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 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
其他回答
迅速:
let fetchRequest = NSFetchRequest()
fetchRequest.entity = NSEntityDescription.entityForName(entityName, inManagedObjectContext: context)
fetchRequest.includesPropertyValues = false
var error:NSError?
if let results = context.executeFetchRequest(fetchRequest, error: &error) as? [NSManagedObject] {
for result in results {
context.deleteObject(result)
}
var error:NSError?
if context.save(&error) {
// do something after save
} else if let error = error {
println(error.userInfo)
}
} else if let error = error {
println("error: \(error)")
}
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)")
}
}
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
func deleteAll(entityName: String) {
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: entityName)
let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
deleteRequest.resultType = .resultTypeObjectIDs
guard let context = self.container?.viewContext
else { print("error in deleteAll")
return }
do {
let result = try context.execute(deleteRequest) as? NSBatchDeleteResult
let objectIDArray = result?.result as? [NSManagedObjectID]
let changes: [AnyHashable : Any] = [NSDeletedObjectsKey : objectIDArray as Any]
NSManagedObjectContext.mergeChanges(fromRemoteContextSave: changes, into: [context])
} catch {
print(error.localizedDescription)
}
}
Dave Delongs的Swift 2.0回答让我崩溃了(在iOS 9中)
但这个方法奏效了:
let fetchRequest = NSFetchRequest(entityName: "Car")
let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
do {
try managedObjectContext.executeRequest(deleteRequest)
try managedObjectContext.save()
}
catch let error as NSError {
// Handle error
}
