我使用Core Data在本地持久化Web服务调用的结果。web服务返回完整的对象模型，比如说，“Cars”——可能有2000辆车(我不能让web服务返回小于1辆或ALL辆的任何东西。
下次打开应用程序时，我希望通过再次调用所有汽车的Web服务来刷新Core Data持久化副本，但是为了防止重复，我需要首先清除本地缓存中的所有数据。
是否有一种更快的方法来清除管理对象上下文中特定实体的所有实例(例如，类型为“CAR”的所有实体)，或者我需要查询它们调用，然后遍历结果删除每个，然后保存?
理想情况下，我可以说删除实体所在的所有地方等等。
当前回答
这段代码适用于iOS 9及以下版本
class func deleteAllRecords(in entity : String) // entity = Your_Entity_Name
{
let context = CoreDataStack.getContext() // Note:- Replace your context here with CoreDataStack.getContext()
let deleteFetch = NSFetchRequest<NSFetchRequestResult>(entityName: entity)
if #available(iOS 9, *)
{
let deleteRequest = NSBatchDeleteRequest(fetchRequest: deleteFetch)
do
{
try context.execute(deleteRequest)
try context.save()
}
catch
{
print("There was an error:\(error)")
}
}
else
{
do{
let deleteRequest = try context.fetch(deleteFetch)
for anItem in deleteRequest {
context.delete(anItem as! NSManagedObject)
}
}
catch
{
print("There was an error:\(error)")
}
}
CoreDataStack.saveContext() // Note:- Replace your savecontext here with CoreDataStack.saveContext()
}
其他回答
这是一个类似的问题，有人建议设置一个关系删除规则，这样你只需要删除一个对象。因此，如果您已经或可以创建一个与汽车具有多对关系的实体，并将删除规则设置为级联，当您删除更高的实体时，所有的汽车也将被删除。这可以节省一些处理时间，因为您不必执行加载所有汽车的步骤。在更大的数据集中，这可能是绝对必要的。
在Swift 3中重置实体:
func resetAllRecords(in entity : String) // entity = Your_Entity_Name
{
let context = ( UIApplication.shared.delegate as! AppDelegate ).persistentContainer.viewContext
let deleteFetch = NSFetchRequest<NSFetchRequestResult>(entityName: entity)
let deleteRequest = NSBatchDeleteRequest(fetchRequest: deleteFetch)
do
{
try context.execute(deleteRequest)
try context.save()
}
catch
{
print ("There was an error")
}
}
快速清除DB中的所有对象:
func purgeAllData() {
let uniqueNames = persistentContainer.managedObjectModel.entities.compactMap({ $0.name })
uniqueNames.forEach { (name) in
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: name)
let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
do {
try persistentContainer.viewContext.execute(batchDeleteRequest)
} catch {
let nserror = error as NSError
fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
}
}
}
这段代码适用于iOS 9及以下版本
class func deleteAllRecords(in entity : String) // entity = Your_Entity_Name
{
let context = CoreDataStack.getContext() // Note:- Replace your context here with CoreDataStack.getContext()
let deleteFetch = NSFetchRequest<NSFetchRequestResult>(entityName: entity)
if #available(iOS 9, *)
{
let deleteRequest = NSBatchDeleteRequest(fetchRequest: deleteFetch)
do
{
try context.execute(deleteRequest)
try context.save()
}
catch
{
print("There was an error:\(error)")
}
}
else
{
do{
let deleteRequest = try context.fetch(deleteFetch)
for anItem in deleteRequest {
context.delete(anItem as! NSManagedObject)
}
}
catch
{
print("There was an error:\(error)")
}
}
CoreDataStack.saveContext() // Note:- Replace your savecontext here with CoreDataStack.saveContext()
}
iOS 9及以上版本:
iOS 9添加了一个名为NSBatchDeleteRequest的新类，允许您轻松删除匹配谓词的对象，而无需将它们全部加载到内存中。下面是你如何使用它:
斯威夫特5
let fetchRequest: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest(entityName: "Car")
let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
do {
try myPersistentStoreCoordinator.execute(deleteRequest, with: myContext)
} catch let error as NSError {
// TODO: handle the error
}
objective - c
NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"Car"];
NSBatchDeleteRequest *delete = [[NSBatchDeleteRequest alloc] initWithFetchRequest:request];
NSError *deleteError = nil;
[myPersistentStoreCoordinator executeRequest:delete withContext:myContext error:&deleteError];
关于批量删除的更多信息可以在WWDC 2015的“Core Data新内容”会议中找到(开始时间约为14:10)。
iOS 8及更早版本:
获取并删除它们:
NSFetchRequest *allCars = [[NSFetchRequest alloc] init];
[allCars setEntity:[NSEntityDescription entityForName:@"Car" inManagedObjectContext:myContext]];
[allCars setIncludesPropertyValues:NO]; //only fetch the managedObjectID
NSError *error = nil;
NSArray *cars = [myContext executeFetchRequest:allCars error:&error];
[allCars release];
//error handling goes here
for (NSManagedObject *car in cars) {
[myContext deleteObject:car];
}
NSError *saveError = nil;
[myContext save:&saveError];
//more error handling here
