Translating Objective-C to RubyMotion code

getDataPartAtOffset.m

- (NSData *)getDataPartAtOffset:(NSInteger)offset  {
__block NSData *chunkData = nil;
if (fileAsset_){
    static const NSUInteger BufferSize = PART_SIZE; // 5 MB chunk
    ALAssetRepresentation *rep = [fileAsset_ defaultRepresentation];
    uint8_t *buffer = calloc(BufferSize, sizeof(*buffer));
    NSUInteger bytesRead = 0;
    NSError *error = nil;

    @try
    {
        bytesRead = [rep getBytes:buffer fromOffset:offset length:BufferSize error:&error];
        chunkData = [NSData dataWithData:[NSData dataWithBytesNoCopy:buffer length:bytesRead freeWhenDone:NO]];
    }
    @catch (NSException *exception)
    {
        free(buffer);
        chunkData = nil;
        // Handle the exception here...
    }

    free(buffer);
} else {
    NSLog(@"failed to retrive Asset");
}
return chunkData;
}