系统转换方法，自动输出合适的单位

NSDictionary *fileInfo = [[NSFileManager defaultManager] attributesOfItemAtPath:filePath error:nil];
NSString *fileSize = [NSByteCountFormatter stringFromByteCount:[fileInfo[NSFileSize] longLongValue] countStyle:NSByteCountFormatterCountStyleFile];

自定义转换方法，按需求选择单位

- (NSString *)transformedValue:(id)value {
    CGFloat convertedValue = [value doubleValue];
    NSInteger multiplyFactor = 0;

    NSArray *tokens = @[@"bytes", @"KB", @"MB", @"GB", @"TB", @"PB", @"EB", @"ZB", @"YB"];
    while (convertedValue > 1024) {
        convertedValue /= 1024;
        multiplyFactor++;
    }
    return [NSString stringWithFormat:@"%4.2f %@",convertedValue, tokens[multiplyFactor]];
}