保存子上下文时_PFFaultHandlerLookupRow崩溃

Question

当我在一个子上下文中更改托管对象(之后保存)并在其他子上下文中删除(首先保存)时,我的应用程序崩溃了.
如何重现:1.
使用"空应用程序"模板创建新项目并启用核心数据.
2.将managedObjectContext getter更改为以下(我更改了并发类型)

- (NSManagedObjectContext *)managedObjectContext
{
    if (_managedObjectContext != nil) {
        return _managedObjectContext;
    }

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil) {
        _managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
        [_managedObjectContext setPersistentStoreCoordinator:coordinator];
    }
    return _managedObjectContext;
}

3.请将didfinishLaunching方法替换为以下

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];

    {
        //insert
        NSManagedObjectContext *insertingContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
        [insertingContext setParentContext:self.managedObjectContext];
        [insertingContext performBlockAndWait:^{
            Test *test = (Test *)[NSEntityDescription insertNewObjectForEntityForName:@"Test" inManagedObjectContext:insertingContext];
            test.test=@"test";
            [insertingContext save:nil];
            [self.managedObjectContext performBlockAndWait:^{
                [self.managedObjectContext save:nil];
            }];
            NSLog(@"inserted and saved to persistance store");
        }];
    }

    {
        //get the mo and change the property
        NSManagedObjectContext *acceesingContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
        {
            [acceesingContext setParentContext:self.managedObjectContext];
            [acceesingContext performBlockAndWait:^{
                NSFetchRequest *request = [[NSFetchRequest alloc] init] ;
                [request setEntity:[NSEntityDescription entityForName:@"Test" inManagedObjectContext:acceesingContext]];
                NSArray *results = [acceesingContext executeFetchRequest:request error:nil];
                if ([results count] > 0 )
                {
                    Test *test= [results objectAtIndex:0];
                    test.test=@"Hello";
                    NSLog(@"accessed and changed the property so that fault is fired");
                }

            }];
        }
        {
            NSManagedObjectContext *deletingContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
            [deletingContext setParentContext:self.managedObjectContext];
            [deletingContext performBlockAndWait:^{
                NSFetchRequest *request = [[NSFetchRequest alloc] init] ;
                [request setEntity:[NSEntityDescription entityForName:@"Test" inManagedObjectContext:deletingContext]];
                NSArray *results = [deletingContext executeFetchRequest:request error:nil];
                if ([results count] > 0 )
                {
                    Test *test= [results objectAtIndex:0];
                    [deletingContext deleteObject:test];
                    [deletingContext save:nil];
                    [self.managedObjectContext performBlockAndWait:^{
                        [self.managedObjectContext save:nil];
                    }];
                    NSLog(@"deleted and saved to persistance store");
                }
            }];
        }

        [acceesingContext performBlock:^{
            // it is crashing here, please help.
            [acceesingContext save:nil];
        }];
    }

    return YES;
}

4.最后使用属性"test"(NSString)添加名为"Test"的实体并运行应用程序

我面临的问题是,当一个子moc已经获取了一个托管对象并更改了其中的属性时,而其他子项删除并将更改保存到持久性存储中.保存已修改mo的moc时发生崩溃.

崩溃报告

*** Terminating app due to uncaught exception 'NSObjectInaccessibleException', reason: 'CoreData could not fulfill a fault for '0xd0000000001c0000 ''
*** First throw call stack:
(
    0   CoreFoundation                      0x0000000101bf0795 __exceptionPreprocess + 165
    1   libobjc.A.dylib                     0x0000000101953991 objc_exception_throw + 43
    2   CoreData                            0x0000000100278a93 _PFFaultHandlerLookupRow + 1075
    3   CoreData                            0x00000001003063a3 -[NSManagedObject(_NSInternalMethods) _updateFromRefreshSnapshot:includingTransients:] + 243
    4   CoreData                            0x00000001002aa563 -[NSManagedObjectContext(_NestedContextSupport) _copyChildObject:toParentObject:fromChildContext:] + 771
    5   CoreData                            0x00000001002aa01b -[NSManagedObjectContext(_NestedContextSupport) _parentProcessSaveRequest:inContext:error:] + 1019
    6   CoreData                            0x0000000100310243 __82-[NSManagedObjectContext(_NestedContextSupport) executeRequest:withContext:error:]_block_invoke + 563
    7   libdispatch.dylib                   0x0000000101fc205a _dispatch_barrier_sync_f_slow_invoke + 45
    8   libdispatch.dylib                   0x0000000101fd16fd _dispatch_client_callout + 8
    9   libdispatch.dylib                   0x0000000101fc146c _dispatch_main_queue_callback_4CF + 354
    10  CoreFoundation                      0x0000000101c4e729 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9
    11  CoreFoundation                      0x0000000101b9b9a4 __CFRunLoopRun + 1764
    12  CoreFoundation                      0x0000000101b9aed3 CFRunLoopRunSpecific + 467
    13  GraphicsServices                    0x0000000103b893a4 GSEventRunModal + 161
    14  UIKit                               0x00000001005bba63 UIApplicationMain + 1010
    15  TestCrash                           0x00000001000040a3 main + 115
    16  libdyld.dylib                       0x000000010227e7e1 start + 0
    17  ???                                 0x0000000000000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type _NSCoreDataException

Answer 1

这里的关键不是堆栈跟踪中的那一行,而是异常消息:

*** Terminating app due to uncaught exception 'NSObjectInaccessibleException', reason: 'CoreData could not fulfill a fault for '0xd0000000001c0000 ''

如果Core Data抱怨对象不可访问,则意味着它无法在持久性存储中找到该对象.让我们看看为什么会在您的代码中发生这种情况:

1. 您可以使用insertingContext并在父级中创建实例并保存更改.此时,您的一个对象位于持久性存储文件中.
2. 您使用acceesingContext并获取此实例并对其进行更改,但您不保存更改.此时,acceesingContext对刚刚获取的对象进行内存中未保存的更改.
3. 您使用deletingContext并删除它来获取对象.然后保存deletingContext父级中的更改.此时,您已从持久性存储文件中删除了该对象.但是,这是至关重要的,acceesingContext仍然有该对象的未保存更改.
4. 您保存更改acceesingContext.由于acceesingContext对您提取的对象进行了未保存的更改,因此它会尝试更新该对象.但是持久性存储中不存在该对象,因为您删除了它.由于acceesingContext无法更新不存在的对象,因此会引发异常并且应用程序崩溃.

使用嵌套的托管对象上下文时要记住的一件事是,保存更改只会将更改推向一个方向 - 朝向父级.如果你有两个兄弟上下文(在这种情况下acceesingContext和deletingContext),对一个孩子保存更改不会自动更新等.

由于这显然是演示代码(感谢BTW,使其更容易遵循!)很难完全确定您需要在真实的应用程序代码中做什么.一种典型的方法是监听NSManagedObjectContextDidSaveNotification然后使用mergeChangesFromContextDidSaveNotification:将更改从一个上下文应用到另一个上下文.这样,当一个上下文删除对象时,您可以更新其他上下文以反映该事实.这使得可以使多个托管对象上下文保持彼此同步.