[iOS App] 'NSInvalidArgumentException', reason : unrecognized selector sent to Instance

I am going through iOS App development recently. I use modal segue to move from parent ViewController to child ViewController, as shown in the picture below.

When i return from child ViewController to parent ViewController.  There is an error.

2014-11-20 09:00:48.229 Heritage[64395:60b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[XYZContinentTableViewController topViewController]: unrecognized selector sent to instance 0x8c60050'

The error shows up when i have 3 level of ViewController, and specifically, from second level ViewController to root ViewController.

I found out the root cause. It is because the unwind segue

- (IBAction)unwindToRegion:(UIStoryboardSegue *)segue

{  }

is mixed up with prepareForSegue.

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender

{
    ....
}

   

To fix the error, in prepareForSegue, add the segue identifier checking. Make sure prepareForSegue is only processing the forward segue (In our case, the UserShowSegueCountry).

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender

{

    

    if ([[segue identifier] isEqualToString:@"UserShowSegueCountry"])

    {

    UINavigationController *nav = segue.destinationViewController;

    XYZCountryTableViewController *vcToPushTo = (XYZCountryTableViewController *)nav.topViewController;

        

    }

    

}

IP address conflict and its detection

We have an IP network, with devices getting their IP address from DHCP server. A problem could arise when a device is off the network, then reconnect to the network, and its IP address is the same as other device which has just joined the network. This could be due to DHCP server thinks that the DHCP lease has expired, but the device doesn't. This is a problem of IP address conflict.

To detect IP address conflict, we can use two methods

1) ARP probe: It is an ARP request. The sending device fills the source MAC address with the hardware address of its interface, and the source IP address must be all ZEROes. The destination MAC address should be all ZEROes. The destination IP address is the address being probe. If the sending device receives any ARP on the interface where probe is being performed, where the packet's source IP is the IP being probed for, then this IP is used by other host. So it is an IP conflict.

2) Gratuitous ARP: It could mean both gratuitous ARP request or gratuitous ARP reply. Gratuitous in this case means a request/reply is not required, but could be used in some cases. In a gratuitous ARP request,  the source and destination IP are both set to the IP of the device issuing the packet, and the destination MAC is the broadcast address ff:ff:ff:ff:ff:ff. Ordinarily, no reply packet will occur. A gratuitous ARP reply is a reply to which no request has been made. When a device receives an ARP request containing a source IP that matches its own, then it knows there is an IP conflict.

Java Serializer

In Computer Science, serialisation is a process of translating data structures or objects into a byte stream that can be stored (in a file or memory buffer), or can be transmitted across a network link, and the byte stream can be re-read and re-constructed into the same clone.  Serialisation is a way to prevent the byte ordering (endianness), memory layout problem.

In Java, the java.io.serializable implement the serialisation. You can write a Java class to implement the Serializable interface.

class IsSerializable implements Serializable  

{  

    private String plug = "Plugin to Java!";  

    public IsSerializable()  

    {  

        try  

        {  

            FileOutputStream out = new FileOutputStream("/mnt/sdcard/yes-out.txt");  

            ObjectOutputStream oos = new ObjectOutputStream(out);  

            oos.writeObject(this);  

        }  

        catch (Exception e)  

        {  

            e.printStackTrace();  

        }  

    }  

}  

@Override

protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        IsSerializable is = new IsSerializable();  

        ...

}

If you run this code, there is an error message:

java.io.NotSerializableException: com.example.xxxxx.MainActivity.

The reason is that you can't get the whole class to be serialised.

Solution:
If we replace oos.writeObject(this); with oos.writeObject(plug);  the problem is fixed.