace persistent store.")
        print(error.localizedDescription)
    }
}