如何判断“NSManagedObject”是否已被删除？

Question

我有一个已被删除的 NSManagedObject，并且包含该托管对象的上下文已被保存。据我所知，如果 Core Data 要求持久存储在下一次保存操作期间删除对象，则 isDeleted 返回 YES。但是，由于保存已经发生，isDeleted 返回NO。

有什么好方法可以判断 NSManagedObject 在保存其包含的上下文之后是否已被删除？

（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，该后台线程使用performSelectorOnMainThread执行删除和保存： withObject:waitUntilDone:。）

Answer 1

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 Apple 关于 managedObjectContext 的文档 ...

如果以下情况，此方法可能返回 nil
接收者已从其中删除
上下文。

如果接收器出现故障，请致电
此方法不会导致其触发。

这两件事似乎都是好事。

更新：如果您尝试测试专门使用objectWithID:检索的托管对象是否已被删除，请查看戴夫·加拉格尔的回答。他指出，如果您使用已删除对象的 ID 调用 objectWithID:，则返回的对象将出现错误，没有其 managedObjectContext 设置为零。因此，您不能简单地检查其 managementObjectContext 来测试它是否已被删除。如果可以的话，请使用 existingObjectWithID:error:。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。请参阅他的回答细节。

Answer 2

更新：基于James Huddleston在下面讨论中的想法改进的答案。

- (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/不推荐的答案：

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 -managementObjectContext 是否返回 nil 是查看缓存是否存在的“相当好的”方法。 /stale NSManagedObject 已从持久存储中删除，但它并不总是准确，正如 Apple 在其文档中所述：

如果接收者已从其接收者中删除，此方法可能返回nil
上下文。

什么时候不会返回nil？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.");

这是打印输出：

2 - Not deleted.
3 - Not deleted.
4 - Deleted.
5 - Not deleted.
6 - Deleted.

如您所见，如果 NSManagedObject 有已从持久存储中删除。

Answer 3

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，正确答案是：

if ([moc existingObjectWithID:object.objectID error:NULL])
{
    // object is valid, go ahead and use it
}

该答案不适用的唯一情况是：

1. 如果您的目标是 Mac OS 10.5 或更早版本
2. 如果您的目标是 iOS 2.0 或更早版本
3. 如果对象/上下文尚未保存（在这种情况下，您要么不关心，因为它不会抛出 NSObjectInaccessibleException，或者您可以使用 object.isDeleted）

Answer 4

由于我最近在依赖 Core Data 进行持久性的 iOS 应用程序中实现 iCloud 的经验，我意识到最好的方法是观察框架的通知。至少，比依赖一些晦涩的方法要好，这些方法可能会或可能不会告诉您某些托管对象是否已被删除。

对于“纯”核心数据应用程序，您应该在主线程上观察 NSManagedObjectContextObjectsDidChangeNotification 。通知的用户信息字典包含插入、删除和更新的托管对象的 objectID 集。

如果您在这些集合之一中找到托管对象的 objectID，那么您可以以某种不错的方式更新您的应用程序和 UI。

就是这样...有关更多信息，请参阅 Apple 的核心数据编程指南，核心数据并发章节。有一节“使用通知跟踪其他线程中的更改”，但不要忘记检查上一节“使用线程限制支持并发”。

Answer 5

在 Swift 3、Xcode 7.3 中验证

您还可以简单地 PRINT 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：（ Book 和 Member 是 2 个不同的对象）

 print(book.managedObjectContext)
 print(member.managedObjectContext)

如果上下文存在但存在，它会打印类似这样的内容不同的

0x7fe758c307d0
0x7fe758c15d70