iOS获取硬盘可用空间的正确姿势

//获取可用容量
+ (NSNumber *)freeDiskSpace
{
    NSURL *fileURL = [[NSURL alloc] initFileURLWithPath:NSTemporaryDirectory()];
    NSDictionary *results = [fileURL resourceValuesForKeys:@[NSURLVolumeAvailableCapacityForImportantUsageKey] error:nil];
    return results[NSURLVolumeAvailableCapacityForImportantUsageKey];
  //下面在iOS11.3的系统获取可用空间出现异常
//    NSDictionary *fattributes = [[NSFileManager defaultManager] attributesOfFileSystemForPath:NSHomeDirectory() error:nil];
//    return [fattributes objectForKey:NSFileSystemFreeSize];
}