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POLoading UIImage in background thread causes ImageIO error
 

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  1. POLoading UIImage in background thread causes ImageIO error
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    <p>I am working on an iOS project that loads UIImages from URLs in a background thread and displays them in a horizontally paged scroll view. This behavior should mimic that of the native photo library application for iPhone. I have everything setup in a way that I believe should work, but upon drawing the UIImages in the main UI graphics context, I occasionally see this message print out in the console:</p> <pre><code>ImageIO: &lt;ERROR&gt; CGImageReadSessionGetCachedImageBlockData *** CGImageReadSessionGetCachedImageBlockData: bad readSession [0x685e590] </code></pre> <p>Despite this error, it appears that the app functions perfectly fine, and that the images draw as expected. Should I simply ignore this "ERROR," or should I change my method for loading images in the background thread? Currently, I load the image in a background thread using the UIImage <code>initWithData:</code> method:</p> <pre><code>- (void)backgroundLoadThread:(NSURL *)url { @autoreleasepool { NSData * imageData = [NSData dataWithContentsOfURL:url]; UIImage * theImage = [[UIImage alloc] initWithData:imageData]; if ([[NSThread currentThread] isCancelled]) { #if !__has_feature(objc_arc) [theImage release]; #endif return; } [self performSelectorOnMainThread:@selector(loadComplete:) withObject:theImage waitUntilDone:NO]; } } </code></pre> <p>My <code>loadComplete:</code> method simply retains the image passed as an instance variable, which is later drawn in the view's <code>drawRect:</code> method. In <code>drawRect:</code>, I am using the UIImage <code>drawInRect:</code> method, which appears to be the perpetrator for the error message in the console. However, drawing the image's underlying CGImageRef using <code>CGContextDrawImage</code> still causes the same error. The full source code can be found in this project's Github repository, in the file <a href="https://github.com/unixpickle/ImageSlideshow/blob/master/ImageSlideshow/ANAsyncImageView.m" rel="nofollow">ANAsyncImageView.m</a>.</p> <p>EDIT: I have now changed the background thread to load a CGImage from the file, and then callback to the main thread, passing the CGImageRef using toll-free bridging. Unfortunately, this still creates the same issue, proving to me that UIImage's <code>imageWithCGImage:</code> must in some way keep a reference to the original CGImage. Check out the code in the file linked above (to the Github repository) for the full code:</p> <pre><code>NSData * imageData = [NSData dataWithContentsOfURL:url]; CGDataProviderRef provider = CGDataProviderCreateWithCFData((CFDataRef)imageData); CGImageRef loaded = CGImageCreateWithPNGDataProvider(provider, NULL, false, kCGRenderingIntentDefault); CGDataProviderRelease(provider); id imageObj = (id)loaded; [self performSelectorOnMainThread:@selector(loadComplete:) withObject:imageObj waitUntilDone:NO]; CGImageRelease(loaded); </code></pre>
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    1. COShouldn't you be releasing that image regardless of whether the thread has been cancelled? `performSelectorOnMainThread:withObject:waitUntilDone:` will retain and release the object for you, so you don't need to worry about it dying out from under you.
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      1. USPeter Hosey
 

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