iOS11系统以上， 获取系统剩余空间不对

iOS11系统以下获取系统剩余空间有2种方法：

方法一：

#include <sys/param.h>
#include <sys/mount.h>

+ (NSString *)freeDiskSpaceInBytes{
    struct statfs buf;
    unsigned long long freeSpace = -1;
    if (statfs("/var", &buf) >= 0) {
        freeSpace = (unsigned long long)(buf.f_bsize * buf.f_bavail);
    }
     NSString *str = [NSString stringWithFormat:@"系统剩余空间为：%0.2lld MB",freeSpace/1024/1024];
    return str;
}

方法二：

  /// 总大小
    float totalsize = 0.0;
    /// 剩余大小
    float freesize = 0.0;
    /// 是否登录
    NSError *error = nil;
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSDictionary *dictionary = [[NSFileManager defaultManager] attributesOfFileSystemForPath:[paths lastObject] error: &error];
    if (dictionary)
    {
        NSNumber *_free = [dictionary objectForKey:NSFileSystemFreeSize];
        freesize = [_free unsignedLongLongValue]*1.0/(1024);

        NSNumber *_total = [dictionary objectForKey:NSFileSystemSize];
        totalsize = [_total unsignedLongLongValue]*1.0/(1024);
    } else
    {
        NSLog(@"Error Obtaining System Memory Info: Domain = %@, Code = %ld", [error domain], (long)[error code]);
    }
    NSLog(@"totalsize = %.2f, freesize = %f",totalsize/1024/1024/1024, freesize/1024);

但是在iOS11以上使用这2个方法获取剩余空间不准确了，或者跟系统不一样 iOS11上可以使用以下方法：

NSURL *fileURL = [[NSURL alloc] initFileURLWithPath:NSTemporaryDirectory()];
    NSDictionary *results = [fileURL resourceValuesForKeys:@[NSURLVolumeAvailableCapacityForImportantUsageKey] error:nil];
    NSLog(@"剩余空间:%@",results[NSURLVolumeAvailableCapacityForImportantUsageKey]);
// 这里拿到的值的单位是bytes，iOS11是这样算的1000MB = 1，1000进制算的

注意是1000位单位了。不是1024。