ios - 受管对象上下文未合并来自后台上下文的更改

Question

我有一个双重托管对象上下文设置，其中有一个父/子MOC关系。父级负责直接写入专用队列(NSPrivateQueueConcurrencyType)上的数据库，然后我有一个子级上下文，负责为主线程(NSMainQueueConcurrencyType)提取和保存数据。

当我对主队列上下文进行更改并将其保存后，上下文将按应有的方式工作，然后将更改合并到后台队列上下文中，并在后台线程上将其写入数据库。

我遇到的问题是当我直接将数据写入后台队列上下文，然后尝试将其合并到主队列上下文中时。这些对象已正确存储在商店中，并且当我合并更改时，它似乎可以正常工作。但是，如果我尝试在将数据保存到存储后直接针对主队列上下文发出NSFetchRequest，则数据是过时的，而不是正确的更新数据。

下面的代码摘录应该有助于您了解我在做什么。需要注意的一些事情是从JSON返回NSManagedObjects的逻辑。此外，我已经测试过，并且mergeChangesFromContextDidSaveNotification在UI尝试针对主队列MOC发出任何获取请求之前运行。你们都有什么想法吗？

// Context Setup
- (NSManagedObjectContext *)backgroundWriterManagedObjectContext
{
    if (_backgroundWriterManagedObjectContext != nil)
        return _backgroundWriterManagedObjectContext;

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil)
    {
        _backgroundWriterManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
        _backgroundWriterManagedObjectContext.persistentStoreCoordinator = coordinator;
        _backgroundWriterManagedObjectContext.mergePolicy = NSMergeByPropertyStoreTrumpMergePolicy;
        _backgroundWriterManagedObjectContext.undoManager = nil;
    }
    return _backgroundWriterManagedObjectContext;
}

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

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil)
    {
        _managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
        _managedObjectContext.parentContext = _backgroundWriterManagedObjectContext;
        _managedObjectContext.mergePolicy = NSMergeByPropertyStoreTrumpMergePolicy;
        _managedObjectContext.stalenessInterval = 0.0;
    }
    return _managedObjectContext;
}

// save MOC
- (void)saveManagedObjectContext
{
    @try
    {
        // perform synchronous process to save to the main MOC
        [_managedObjectContext performBlockAndWait:^(void)
        {
            __block NSError *error = nil;

            // push any changes in the main context to the background writer context
            if (![_managedObjectContext tryLock])
                [VS_Log VS_LogError:[NSString stringWithFormat:@"VS_CoreDataManger - Unable To Lock Managed Object Context: %@", error.localizedDescription]];

            if (![_managedObjectContext save:&error])
                [VS_Log VS_LogError:[NSString stringWithFormat:@"VS_CoreDataManger - Error Saving Managed Object Context: %@", error.localizedDescription]];

            [_managedObjectContext unlock];

            // save the background writer context
            [_backgroundWriterManagedObjectContext performBlock:^(void)
            {
                error = nil;

                if (![_backgroundWriterManagedObjectContext tryLock])
                    [VS_Log VS_LogError:[NSString stringWithFormat:@"VS_CoreDataManger - Unable To Lock Background Writer Managed Object Context: %@", error.localizedDescription]];

                if (![_backgroundWriterManagedObjectContext save:&error])
                    [VS_Log VS_LogError:[NSString stringWithFormat:@"VS_CoreDataManger - Error Saving Background Writer Managed Object Context: %@", error.localizedDescription]];

                [_backgroundWriterManagedObjectContext unlock];
            }];
        }];
    }
    @catch (NSException *exception)
    {
        [VS_Log VS_LogException:exception];
    }
}

// This code runs when attempting to save objects from a web service call
[_backgroundWriterManagedObjectContext performBlock:^(void)
    {
        // create a new process object and add it to the dictionary
        __block VS_CoreDataRequest *request = [[VS_CoreDataRequest alloc] init];

        // .. LOGIC HERE DESERIALIZES THE JSON AND RETURNS AN ARRAY OF NSMANAGEDOBJECTS

         // perform synchronous process to save to the main MOC
         @try
         {
             NSError *error = nil;

             [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mergeChangesFromContextDidSaveNotification:) name:NSManagedObjectContextDidSaveNotification object:_backgroundWriterManagedObjectContext];

             if (![_backgroundWriterManagedObjectContext tryLock])
                 [VS_Log VS_LogError:[NSString stringWithFormat:@"VS_CoreDataManger - Unable To Lock Background Writer Managed Object Context: %@", error.localizedDescription]];

             if (![_backgroundWriterManagedObjectContext save:&error])
                 [VS_Log VS_LogError:[NSString stringWithFormat:@"VS_CoreDataManger - Error Saving Background Writer Managed Object Context: %@", error.localizedDescription]];

             [_backgroundWriterManagedObjectContext unlock];

             // submit changes back to the forground context
             [_managedObjectContext performBlock:^(void)
             {
                 NSMutableArray *objects = [[NSMutableArray alloc] init];

                 // iterate through the updated objects and find them in the main thread MOC
                 for (VS_BaseManagedObject *object in request.objects)
                 {
                     // get the object from the main managed object context
                     NSError *error;
                     NSManagedObject *obj = [_managedObjectContext existingObjectWithID:object.objectID error:&error];

                     if (error)
                         [VS_Log VS_LogError:[NSString stringWithFormat:@"VS_CoreDataManager - Error: %@", error.localizedDescription]];

                     if (obj)
                     {
                         [_managedObjectContext refreshObject:obj mergeChanges:YES];
                         [objects addObject:obj];
                     }
                 }

                 // RETURN FROM METHOD SO UI CAN REGAIN CONTROL
             }];
         }
         @catch (NSException *exception)
         {
            [VS_Log VS_LogException:exception];
            return;
         }
     }];

// Merges changes from parent to child context
- (void)mergeChangesFromContextDidSaveNotification:(NSNotification *)notification
{
    // remove observer
    [[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:_backgroundWriterManagedObjectContext];

    // merge the changes
    [_managedObjectContext mergeChangesFromContextDidSaveNotification:notification];
}

Answer 1

您不是在调用流程挂起的更改。