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POCalling ALAssetReprsentation's metadata method several hundred times will fail
 

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  1. POCalling ALAssetReprsentation's metadata method several hundred times will fail
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    <p>I am writing a tiny iPhone app to retrieve metadata, such as EXIF info, for all photos stored in the iPhone, and ran into a weird issue when calling the Assets Library Framework API. Basically, if I am calling ALAssetReprsentation's metadata method (http://developer.apple.com/library/ios/documentation/AssetsLibrary/Reference/ALAssetRepresentation_Class/Reference/Reference.html#//apple_ref/occ/instm/ALAssetRepresentation/metadata) for several hundred times (even for the same ALAssetReprsentation object), the API will report an error and return null instead of photo's metadata.</p> <p>Here is the code to reproduce this issue:</p> <pre><code>ALAsset *photo = ... // fetch a photo asset via Assets Library Framework int i = 0; ALAssetRepresentation *representation = [photo defaultRepresentation]; NSDictionary *metadata; while (i&lt;600) { i++; metadata = [representation metadata]; NSLog(@"photo %d indexed %@", i, metadata); } </code></pre> <p>Here is the output for the code above. In the beginning of the output, everything is okay, but after 500+ times, the metadata API will report error like "ImageIO: CGImageSourceCreateWithData data parameter is nil".</p> <pre><code>... 2011-12-29 21:46:17.106 MyApp[685:707] photo 578 indexed { ColorModel = RGB; DPIHeight = 72; DPIWidth = 72; ... } ... ImageIO: &lt;ERROR&gt; CGImageSourceCreateWithData data parameter is nil 2011-12-29 21:46:17.151 MyApp[685:707] photo 579 indexed (null) ImageIO: &lt;ERROR&gt; CGImageSourceCreateWithData data parameter is nil 2011-12-29 21:46:17.177 MyApp[685:707] photo 580 indexed (null) </code></pre> <p>I am testing in an iPhone 3GS with iOS 5.0.1. And I am developing with Xcode 4.2 with ARC (automatical reference counting) enabled. And I can only reproduce this issue when deploying the app to the iPhone 3GS device, but cannot reproduce this issue when using iOS simulator with the same code (at least I don't reproduce this issue after calling the API over 1800 times in iOS simulator).</p> <p>Any help is appreciated. Thanks.</p>
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    1. COHave you checked whether it's a memory issue? E.g. check with Instruments and/or use an extra autorelease pool inside the loop.
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      1. USOle Begemann
    2. COI am using ARC, do you have any idea in which case there will be memory issue if ARC is used? I will try to use Instruments to do the profiling soon (I am not familiar with Instruments yet). Thanks.
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    3. COARC has nothing to do with it. You can still run out of memory if you allocate too many autoreleased objects in a tight loop. Try modifying your loop like this: `while (i<600) { @autoreleasepool { i++; metadata = [representation metadata]; NSLog(@"photo %d indexed %@", i, metadata); } }`
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