Thread and Handler

The graphic interface cannot be modified by another thread, so when you use a different thread to make calculations or another task and you want to show the result in the activity, you will need a handler.

import android.os.Handler;
import android.os.Message;
import java.lang.ref.WeakReference;

public class TimeHandler extends Handler {
    private final WeakReference<MainActivity> mainActivityWeakReference;


public TimeHandler(MainActivity mainActivity){
        mainActivityWeakReference= new WeakReference<MainActivity>(mainActivity);
    }
@Override    
 public void handleMessage(Message ms){


MainActivity activity=mainActivityWeakReference.get();
        if (activity!=null){
            activity.showElapsedTime();//this is the function in your activity which updates.        }
    }
}

you need to declare a handler variable

public static TimeHandler handler;
and initiate it inside the activity which receives the messages.


Val.handler=new TimeHandler(this);

In this case I have declared handler as static inside a class named Val. 
(I store here some values)
Then, when your processing thread needs to change the call it from a thread

public class TimeThread extends Thread {
    public boolean running;
    public void TimeTread(){
    }
    public void run(){
        Log.i("main","thread begins");
        while (running){
            try {
                sleep(200);
                Val.handler.sendEmptyMessage(0);
            }
            catch (Exception e){}
        }
        Log.i("main","thread ends");
    }
}

here I use an empty message
you can send a more complex message via the Message class


        Val.handler.sendMessage(m);


to make the message see this.

To use the thread:

declare it

TimeThread myThread;

Initialize it and start runnig

myThread=new TimeThread();
myThread.running = true;
myThread.start();
"running" is a boolean variable to loop the activity of the thread.




And when the task is completed, stop the thread:

if (myThread.isAlive()){
    boolean repeat=false;
    myThread.running=false;
    while (repeat){
        try {
            myThread.join();
            repeat=false;
        }
        catch (Exception e){}
    }
}