iOS中生成随机的UUID

 + (NSString *)uuidString
 {
     CFUUIDRef uuid_ref = CFUUIDCreate(NULL);
     CFStringRef uuid_string_ref= CFUUIDCreateString(NULL, uuid_ref);
     NSString *uuid = [NSString stringWithString:(__bridge NSString *)uuid_string_ref];
     CFRelease(uuid_ref);
     CFRelease(uuid_string_ref);
     return [uuid lowercaseString];
 }