小编eve*_*ive的帖子

使用,当我遇到有趣的行为来的NSManagedObjectContext的performBlock:与通知中心.

从主UI线程我触发异步数据下载(使用NSURLConnection的connectionWithRequest:).当数据到达时,将调用以下委托方法:

- (void)downloadCompleted:(NSData *)data
{
    NSArray *new_data = [NSJSONSerialization JSONObjectWithData:data options:0 error:nil];

    self.backgroundObjectContext = [[NSManagedObjectContext alloc]   
                          initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    self.backgroundObjectContext.persistentStoreCoordinator = self.persistentStoreCoordinator;

    [self.backgroundObjectContext performBlockAndWait:^{
        [self saveToCoreData:new_data];
    }];
}

该savetoCoreData:方法只是将新数据保存到后台上下文:

- (void)saveToCoreData:(NSArray*)questionsArray
{
    for (NSDictionary *questionDictionaryObject in questionsArray) {
        Question *newQuestion = [NSEntityDescription 
                      insertNewObjectForEntityForName:@"Question" 
                               inManagedObjectContext:self.backgroundObjectContext];

        newQuestion.content = [questionDictionaryObject objectForKey:@"content"];            
    }

    NSError *savingError = nil;
    [self.backgroundObjectContext save:&savingError];
}

在视图控制器中,viewDidLoad我将观察者添加到通知中心:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(contextChanged:) 
                                             name:NSManagedObjectContextDidSaveNotification
                                           object:nil];

然后在contexChanged:我合并背景上下文与主上下文,以便调用我的NSFetchedResultsController的委托方法,我的视图可以更新:

- (void)contextChanged:(NSNotification*)notification
{ …