托管对象上下文不合并来自后台上下文的更改

Posted 2023-03-04

【中文标题】托管对象上下文不合并来自后台上下文的更改

【英文标题】：Managed object context not merging changes from background context

【发布时间】：2013-05-10 12:56:31

【问题描述】：

我有一个双重托管对象上下文设置，其中我有一个父/子 MOC 关系。父级负责直接写入私有队列（NSPrivateQueueConcurrencyType）上的数据库，然后我有一个子上下文负责为主线程（NSMainQueueConcurrencyType）拉取和保存数据。

当我对主队列上下文进行更改并保存它时，上下文会正常工作，然后将更改合并到后台队列上下文并在后台线程上写入数据库。

我遇到的问题是当我将数据直接写入后台队列上下文，然后尝试将其合并到主队列上下文时。对象正确存储在商店中，当我合并更改时，它似乎可以工作。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，那么数据是陈旧的，而不是正确的更新数据。

以下是代码摘录，可以帮助大家了解我在做什么。需要注意的是从 JSON 返回 NSManagedObjects 的逻辑可以正常工作。此外，我已经测试并且在 UI 尝试对主队列 MOC 发出任何获取请求之前运行 mergeChangesFromContextDidSaveNotification。大家有什么想法吗？

// 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];

【问题讨论】：

在您提供的信息中有些东西没有意义，因为您的代码和概念看起来不错。您是否使用 FRC 将数据提供给视图？如果是这样，我怀疑它与您的 FRC 有更多关系。我不确定独立提取是否会根据您的需要更新 MOC。我会调查的。

【参考方案1】：

您没有调用进程挂起的更改。