Question

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

Then I call this from main thread

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

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];
}

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

are called. However if I replace

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

by

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

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?

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

Thanks in advance.

Answer 1

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];

You can keep the thread from exiting with:

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

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

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());
}

Learn more about Run Loops in the Threading Guide and RunLoop Reference.

Answer 2

You can start the connection on a background thread but you have to ensure the delegate methods are called on main thread. This cannot be done with

[[NSURLConnection alloc] initWithRequest:urlRequest 
                                delegate:self];

since it starts immediately.

Do this to configure the delegate queue and it works even on secondary threads:

NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:urlRequest 
                                                              delegate:self 
                                                      startImmediately:NO];
[connection setDelegateQueue:[NSOperationQueue mainQueue]];
[connection start];

Answer 3

NSURLRequests are completely asynchronous anyway. If you need to make an NSURLRequest from a thread other than the main thread, I think the best way to do this is just make the NSURLRequest from the main thread.

// Code running on _not the main thread_:
[self performSelectorOnMainThread:@selector( SomeSelectorThatMakesNSURLRequest ) 
      withObject:nil
      waitUntilDone:FALSE] ; // DON T block this thread until the selector completes.

All this does is shoot off the HTTP request from the main thread (so that it actually works and doesn t mysteriously disappear). The HTTP response will come back into the callbacks as usual.

If you want to do this with GCD, you can just go

// From NOT the main thread:
dispatch_async( dispatch_get_main_queue(), ^{ //
  // Perform your HTTP request (this runs on the main thread)
} ) ;

The MAIN_QUEUE runs on the main thread.

So the first line of my HTTP get function looks like:

void Server::get( string queryString, function<void (char*resp, int len) > onSuccess, 
                  function<void (char*resp, int len) > onFail )
{
    if( ![NSThread isMainThread] )
    {
        warning( "You are issuing an HTTP request on NOT the main thread. "
                 "This is a problem because if your thread exits too early, "
                 "I will be terminated and my delegates won t run" ) ;

        // From NOT the main thread:
        dispatch_async( dispatch_get_main_queue(), ^{
          // Perform your HTTP request (this runs on the main thread)
          get( queryString, onSuccess, onFail ) ; // re-issue the same HTTP request, 
          // but on the main thread.
        } ) ;

        return ;
    }
    // proceed with HTTP request normally
}

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