从后台线程向服务器的异步请求 [英] Asynchronous request to the server from background thread

问题描述

当我尝试从后台线程向服务器发出异步请求时，我遇到了问题。我从来没有得到过这些要求的结果。显示问题的简单示例：

I've got the problem when I tried to do asynchronous requests to server from background thread. I've never got results of those requests. Simple example which shows the problem:

@protocol AsyncImgRequestDelegate
-(void) imageDownloadDidFinish:(UIImage*) img;
@end


@interface AsyncImgRequest : NSObject
{
 NSMutableData* receivedData;
 id<AsyncImgRequestDelegate> delegate;
}

@property (nonatomic,retain) id<AsyncImgRequestDelegate> delegate;

-(void) downloadImage:(NSString*) url ;

@end



@implementation AsyncImgRequest
-(void) downloadImage:(NSString*) url 
{  
 NSURLRequest *theRequest=[NSURLRequest requestWithURL:[NSURL URLWithString:url]
             cachePolicy:NSURLRequestUseProtocolCachePolicy
            timeoutInterval:20.0];
 NSURLConnection *theConnection=[[NSURLConnection alloc] initWithRequest:theRequest delegate:self];
 if (theConnection) {
  receivedData=[[NSMutableData data] retain];
 } else {
 }  

}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
  [delegate imageDownloadDidFinish:[UIImage imageWithData:receivedData]];
  [connection release];
  [receivedData release];
}
@end

然后我从主线程中调用它

Then I call this from main thread

asyncImgRequest = [[AsyncImgRequest alloc] init];
asyncImgRequest.delegate = self; 
[self performSelectorInBackground:@selector(downloadImage) withObject:nil];

方法downloadImage如下：

method downloadImage is listed below:

-(void) downloadImage
{
 NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
 [asyncImgRequest downloadImage:@"http://photography.nationalgeographic.com/staticfiles/NGS/Shared/StaticFiles/Photography/Images/POD/l/leopard-namibia-sw.jpg"];
 [pool release];
}

问题是从不调用方法imageDownloadDidFinish。此外没有方法

The problem is that method imageDownloadDidFinish is never called. Moreover none of methods

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse*)response

被调用。但是如果我用对象替换

are called. However if I replace

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

by

 [self performSelector:@selector(downloadImage) withObject:nil]; 

一切正常。我假设后台线程在异步请求完成它之前死亡，这导致了问题，但我不确定。我对这个假设是对的吗？有没有办法避免这个问题？

everything is working correct. I assume that the background thread dies before async request is finished it job and this causes the problem but I'm not sure. Am I right with this assumptions? Is there any way to avoid this problem?

我知道我可以使用同步请求来避免这个问题，但这只是一个简单的例子，真实情况更复杂。

I know I can use sync request to avoid this problem but it's just simple example, real situation is more complex.

提前致谢。

推荐答案

是的，线程正在退出。你可以通过添加：

Yes, the thread is exiting. You can see this by adding:

-(void)threadDone:(NSNotification*)arg
{
    NSLog(@"Thread exiting");
}

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(threadDone:)
                                             name:NSThreadWillExitNotification
                                           object:nil];

您可以保持线程退出：

-(void) downloadImage
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    [self downloadImage:urlString];

    CFRunLoopRun(); // Avoid thread exiting
    [pool release];
}

但是，这意味着线程永远不会退出。所以你需要在完成后停止它。

However, this means the thread will never exit. So you need to stop it when you're done.

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    CFRunLoopStop(CFRunLoopGetCurrent());
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    CFRunLoopStop(CFRunLoopGetCurrent());
}

在线程指南和 RunLoop参考。

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