【问题标题】：Empty Core Data Table空核心数据表
【发布时间】：2019-12-27 11:10:34
【问题描述】：

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，但出现以下错误：

未解析的标识符 managedObjectContext

let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "UserAccountTbl")
let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)

do {
   try managedObjectContext.executeRequest(batchDeleteRequest)
} catch {
   // Error Handling
}

【问题讨论】：

标签： swift xcode core-data

【解决方案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

强制解包非常好，如果缺少AppDelegate，应用程序甚至都不会启动。

但有一点需要注意：

如果您在托管对象上下文中调用 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，如果您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)
}

【讨论】：

【解决方案2】：

似乎您还没有声明您的 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
}

【讨论】：

【解决方案3】：

由于您的错误，您没有从 appDelegate 中的容器声明上下文。你可以试试下面的代码：

    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
    }

【讨论】：