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POIs ALAssetsLibrary thread safe (Deadlock occurs when using multiple threads)
 

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  1. POIs ALAssetsLibrary thread safe (Deadlock occurs when using multiple threads)
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    <p>I'm currenly working on a tiny project which amis to loading ALL gallery photos into my app to show some fancy effect. Unfortunately, these default thumbnail provided by system cannot meet my requirement. So I try to create my own thumbnails using "fullScreenImage". To speed up the process, I load fullScreenImage using background operations. The main methods are:</p> <pre><code>- (void)getFullScreenImage:(NSURL *)url success:(void(^)(UIImage *))callback { NSLog(@"Requesting %@", url); [assetsLibraryInstance assetForURL:url resultBlock:^(ALAsset *asset) { callback(asset.defaultRepresentation.fullScreenImage); } failureBlock:nil]; } - (void)processURLs:(NSArray *)urls { for (NSURL *url in urls) { dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0) ^{ [self getFullScreenImage:url success:^(UIImage *img) { NSLog(@"Got image %@", img); }] ; }); } } </code></pre> <p>Only the "Requesting..." log is printed in console, the "getFullScreenImage" method is locked, no any output.</p> <p>I tried below methods to work around this issue:</p> <ol> <li>Not sharing assetsLibraryInstance (Didn't work)</li> <li>Don't dispatch_async when enumeate urls in "processURLs". (Did work, but I don't want to use a signle thread to process all URLs)</li> <li>Not using global queue, using main queue (Did work, but all these "fullScreenImage" works were doing on UI thread, making UI non-responsive)</li> <li>Using a private queue created with "dispatch_queue_create". (Didn't work)</li> </ol> <p>So, is ALAssetsLibrary thread safe? I guess it's not... Or, is there any better way I can use to:</p> <ol> <li>Load fullScreenImage in background</li> <li>Multiple threading</li> </ol> <p>Thanks!</p>
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    1. USxiaowl
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    1. CODid you ever get an answer? I just encounter the same issue and would like to see if you found any solutions?
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      1. USLuis Artola
    2. CO@LuisArtola No, I didn't got any response from SO. But I finally solve d it myself. The idea is making `assetForURL` and `defaultRepresentation.fullScreenImage` a "atomic" operation, to make sure there is no other `assetForURL` call happened before `fullScreenImage` returns. And actually, there is some internal lock under assetslibrary, thus multiple threading doesn't benefit much...
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      1. POIs ALAssetsLibrary thread safe (Deadlock occurs when using multiple threads)
      1. USxiaowl
    3. COThanks for the pointers @xiaowl! I'll give it a try.
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