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

    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)
    }
}

其他回答

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"];

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)")
        }
    }

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)")
    }
}

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。

延续戴夫·德隆的回答。

Swift版本,照顾iOS 9和以前的版本以及。我还介绍了错误处理:

let appDelegate: AppDelegate = UIApplication.sharedApplication().delegate as!应用委托

    let fetchRequest = NSFetchRequest(entityName: "Car")
    if #available(iOS 9.0, *) {
        let delete = NSBatchDeleteRequest(fetchRequest: fetchRequest)
        do {
            try appDelegate.persistentStoreCoordinator.executeRequest(delete, withContext: appDelegate.managedObjectContext)
        } catch let error as NSError {
            print("Error occured while deleting: \(error)")
        }
    } else {
        // Fallback on earlier versions
        let carRequest = NSFetchRequest()
        carRequest.entity = NSEntityDescription.entityForName("Cars", inManagedObjectContext: appDelegate.managedObjectContext)
        carRequest.includesPropertyValues = false

        do {
            let cars: NSArray = try appDelegate.managedObjectContext.executeFetchRequest(carRequest)

            for car in cars {
                appDelegate.managedObjectContext.delete(car)
            }

            try appDelegate.managedObjectContext.save()

        } catch let error as NSError {
            print("Error occured while fetching or saving: \(error)")
        }
    }