Core data 如何判断“NSManagedObject”是否已被删除?
我有一个已删除的Core data 如何判断“NSManagedObject”是否已被删除?,core-data,ios,nsmanagedobject,Core Data,Ios,Nsmanagedobject,我有一个已删除的NSManagedObject,包含该托管对象的上下文已保存。我知道isDeleted返回YES如果核心数据将在下一次保存操作期间要求持久存储删除对象。但是,由于保存已经发生,isDeleted返回NO 保存NSManagedObject的包含上下文后,如何判断该对象是否已被删除 (如果您想知道,为什么引用已删除托管对象的对象尚未意识到删除,这是因为删除和上下文保存是由后台线程启动的,该线程使用performSelectorOnMainThread:withObject:wait
NSManagedObject
,包含该托管对象的上下文已保存。我知道isDeleted
返回YES
如果核心数据将在下一次保存操作期间要求持久存储删除对象。但是,由于保存已经发生,isDeleted
返回NO
保存NSManagedObject
的包含上下文后,如何判断该对象是否已被删除
(如果您想知道,为什么引用已删除托管对象的对象尚未意识到删除,这是因为删除和上下文保存是由后台线程启动的,该线程使用
performSelectorOnMainThread:withObject:waitUntilDone:
执行删除和保存操作)检查托管对象的上下文似乎有效:
if (managedObject.managedObjectContext == nil) {
// Assume that the managed object has been deleted.
}
来自苹果公司关于managedObjectContext
的文档
如果
接收器已从其目录中删除
上下文
如果接收器出现故障,则呼叫
此方法不会导致它起火
这两个似乎都是好事
更新:如果您试图测试是否已删除专门使用
objectWithID:
检索的托管对象,请签出。他指出,如果您使用已删除对象的ID调用objectWithID:
,则返回的对象将是一个未将其managedObjectContext
设置为nil的错误。因此,您不能简单地检查它的managedObjectContext
来测试它是否已被删除。如果可以,请使用ID:error:现有对象。如果不是,例如,您的目标是Mac OS 10.5或iOS 2.0,则需要执行其他操作来测试删除。有关详细信息,请参见。更新:一个改进的答案,基于下面讨论中的詹姆斯·哈德尔斯顿的想法
- (BOOL)hasManagedObjectBeenDeleted:(NSManagedObject *)managedObject {
/*
Returns YES if |managedObject| has been deleted from the Persistent Store,
or NO if it has not.
NO will be returned for NSManagedObject's who have been marked for deletion
(e.g. their -isDeleted method returns YES), but have not yet been commited
to the Persistent Store. YES will be returned only after a deleted
NSManagedObject has been committed to the Persistent Store.
Rarely, an exception will be thrown if Mac OS X 10.5 is used AND
|managedObject| has zero properties defined. If all your NSManagedObject's
in the data model have at least one property, this will not be an issue.
Property == Attributes and Relationships
Mac OS X 10.4 and earlier are not supported, and will throw an exception.
*/
NSParameterAssert(managedObject);
NSManagedObjectContext *moc = [self managedObjectContext];
// Check for Mac OS X 10.6+
if ([moc respondsToSelector:@selector(existingObjectWithID:error:)])
{
NSManagedObjectID *objectID = [managedObject objectID];
NSManagedObject *managedObjectClone = [moc existingObjectWithID:objectID error:NULL];
if (!managedObjectClone)
return YES; // Deleted.
else
return NO; // Not deleted.
}
// Check for Mac OS X 10.5
else if ([moc respondsToSelector:@selector(countForFetchRequest:error:)])
{
// 1) Per Apple, "may" be nil if |managedObject| deleted but not always.
if (![managedObject managedObjectContext])
return YES; // Deleted.
// 2) Clone |managedObject|. All Properties will be un-faulted if
// deleted. -objectWithID: always returns an object. Assumed to exist
// in the Persistent Store. If it does not exist in the Persistent
// Store, firing a fault on any of its Properties will throw an
// exception (#3).
NSManagedObjectID *objectID = [managedObject objectID];
NSManagedObject *managedObjectClone = [moc objectWithID:objectID];
// 3) Fire fault for a single Property.
NSEntityDescription *entityDescription = [managedObjectClone entity];
NSDictionary *propertiesByName = [entityDescription propertiesByName];
NSArray *propertyNames = [propertiesByName allKeys];
NSAssert1([propertyNames count] != 0, @"Method cannot detect if |managedObject| has been deleted because it has zero Properties defined: %@", managedObject);
@try
{
// If the property throws an exception, |managedObject| was deleted.
(void)[managedObjectClone valueForKey:[propertyNames objectAtIndex:0]];
return NO; // Not deleted.
}
@catch (NSException *exception)
{
if ([[exception name] isEqualToString:NSObjectInaccessibleException])
return YES; // Deleted.
else
[exception raise]; // Unknown exception thrown.
}
}
// Mac OS X 10.4 or earlier is not supported.
else
{
NSAssert(0, @"Unsupported version of Mac OS X detected.");
}
}
旧的/贬值的答案: 我写了一个稍微好一点的方法
self
是您的核心数据类/控制器
- (BOOL)hasManagedObjectBeenDeleted:(NSManagedObject *)managedObject
{
// 1) Per Apple, "may" be nil if |managedObject| was deleted but not always.
if (![managedObject managedObjectContext])
return YES; // Deleted.
// 2) Clone |managedObject|. All Properties will be un-faulted if deleted.
NSManagedObjectID *objectID = [managedObject objectID];
NSManagedObject *managedObjectClone = [[self managedObjectContext] objectWithID:objectID]; // Always returns an object. Assumed to exist in the Persistent Store. If it does not exist in the Persistent Store, firing a fault on any of its Properties will throw an exception.
// 3) Fire faults for Properties. If any throw an exception, it was deleted.
NSEntityDescription *entityDescription = [managedObjectClone entity];
NSDictionary *propertiesByName = [entityDescription propertiesByName];
NSArray *propertyNames = [propertiesByName allKeys];
@try
{
for (id propertyName in propertyNames)
(void)[managedObjectClone valueForKey:propertyName];
return NO; // Not deleted.
}
@catch (NSException *exception)
{
if ([[exception name] isEqualToString:NSObjectInaccessibleException])
return YES; // Deleted.
else
[exception raise]; // Unknown exception thrown. Handle elsewhere.
}
}
正如James Huddleston在他的回答中提到的,检查NSManagedObject的
-managedObjectContext
是否返回nil
是一种“非常好的”方式,可以查看缓存/过时的NSManagedObject是否已从持久存储中删除,但它并不总是像苹果在其文档中所说的那样准确:
如果接收者已从其数据库中删除,此方法可能返回nil
上下文
什么时候不归零?如果您使用已删除的NSManagedObject的-objectID
获取不同的NSManagedObject,如下所示:
// 1) Create a new NSManagedObject, save it to the Persistant Store.
CoreData *coreData = ...;
NSManagedObject *apple = [coreData addManagedObject:@"Apple"];
[apple setValue:@"Mcintosh" forKey:@"name"];
[coreData saveMOCToPersistentStore];
// 2) The `apple` will not be deleted.
NSManagedObjectContext *moc = [apple managedObjectContext];
if (!moc)
NSLog(@"2 - Deleted.");
else
NSLog(@"2 - Not deleted."); // This prints. The `apple` has just been created.
// 3) Mark the `apple` for deletion in the MOC.
[[coreData managedObjectContext] deleteObject:apple];
moc = [apple managedObjectContext];
if (!moc)
NSLog(@"3 - Deleted.");
else
NSLog(@"3 - Not deleted."); // This prints. The `apple` has not been saved to the Persistent Store yet, so it will still have a -managedObjectContext.
// 4) Now tell the MOC to delete the `apple` from the Persistent Store.
[coreData saveMOCToPersistentStore];
moc = [apple managedObjectContext];
if (!moc)
NSLog(@"4 - Deleted."); // This prints. -managedObjectContext returns nil.
else
NSLog(@"4 - Not deleted.");
// 5) What if we do this? Will the new apple have a nil managedObjectContext or not?
NSManagedObjectID *deletedAppleObjectID = [apple objectID];
NSManagedObject *appleClone = [[coreData managedObjectContext] objectWithID:deletedAppleObjectID];
moc = [appleClone managedObjectContext];
if (!moc)
NSLog(@"5 - Deleted.");
else
NSLog(@"5 - Not deleted."); // This prints. -managedObjectContext does not return nil!
// 6) Finally, let's use the method I wrote, -hasManagedObjectBeenDeleted:
BOOL deleted = [coreData hasManagedObjectBeenDeleted:appleClone];
if (deleted)
NSLog(@"6 - Deleted."); // This prints.
else
NSLog(@"6 - Not deleted.");
这是打印件:
2 - Not deleted.
3 - Not deleted.
4 - Deleted.
5 - Not deleted.
6 - Deleted.
如您所见,
-managedObjectContext
如果NSManagedbject已从持久性存储中删除,则不会始终返回nil。由于我最近在iOS应用程序中实现iCloud的经验,该应用程序依赖于核心数据来实现持久性,我意识到最好的方法是观察框架的通知。至少,这比依赖一些模糊的方法要好,这些方法可能会,也可能不会告诉您是否删除了某个托管对象
对于“纯”核心数据应用程序,您应该在主线程上观察NSManagedObjectContextObjectsIDChangeNotification。通知的用户信息字典包含已插入、删除和更新的托管对象objectID集合
如果您在其中一个集合中找到托管对象的objectID,那么您可以以某种好的方式更新应用程序和UI
就这样。。。欲了解更多信息,请参阅苹果的核心数据编程指南《与核心数据并发》一章。有一节“使用通知跟踪其他线程中的更改”,但不要忘记检查前一节“使用线程限制支持并发”。我担心其他答案中的讨论实际上隐藏了正确答案的简单性。在几乎所有情况下,正确答案都是:
if ([moc existingObjectWithID:object.objectID error:NULL])
{
// object is valid, go ahead and use it
}
此答案不适用于以下情况:
NSObjectInaccessibleException
,或者您可以使用object.isDeleted
)在Swift 3,Xcode 7.3中验证 您还可以简单地
打印每个上下文的内存引用并检查
(a) if the context exists,
(b) if the contexts of 2 objects are different
例如:(书和成员是两个不同的对象)
如果上下文存在但不同,它会打印类似的内容
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0x7fe758c15d70
有趣的是,虽然看起来这对没有属性的对象不起作用。另外,为什么不使用existingObjectWithID:error:
而不是objectWithID:
,只需检查返回值是否等于nil
?啊,你是对的,-existingObjectWithID:error:
是一种更好的方法!)我写的答案是与MacOSX10.5+兼容,所以我忽略了这个方法,它只有10.6+版本。是的,我的答案不适用于没有任何属性的对象,尽管数据模型中不太可能有空对象。你是对的。对象不太可能没有包含关系的属性。出于某种原因,我只考虑属性。隐马尔可夫模型。。。是否有一种方法可以在不检查所有属性的情况下快速评估ID为的objectWithID:
返回的故障?(对于尚未删除的对象,访问每个属性的代价可能会很高。)如果有一个方法会触发错误,您可以对objectWithID:
返回的对象调用该方法,以查看它是否确实存在。我寻找这样一种方法,但没有发现任何明显的问题。我想更好的优化方法是只查询一个
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