核心数据：删除实体所有实例的最快方法

Question

核心数据：删除实体所有实例的最快方法

我正在使用Core Data在本地持久保存Web服务调用的结果。Web服务返回完整的对象模型，比方说，“汽车” - 可能是大约2000个（我不能让Web服务返回任何小于1或所有汽车。

下次打开我的应用程序时，我想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。

是否有更快的方法来清除托管对象上下文中特定实体的所有实例（例如“CAR”类型的所有实体），或者我是否需要查询它们，然后遍历结果以删除每个实例，然后保存？

理想情况下，我可以说删除所有实体是Blah的地方。

Answer 1

iOS 9及更高版本：iOS 9添加了一个名为的新类NSBatchDeleteRequest，它允许您轻松删除与谓词匹配的对象，而无需将它们全部加载到内存中。这是你如何使用它：斯威夫特2let fetchRequest = NSFetchRequest(entityName: "Car")let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)do {    try myPersistentStoreCoordinator.executeRequest(deleteRequest, withContext: myContext)} catch let error as NSError {    // TODO: handle the error}Objective-C的NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"Car"];NSBatchDeleteRequest *delete = [[NSBatchDeleteRequest alloc] initWithFetchRequest:request];NSError *deleteError = nil;[myPersistentStoreCoordinator executeRequest:delete withContext:myContext error:&deleteError];有关批量删除的更多信息可以在WWDC 2015的“核心数据中的新内容”会话中找到（从~14：10开始）。iOS 8及更早版本：获取所有并删除所有：NSFetchRequest *allCars = [[NSFetchRequest alloc] init];[allCars setEntity:[NSEntityDescription entityForName:@"Car" inManagedObjectContext:myContext]];[allCars setIncludesPropertyValues:NO]; //only fetch the managedObjectIDNSError *error = nil;NSArray *cars = [myContext executeFetchRequest:allCars error:&error];[allCars release];//error handling goes herefor (NSManagedObject *car in cars) {  [myContext deleteObject:car];}NSError *saveError = nil;[myContext save:&saveError];//more error handling here

Answer 2

更清洁和通用：添加此方法：- (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];}

Answer 3

在Swift 3中重置实体：func&nbsp;resetAllRecords(in&nbsp;entity&nbsp;:&nbsp;String)&nbsp;//&nbsp;entity&nbsp;=&nbsp;Your_Entity_Name &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;let&nbsp;context&nbsp;=&nbsp;(&nbsp;UIApplication.shared.delegate&nbsp;as!&nbsp;AppDelegate&nbsp;).persistentContainer.viewContext &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;let&nbsp;deleteFetch&nbsp;=&nbsp;NSFetchRequest<NSFetchRequestResult>(entityName:&nbsp;entity) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;let&nbsp;deleteRequest&nbsp;=&nbsp;NSBatchDeleteRequest(fetchRequest:&nbsp;deleteFetch) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;do &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try&nbsp;context.execute(deleteRequest) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try&nbsp;context.save() &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;catch &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;print&nbsp;("There&nbsp;was&nbsp;an&nbsp;error") &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} &nbsp;&nbsp;&nbsp;&nbsp;}