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Downscaling ALAssets
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// See http://mindsea.com/2012/12/18/downscaling-huge-alassets-without-fear-of-sigkill for details
 
#import <AssetsLibrary/AssetsLibrary.h>
#import <ImageIO/ImageIO.h>
 
// Helper methods for thumbnailForAsset:maxPixelSize:
static size_t getAssetBytesCallback(void *info, void *buffer, off_t position, size_t count) {
ALAssetRepresentation *rep = (__bridge id)info;
NSError *error = nil;
size_t countRead = [rep getBytes:(uint8_t *)buffer fromOffset:position length:count error:&error];
if (countRead == 0 && error) {
// We have no way of passing this info back to the caller, so we log it, at least.
NSLog(@"thumbnailForAsset:maxPixelSize: got an error reading an asset: %@", error);
}
return countRead;
}
 
static void releaseAssetCallback(void *info) {
// The info here is an ALAssetRepresentation which we CFRetain in thumbnailForAsset:maxPixelSize:.
// This release balances that retain.
CFRelease(info);
}
 
// Returns a UIImage for the given asset, with size length at most the passed size.
// The resulting UIImage will be already rotated to UIImageOrientationUp, so its CGImageRef
// can be used directly without additional rotation handling.
// This is done synchronously, so you should call this method on a background queue/thread.
- (UIImage *)thumbnailForAsset:(ALAsset *)asset maxPixelSize:(NSUInteger)size {
NSParameterAssert(asset != nil);
NSParameterAssert(size > 0);
ALAssetRepresentation *rep = [asset defaultRepresentation];
CGDataProviderDirectCallbacks callbacks = {
.version = 0,
.getBytePointer = NULL,
.releaseBytePointer = NULL,
.getBytesAtPosition = getAssetBytesCallback,
.releaseInfo = releaseAssetCallback,
};
CGDataProviderRef provider = CGDataProviderCreateDirect((void *)CFBridgingRetain(rep), [rep size], &callbacks);
CGImageSourceRef source = CGImageSourceCreateWithDataProvider(provider, NULL);
CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(source, 0, (__bridge CFDictionaryRef) @{
(NSString *)kCGImageSourceCreateThumbnailFromImageAlways : @YES,
(NSString *)kCGImageSourceThumbnailMaxPixelSize : [NSNumber numberWithInt:size],
(NSString *)kCGImageSourceCreateThumbnailWithTransform : @YES,
});
CFRelease(source);
CFRelease(provider);
 
if (!imageRef) {
return nil;
}
UIImage *toReturn = [UIImage imageWithCGImage:imageRef];
CFRelease(imageRef);
return toReturn;
}

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