Question

I am working on an iOS app which dispatch quite a number of tasks to my serial queue. The task is to download images from my web server, save it to disk, and later displayed on UIImageView. However, [NSURLConnection sendAsynchrousRequest] will keep eating up more and more memory until iOS kill my process.

The downloader method looks like this:

// dispatch_queue_t is created once by: m_pRequestQueue = dispatch_queue_create( "mynamespace.app", DISPATCH_QUEUE_SERIAL);

- (void) downloadImageInBackgroundWithURL:(NSString*) szUrl {
__block typeof(self) bSelf = self;
__block typeof(m_pUrlRequestQueue) bpUrlRequestQueue = m_pRequestQueue;

dispatch_async( m_pRequestQueue, ^{ 
    NSAutoreleasePool *pAutoreleasePool = [[NSAutoreleasePool alloc] init];
    NSURLRequest *pRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:szUrl]
                                              cachePolicy:NSURLRequestReloadIgnoringCacheData
                                          timeoutInterval:URL_REQUEST_TIMEOUT];

    [NSURLConnection sendAsynchronousRequest:pRequest queue:bpUrlRequestQueue completionHandler:^(NSURLResponse *pResponse, NSData *pData, NSError *pError) {
        NSAutoreleasePool *pPool = [[NSAutoreleasePool alloc] init];
        if ( pError != nil ) {
        } else {                
            // convert image to png format
            UIImage *pImg = [UIImage imageWithData:pData];
            NSData *pDataPng = UIImagePNGRepresentation(pImg);
            bool bSaved = [[NSFileManager defaultManager] createFileAtPath:szCacheFile contents:pDataPng attributes:nil];                    
        }

            __block typeof(pDataPng) bpDataPng = pDataPng;
            __block typeof(pError) bpError = pError;
            dispatch_sync( dispatch_get_main_queue(), ^ {
                NSAutoreleasePool *autoreleasepool = [[NSAutoreleasePool alloc] init];
                UIImage *pImage = [[UIImage alloc] initWithData:bpDataPng];

                // display the image

                [pImage release];
//                    NSLog( @"image retain count: %d", [pImage retainCount] ); // 0, bad access

                [autoreleasepool drain];
            });
        }
        [pPool drain];
    }]; // end sendAsynchronousRequest

    [pAutoreleasePool drain];

}); // end dispatch_async
} // end downloadImageInBackgroundWithURL

I am quite sure it is something inside [NSURLConnection sendAsynchronousRequest] as the profiler is showing that the function is the one eating up all the memory…

However, I am also not very sure about the dispatch_*** and block things, I’ve always used C and C++ code with pthread before, but after reading from Apple’s documentation on migrating away from thread, I decided to give GCD a try, objective-c is so troublesome and I’m not sure how to release the NSData *pData and NSURLResponse *pResponse as it crash whenever I do it.

Please advice… really need help to learn and appreciate objective-c…

ADDITIONAL EDIT:

Thanks to @robhayward, I put the pImg and pDataPng outside as __block variable, use his RHCacheImageView way of downloading data ( NSData initWithContentOfURL )

Thanks as well to @JorisKluivers, the first UIImage can actually be reused to display as UIImageView recognized both jpg and png format, just my later processing requires png format and I am reading from the disk later just when required

Answer 1

I would firstly put it down to the image and data objects that you are creating:

UIImage *pImg = [UIImage imageWithData:pData];
NSData *pDataPng = UIImagePNGRepresentation(pImg);

Which might be hanging around too long, perhaps put them outside the block, as they are probably being created/released on different threads:

__block UIImage *pImg = nil;
__block NSData *pDataPng = nil;
[NSURLConnection sendAsynchronousRequest..

(Also consider using ARC if you can)

I have some code on Github that does a similar job without this issue, feel free to check it out:

https://github.com/robinhayward/RHCache/blob/master/RHCache/RHCache/Helpers/UIImageView/RHCacheImageView.m