空核心数据表

Question

我想从选定的核心数据实体中删除所有数据。我正在使用以下代码，但收到以下错误：

未解析的标识符托管对象上下文

let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "UserAccountTbl")
let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
do {
try managedObjectContext.executeRequest(batchDeleteRequest)
} catch {
// Error Handling
}

Answer 1

首先 -正如其他答案中已经提到的 - 你必须从AppDelegate获得NSManagedObjectContext

let appDelegate = UIApplication.shared.delegate as! AppDelegate
let managedObjectContext = appDelegate.persistentContainer.viewContext 

强制展开完全没问题，如果缺少AppDelegate应用程序甚至无法启动。

但有一个警告：

如果在托管对象上下文上调用execute，则必须将更改合并到上下文中，因为执行操作不会修改上下文的内容。

let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "UserAccountTbl")
let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
batchDeleteRequest.resultType = .resultTypeObjectIDs
do {
let result = try managedObjectContext.execute(batchDeleteRequest) as! NSBatchDeleteResult
let changes = [NSDeletedObjectsKey : result.result as! [NSManagedObjectID]]
NSManagedObjectContext.mergeChanges(fromRemoteContextSave: changes, into: [managedObjectContext])
} catch {
print(error)
}

但是，如果在持久存储协调器上调用execute，则可以避免merge步骤

let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "UserAccountTbl")
let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
do {
let persistentStoreCoordinator = managedObjectContext.persistentStoreCoordinator!
try persistentStoreCoordinator.execute(batchDeleteRequest, with: managedObjectContext)        
} catch {
print(error)
}

Answer 2

似乎您尚未声明您的 managedObjectCotext 并尝试访问它。试试这个：

let managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "UserAccountTbl")
let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
do {
try managedObjectContext.executeRequest(batchDeleteRequest)
} catch {
// Error Handling
}

Answer 3

从错误来看，您尚未从 appDelegate 中的容器声明上下文。您可以尝试使用以下代码：

guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else { return }
//We need to create a context from this container
let managedObjectContext = appDelegate.persistentContainer.viewContext 
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "UserAccountTbl")
let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
do {
try managedObjectContext.executeRequest(batchDeleteRequest)
} catch {
// Error Handling
}