Asynchronous request to the server from background thread

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];