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

一个很好的答案已经贴出来了,这只是一个推荐!

一个好方法是添加一个类别到NSManagedObject和实现一个方法,就像我做的:

头文件(例如NSManagedObject+Ext.h)

@interface NSManagedObject (Logic)

+ (void) deleteAllFromEntity:(NSString*) entityName;

@end

代码文件:(例如NSManagedObject+Ext.m)

@implementation NSManagedObject (Logic)

+ (void) deleteAllFromEntity:(NSString *)entityName {
    NSManagedObjectContext *managedObjectContext = [AppDelegate managedObjectContext];
    NSFetchRequest * allRecords = [[NSFetchRequest alloc] init];
    [allRecords setEntity:[NSEntityDescription entityForName:entityName inManagedObjectContext:managedObjectContext]];
    [allRecords setIncludesPropertyValues:NO];
    NSError * error = nil;
    NSArray * result = [managedObjectContext executeFetchRequest:allRecords error:&error];
    for (NSManagedObject * profile in result) {
        [managedObjectContext deleteObject:profile];
    }
    NSError *saveError = nil;
    [managedObjectContext save:&saveError];
}

@end

... 你唯一要做的就是从应用程序委托获取managedObjectContext,或者你在哪里都有它;)

之后你可以这样使用它:

[NSManagedObject deleteAllFromEntity:@"EntityName"];

进一步的优化可以是删除entityname的参数,而从clazzname获取名称。这将导致使用:

[ClazzName deleteAllFromEntity];

一个更干净的impl(作为NSManagedObjectContext的类别):

@implementation NSManagedObjectContext (Logic)

- (void) deleteAllFromEntity:(NSString *)entityName {
    NSFetchRequest * allRecords = [[NSFetchRequest alloc] init];
    [allRecords setEntity:[NSEntityDescription entityForName:entityName inManagedObjectContext:self]];
    [allRecords setIncludesPropertyValues:NO];
    NSError * error = nil;
    NSArray * result = [self executeFetchRequest:allRecords error:&error];
    for (NSManagedObject * profile in result) {
        [self deleteObject:profile];
    }
    NSError *saveError = nil;
    [self save:&saveError];
}

@end

那时的用法:

[managedObjectContext deleteAllFromEntity:@"EntityName"];

其他回答

斯威夫特3。X和Swift 4。X,简单。只改变你的表格

    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "YourTable")
    fetchRequest.returnsObjectsAsFaults = false

    do
    {
        let results = try context.fetch(fetchRequest)
        for managedObject in results
        {
            let managedObjectData:NSManagedObject = managedObject as! NSManagedObject
            context.delete(managedObjectData)
        }
    } catch let error as NSError {
        print("Detele all my data in \(entity) error : \(error) \(error.userInfo)")
    }

为什么不将接收到的数据与现有缓存一起折叠呢?否则它就不是真正的“刷新”,而是“重新开始”,你也可以删除SQLLite文件并重新开始(假设你没有持久化其他数据)。

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

iOS 9.0及以上版本:

NSBatchDeleteRequest用于删除核心数据中的记录。它的工作速度非常快,从一个实体删除所有记录所需的时间更少。它需要NSFetchRequest作为参数。如果你想从一个实体删除所有记录,你可以使用它,它为我工作。

let manageObject:NSManagedObjectContext = appDelegateObject.managedObjectContext

let fetchRequest = NSFetchRequest(entityName: “EnityName”)

let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)

let persistCor:NSPersistentStoreCoordinator = appDelegateObject.persistentObject
 do {
        try persistCor.executeRequest(deleteRequest, withContext: manageObject)
        try manageObject.save()
    } 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
    }