系统内存

+(double)availableMemory{
     vm_statistics_data_t vmStats;
     mach_msg_type_number_t infoCount = HOST_VM_INFO_COUNT;
     kern_return_t kernReturn = host_statistics(mach_host_self(), HOST_VM_INFO, (host_info_t)&vmStats, &infoCount);
 
     if(kernReturn != KERN_SUCCESS)
         return NSNotFound;
 
     return ((vm_page_size * vmStats.free_count) / 1024.0) / 1024.0;
}