iphone - 在后台线程中更新托管对象并在主线程中显示它们，出了什么问题

Question

晚上好，

我在 CoreData 和并发方面遇到了一些问题，所以我尝试了最简单的代码，但它仍然不起作用。你能指出我哪里错了吗？

我创建了一个“DataManager”来更新一个 CoreData 对象

@implementation OBSDataManager

@synthesize persistentStoreCoordinator;

- (OBSDataManager *)initWithPersistentStoreCoordinator:(NSPersistentStoreCoordinator *)aPersistentStoreCoordinator {
   if (self = [super init]) {
      self.persistentStoreCoordinator = aPersistentStoreCoordinator;
   }

   return self;
}

- (void)dealloc {
   [persistentStoreCoordinator release];

   [super dealloc];
}

- (void)start {

   [self performSelectorInBackground:@selector(updateData) withObject:nil];
}

- (void)updateData {
   NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init];
   context.persistentStoreCoordinator = self.persistentStoreCoordinator;

   // get chunk if it exists, or create it
   OBSChunk *chunk = [OBSChunk theChunkInContext:context];
   if (!chunk) {
      chunk = [NSEntityDescription insertNewObjectForEntityForName:@"Chunk"
                                            inManagedObjectContext:context];
   }

   while (1) {
      // update content
      chunk.text = [[NSDate date] description];

      // save it
      NSError *error;
      if ([context save:&error]) {
         NSLog(@"Problem on save");
      }
   }
   [context release];

}

@end

我有一个 View Controller ，它显示我的 CoreData 对象的内容

    @implementation MainViewController

@synthesize managedObjectContext;
@synthesize label;

#pragma mark -
#pragma mark UIViewController
 // Implement viewDidLoad to do additional setup after loading the view, typically from a nib.
- (void)viewDidLoad {
    [super viewDidLoad];

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

- (void)viewDidUnload {
   [super viewDidUnload];

   [[NSNotificationCenter defaultCenter] removeObserver:self
                                                   name:NSManagedObjectContextDidSaveNotification
                                                 object:nil];
}


#pragma mark -
#pragma mark private
- (void)onCoreDataUpdate:(NSNotification *)updateNotification {
   [self.managedObjectContext mergeChangesFromContextDidSaveNotification:updateNotification];

   OBSChunk *chunk = [OBSChunk theChunkInContext:self.managedObjectContext];

   self.label.text = chunk.text;
}

@end

onCoreDataUpdate 方法中获取的 block 对象似乎有错误的数据。

我哪里错了？

问候，昆汀

Answer 1

-onCoreDataUpdate: 正在后台线程上调用。通知是在发送通知的线程上接收的。您需要回调到主线程来实际处理更新。您可以使用类似以下内容来处理此问题:

- (void)onCoreDataUpdate:(NSNotification *)updateNotification {
    if (![NSThread isMainThread]) {
        [self performSelectorOnMainThread:_cmd withObject:updateNotification waitUntilDone:NO];
        return;
    }
    [self.managedObjectContext mergeChangesFromContextDidSaveNotification:updateNotification];

    OBSChunk *chunk = [OBSChunk theChunkInContext:self.managedObjectContext];

    self.label.text = chunk.text;
}