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
iOS 11.3和Swift 4.1
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: entityName)
let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest )
batchDeleteRequest.resultType = .resultTypeCount
do {
let batchDeleteResult = try dataController.viewContext.execute(batchDeleteRequest) as! NSBatchDeleteResult
print("The batch delete request has deleted \(batchDeleteResult.result!) records.")
dataController.viewContext.reset() // reset managed object context (need it for working)
} catch {
let updateError = error as NSError
print("\(updateError), \(updateError.userInfo)")
}
你必须在执行之后调用重置。如果不是，它将不会在表视图上更新。
其他回答
iOS 11.3和Swift 4.1
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: entityName)
let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest )
batchDeleteRequest.resultType = .resultTypeCount
do {
let batchDeleteResult = try dataController.viewContext.execute(batchDeleteRequest) as! NSBatchDeleteResult
print("The batch delete request has deleted \(batchDeleteResult.result!) records.")
dataController.viewContext.reset() // reset managed object context (need it for working)
} catch {
let updateError = error as NSError
print("\(updateError), \(updateError.userInfo)")
}
你必须在执行之后调用重置。如果不是，它将不会在表视图上更新。
iOS 10及以上版本
适用于所有版本。传递实体名称并遍历删除所有条目并保存上下文。
func deleteData(entityToFetch: String, completion: @escaping(_ returned: Bool) ->()) {
let context = NSManagedObjectContext()
context = your managedObjectContext
let fetchRequest = NSFetchRequest<NSFetchRequestResult>()
fetchRequest.entity = NSEntityDescription.entity(forEntityName: entityToFetch, in: context)
fetchRequest.includesPropertyValues = false
do {
let results = try context.fetch(fetchRequest) as! [NSManagedObject]
for result in results {
context.delete(result)
}
try context.save()
completion(true)
} catch {
completion(false)
print("fetch error -\(error.localizedDescription)")
}
}
更干净和通用一点:添加这个方法:
- (void)deleteAllEntities:(NSString *)nameEntity
{
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:nameEntity];
[fetchRequest setIncludesPropertyValues:NO]; //only fetch the managedObjectID
NSError *error;
NSArray *fetchedObjects = [theContext executeFetchRequest:fetchRequest error:&error];
for (NSManagedObject *object in fetchedObjects)
{
[theContext deleteObject:object];
}
error = nil;
[theContext save:&error];
}
在Swift 2.0中:
func deleteAllData(entity: String)
{
let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
let managedContext = appDelegate.managedObjectContext
let fetchRequest = NSFetchRequest(entityName: entity)
fetchRequest.returnsObjectsAsFaults = false
do
{
let results = try managedContext.executeFetchRequest(fetchRequest)
for managedObject in results
{
let managedObjectData:NSManagedObject = managedObject as! NSManagedObject
managedContext.deleteObject(managedObjectData)
}
} catch let error as NSError {
print("Detele all data in \(entity) error : \(error) \(error.userInfo)")
}
}
当最小iOS为9.0时，使用NSBatchDeleteRequest删除多条记录。如果是后台线程，执行NSManagedObjectContext save，否则使用NSFetchRequest获取记录并删除for循环中的所有记录，删除完成后保存。
