ios - 需要帮助设置后台托管对象上下文。在 FRC controllerDidChangeContent 上获取异常

Question

我有一个包含两个托管对象上下文的应用程序:

@property (nonatomic, strong) NSManagedObjectContext *managedObjectContext;
@property (nonatomic, strong) NSManagedObjectContext *backgroundContext;

我的 View Controller 之一是响应 NSFetchedResultsControllerDelegate 的 UITableView Controller 。当我(从远程 Web 服务)获取对象时，我想在后台线程上处理数据，将更改合并到我的主 managedObjectContext 并更新 UI。

在我当前的设置中，当我尝试获取新对象时出现以下异常。

CoreData: error: Serious application error.  An exception was caught from the delegate of   NSFetchedResultsController during a call to -controllerDidChangeContent:.  *** -[__NSArrayM  insertObject:atIndex:]: object cannot be nil with userInfo (null)

此异常引发于:

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {

    [self.tableView endUpdates];

}

我的托管对象上下文是在名为 AppController 的单例类中创建的。目前看起来像这样:

- (id)init {    
    self = [super init];
    if (self) {
        sharedInstance = self;

        // Registers NSManagedObjectContextDidSaveNotification 
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mergeChanges:) name:NSManagedObjectContextDidSaveNotification object:self.managedObjectContext];

    }

    return self;
}

- (void)mergeChanges:(NSNotification *)notification {

    //should tell the main thread moc to run on the main thread, and merge in the changes there
    [self.managedObjectContext  performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:)  withObject:notification waitUntilDone:YES];

}


- (void)saveContext {    
    NSError *childError = nil;
    [self.backgroundContext save:&childError];

    [self.managedObjectContext performBlock:^{
        NSError *parentError = nil;
        [self.managedObjectContext save:&parentError];
    }];
}


// Returns the managed object context for the application.
// If the context doesn't already exist, it is created and bound to the persistent store coordinator for the application.
- (NSManagedObjectContext *)managedObjectContext {
    if (_managedObjectContext != nil) {
        return _managedObjectContext;
    }

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

关于我遗漏或做错了什么的任何想法？

Answer 1

我认为您必须在后台进程中创建 backgroundContext。

并且您需要监听backgroundContext发送的通知:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mergeChanges:) name:NSManagedObjectContextDidSaveNotification object:self.backgroundContext];

在我的例子中，runupdate 在后台进程中运行:

- (void) runUpdate {
    NSManagedObjectContext *managedContext = [[NSManagedObjectContext alloc] init];
    managedContext.persistentStoreCoordinator = ...;
    [updateController registerBackgroundMoc:managedContext];
    //do all update stuff
    NSError *error;
    if (![managedContext save: &error] {
       ...
    }
    [updateController unregisterBackgroundMoc:managedContext];
}

在我的 updateController 中我做了以下事情:

- (void) registerBackgroundMoc: (NSManagedObjectContext *) updateMoc {
    [[NSNotificationCenter defaultCenter] addObserver: self selector: @selector(mergeChanges:) name: NSManagedObjectContextDidSaveNotification object: updateMoc];
}

- (void) unregisterBackgroundMoc: (NSManagedObjectContext *) updateMoc {
    [[NSNotificationCenter defaultCenter] removeObserver: self name: NSManagedObjectContextDidSaveNotification object: updateMoc];
}