Question

I have this code:

package org.example.Threading;


import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class ThreadingActivity extends Activity implements OnClickListener {
    /** Called when the activity is first created. */

    private doinback background = new doinback();
    Handler handle= new Handler(){
    @Override
    public void handleMessage(Message msg) {
        Log.e("INSIDE HANDLER", "About to stop the thread.");
        background.stopThread();
        }};
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);


        Log.e("ONCREATE", "BEFORE Thread");
        background.start();
        Log.e("ONCREATE", "AFTER Thread started");

        View button = (Button)findViewById(R.id.button1);
        button.setOnClickListener(this);
    }

    class doinback extends Thread{
        private volatile boolean isRunning = false;
        public void run() {
            while(isRunning){
                Log.e("INSIDE THREAD", "about to sleep");
                handle.sendMessage(handle.obtainMessage());
            }

        }
        public void stopThread()
        {
            isRunning = false;
        }
        public void startThread(){
            Log.e("INSIDE StartThread","MAKING isRunning True");
            isRunning = true;
        }
        }
    public void onClick(View v) {
        background.startThread();
        Log.e("INSIDE ONCLICK", "made isRunning true, about to leave");

    }
}

I am trying to understand why it does not go back to the thread and if there is something I am doing wrong and if so what is the best way to go back and forth from a thread, for instance with the stopThread() function I change the value of the while variable to stop the thread from running. When I run the code and press the button it does not start the thread. Any ideas why?

Answer 1

When background.start() is called, isRunning is still false, so the thread is started(but didn’t enter the while loop) and stopped immediately.

To ensure the thread to enter the while loop when startThread is called, you can put the following code in the startThread() method:

public void startThread(){
    isRunning = true;
    start();
}

And you don’t call background.start() in onCreate(), just remove that line.

EDIT

If you want to keep a single thread alive, so you can “start” it and “stop” it as many times as you like, you can use the following code:

public void run () {
    while (isRunning) {
        if(!isWorking) {
            try { Thread.sleep(10); } catch (InterruptedException ex) { }
            continue;
        }
        //.. do your work here.
    }
}

You control the running state of the thread with isRunning, and the working state with isWorking.