怎么在一个线程结束后回到主线程？

第一种是Notification，这个比较解耦。

-(void)viewDidLoad
{
    // add a notification observer
    [NSNotificationCenter defaultCenter] addObserver:selector:name:object:]
}
-(void)handle
{
    if (fileNameArr.count == 3) {
        // do something
    }
}
NSMutableArray *fileNameArr = [[NSMutableArray alloc]init];
for (int i= 0; i<[self.imgArr count]; i++) {
    NSData *data = [self.imgArr objectAtIndex:i];
    [manager POST:url 
       parameters:nil 
       constructingBodyWithBlock:
     ^(id<AFMultipartFormData> formData){
         [formData appendPartWithFileData:data 
                                     name:@"pic" 
                                 fileName:fileName 
                                 mimeType:@"image/jpeg"];
     } success:^(AFHTTPRequestOperation *operation,id responseObject) {
         NSLog(@"success:%@",responseObject);
         NSString *str = [[responseObject objectForKey:@"data"] objectForKey:@"fileName"];
         [fileNameArr addObject:str];
         NSString *ids = [[responseObject objectForKey: @"data"] objectForKey:@"ids"];          
         // add here
         [NSNotificationCenter defaultCenter] postNotificationName:object:]
   } failure:^(AFHTTPRequestOperation *operation,NSError *error){
         NSLog(@"%@",error);
      }];
   }

第二种就是，GCD啦，但是代码看起来耦合比较高。

NSMutableArray *fileNameArr = [[NSMutableArray alloc]init];
for (int i= 0; i<[self.imgArr count]; i++) {
    NSData *data = [self.imgArr objectAtIndex:i];
    [manager POST:url 
       parameters:nil
       constructingBodyWithBlock:
    ^(id<AFMultipartFormData> formData){
        [formData appendPartWithFileData:data 
                                    name:@"pic" 
                                fileName:fileName 
                                mimeType:@"image/jpeg"];
    } success:^(AFHTTPRequestOperation *operation,id responseObject){
        NSLog(@"success:%@",responseObject);
        NSString *str = [[responseObject objectForKey:@"data"] objectForKey:@"fileName"];
        [fileNameArr addObject:str];
        NSString *ids = [[responseObject objectForKey: @"data"] objectForKey:@"ids"];          
        // here
        if (fileNameArr.count == 3) {
            dispatch_async(dispatch_get_main_queue(), ^{
                // handle 
            });
        }
   } failure:^(AFHTTPRequestOperation *operation,NSError *error){
          NSLog(@"%@",error);
      }];
   }