查询剩余空间大小,单位为MB
+ (NSUInteger *)freeDiskSpaceMB
{
NSDictionary *fattributes = [[NSFileManager defaultManager] attributesOfFileSystemForPath:NSHomeDirectory() error:nil];
return ([[fattributes objectForKey:NSFileSystemFreeSize] unsignedIntegerValue] / 1024 / 1024);
}