怎么在一个线程结束后回到主线程？

我用一个for循环上传三张照片（用的是afnetworking框架）。
上传它就开了一个线程，上传成功后后台返回一个filename，把filename放进一个数组
我想等三张照片全都上传结束后，对获取到的三个filename进行操作
要怎么做？

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"];          
    }
          failure:^(AFHTTPRequestOperation *operation,NSError *error){
              NSLog(@"%@",error);
          }];
}

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简单点设置一个变量，等加到3了，再处理

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观察者模式。主线程监听子线程执行情况，子线程执行完毕，调用监听器方法即可。

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用Notification写好了。在三个任务都完成之后发送通知，然后执行注册的方法。

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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);
      }];
   }

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没仔细看你为什么要回到主线程哈，要回的话这样就可以啦：

dispatch_sync(dispatch_get_main_queue(), ^(){
    // 这里的代码会在主线程执行
});

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上面的两个答案应该不是题主想要的，上传图片是异步的，但是要等三个全部上传完成才做下一步操作，这是要求同步。通知可以，但GCD可以更方便，用dispatch_group_async可以实现监听一组任务是否完成，完成后得到通知执行其他的操作。这个方法很有用，比如你执行三个下载任务，当三个任务都下载完成后你才通知界面说完成的了,符合你的要求，还可以用信号量来处理，不过这有些老了