iOS NSFileManager获取设备硬盘剩余可用容量不准确问题

方法1. 通用
NSFileManager attributesOfFileSystemForPath: error:

方法2. available(iOS 11.0)
NSURL resourceValuesForKeys: error:

发现问题：方法1获取到的剩余值并不准确，测得使用剩余值远小于实际的手机存储容量剩余。所以使用方法2优先。下面代码中字典信息的Key值可以获取到对应的容量值。

示例代码：
 

#import "UIDevice+DiskSpace.h"


+ (long)freeDiskSpaceInBytes
{
    NSString *path = [MYPath document];
    NSError * error = nil;
    if (@available(iOS 11.0, *)) {
        NSURL * url = [[NSURL alloc]initFileURLWithPath:[NSString stringWithFormat:@"%@",path]];
        NSDictionary<NSURLResourceKey, id> * dict = [url resourceValuesForKeys:@[NSURLVolumeTotalCapacityKey,NSURLVolumeAvailableCapacityForImportantUsageKey] error:&error];
        if (error) {
            NSLog(@"Error Obtaining System Memory Info: Domain = %@, Code = %@", [error domain], @([error code]));
            return 0;
        }
        uint64_t capacity = [dict[NSURLVolumeTotalCapacityKey] longLongValue];
        uint64_t freeSize = [dict[NSURLVolumeAvailableCapacityForImportantUsageKey] longLongValue];
        const uint64_t reserve  = 200 * 1024 * 1024;                  /// 200m保留空间
        const CGFloat  GB       = 1024 * 1024 * 1024;
        NSLog(@"Memory Capacity of %.2f GB with %.2f GB Free memory available.", capacity / GB, (freeSize - reserve) / GB);
        return freeSize - reserve;
    } else {
        NSDictionary *attributes = [[NSFileManager defaultManager] attributesOfFileSystemForPath:path error:&error];
        if (attributes) {
            uint64_t       capacity = [attributes[NSFileSystemSize] unsignedLongLongValue];
            uint64_t       freeSize = [attributes[NSFileSystemFreeSize] unsignedLongLongValue];
            const uint64_t reserve  = 200 * 1024 * 1024;                  /// 200m保留空间
            const CGFloat  GB       = 1024 * 1024 * 1024;
            NSLog(@"Memory Capacity of %.2f GB with %.2f GB Free memory available.", capacity / GB, (freeSize - reserve) / GB);
            return freeSize - reserve;
        }
        NSLog(@"Error Obtaining System Memory Info: Domain = %@, Code = %@", [error domain], @([error code]));
        return 0;
    }
}