Thumbnail generator from assets

gist.m

- (CGImageRef)thumbnailImageForAsset:(ALAsset *)asset
                        maxPixelSize:(NSUInteger)size
{
    NSParameterAssert(asset != nil);
    NSParameterAssert(size > 0);
    
    ALAssetRepresentation *assetRepresentation = [asset defaultRepresentation];
    
    CGDataProviderDirectCallbacks callbacks = {
        .version = 0,
        .getBytePointer = NULL,
        .releaseBytePointer = NULL,
        .getBytesAtPosition = getAssetBytesCallback,
        .releaseInfo = releaseAssetCallback,
    };
    
    CGDataProviderRef dataProvider = CGDataProviderCreateDirect((void *)CFBridgingRetain(assetRepresentation), [assetRepresentation size], &callbacks);
    
    CGImageSourceRef imageSource = CGImageSourceCreateWithDataProvider(dataProvider, NULL);
    
    NSDictionary *thumbnailOptions = @{
                                       (NSString *)kCGImageSourceCreateThumbnailFromImageAlways:
                                           @YES,
                                       (NSString *)kCGImageSourceThumbnailMaxPixelSize:
                                           [NSNumber numberWithInt:size],
                                       (NSString *)kCGImageSourceCreateThumbnailWithTransform:
                                           @YES
                                       };
    
    CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, (__bridge CFDictionaryRef)(thumbnailOptions));
    CFRelease(imageSource);
    CFRelease(dataProvider);
    
    if (!imageRef) {
        return nil;
    }
    
    return imageRef;
}

// 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) {
        NSLog(@"thumbnailForAsset:maxPixelSize: got an error reading an asset: %@", error);
    }
    
    return countRead;
}

static void releaseAssetCallback(void *info) {
    CFRelease(info);
}