dispatch_async中的异步URL请求 [英] Asynchronous url requests inside dispatch_async

问题描述

我正在尝试在特定函数中实现异步url请求，我希望完成所有这些请求，然后执行特定操作，但操作先于请求，即在请求完成之前调用它。

I am trying to implement asynchronous url requests in a particular function, I want all these requests to complete and then do a particular action but the action precedes the requests i.e, it is getting called before the requests complete.

dispatch_queue_t fetchQ = dispatch_queue_create("Featured Doc Downloader", NULL);
        dispatch_async(fetchQ, ^{
            [self myAsyncMultipleURLRequestFunction];
            dispatch_sync(dispatch_get_main_queue(), ^{
                [self updateUIFunction];
            });
        });

-(void)myAsyncMultipleURLRequestFunction
   {
    for (int i=0; i<count; i++) 
     {
     NSURLConnection *loginConnection = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self];          
     }
   }

现在在myAsyncMultipleURLRequestFunction完成所有请求之前调用updateUIFunction。也尝试使用NSOperaitonQueue但不能做我真正想要的。

now updateUIFunction gets called before myAsyncMultipleURLRequestFunction completes all requests. Also tried this with NSOperaitonQueue but could not do what I really want.

[_operationQ addOperationWithBlock: ^ {
     for (int i=0; i<count; i++)
      {
     NSURLConnection *loginConnection = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self];          
      }
    }

[[NSOperationQueue mainQueue] addOperationWithBlock: ^ {
         // updating UI
         [self updateUIFunction];
    }];
}];

我知道这很简单，但我的时间不多了，感谢任何帮助。

I know this is simple but I am running outta time, any help is appreciated.

推荐答案

@tkanzakic在正确的道路上。要使用的正确构造是dispatch_group_t。但实施可以改进。通过使用信号量，您可以异步启动所有下载，并且仍然确保没有太多并发运行。下面是一个代码示例，说明了如何使用dispatch_group_t并使所有下载并行：

@tkanzakic is on the right path. The correct construct to use is the dispatch_group_t. But the implementation could be improved. By using a semaphore you can launch all your downloads asynchronously and still make sure that you don't have too many running concurrently. Here is a code sample that illustrates how you should use dispatch_group_t as well as make all your downloads parallel:

dispatch_queue_t fetchQ = dispatch_queue_create("Featured Doc Downloader", NULL);
dispatch_group_t fetchGroup = dispatch_group_create();

// This will allow up to 8 parallel downloads.
dispatch_semaphore_t downloadSema = dispatch_semaphore_create(8);

// We start ALL our downloads in parallel throttled by the above semaphore.
for (int i=0; i<count; i++) {
    dispatch_group_async(fetchGroup, fetchQ, ^(void) {
        dispatch_semaphore_wait(downloadSema, DISPATCH_TIME_FOREVER);
        NSURLConnection *loginConnection = [[NSURLConnection alloc] initWithRequest:requestArray[i] delegate:self];
        dispatch_semaphore_signal(downloadSema);
    });
}

// Now we wait until ALL our dispatch_group_async are finished.
dispatch_group_wait(fetchGroup, DISPATCH_TIME_FOREVER);

// Update your UI
dispatch_sync(dispatch_get_main_queue(), ^{
    [self updateUIFunction];
});

// Release resources
dispatch_release(fetchGroup);
dispatch_release(downloadSema);
dispatch_release(fetchQ);

这篇关于dispatch_async中的异步URL请求的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！