Question

I just begun with a simple thread handler program on Android. The code works. The only part is on how to shut down the handler. I am trying in vain to close the handler onPause() but it simply keeps running. Any suggestions?

Here is the code:
My Main Activity thread:

package com.test.prog;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.widget.TextView;

public class AndroidThreadProgv1Activity extends Activity {

    private Handler DataHandler;

    private DataUpdater du;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);       
        setContentView(R.layout.main);

        /* Initialize handler to allow UI updating in other threads */
        DataHandler = new Handler();

        startDataUpdater();

    }

    public void onPause(){
        super.onPause();
        du.stopConnection();
    }

    private void startDataUpdater(){
        System.out.println("In startDataUpdater");
        du = new DataUpdater(DataHandler,(TextView) findViewById(R.id.textView1));
        du.start();
    }

}

DataUpdater.java:

package com.test.prog;

import java.io.IOException;
import android.os.Handler;
import android.os.Looper;
import android.widget.TextView;

/*
 * Data updater thread.  Updates the display with the response.
 */
public class DataUpdater extends Thread{

    /*Handler for the UI update*/
    private Handler dataRefreshHandler;

    /*The UI text displaying the data*/
    private TextView dataLabel;

    /*data value*/
    private int data = 0;
    /*data update rate in ms*/
    private static final int dataUpdateRate = 2000;

    private boolean stoppedFlag= false;

    public DataUpdater(Handler handler, TextView tv){
        this.dataRefreshHandler = handler;
        this.dataLabel = tv;
    }


    public void stopConnection()
    {
        if(this.dataRefreshHandler != null) {
            this.dataRefreshHandler.removeMessages(data); // Remove a delayed message  
            System.out.println("In stopConnection"); 
            /*
           if(!stoppedFlag){
                this.dataRefreshHandler.getLooper().quit(); // Warning
                stoppedFlag = true;
            }
            */
        }
    }


    @Override
    public void run(){
        try {
            while(true){
                System.out.println("In run");
                data = data + 1;        
                updateData();               
                Thread.sleep(dataUpdateRate);
            }

        } 
        catch (InterruptedException e) {
            e.printStackTrace();
        }
    }       
    /*
     * Calls the UI Handler and updates the display
     */
    private void updateData(){
        dataRefreshHandler.post(new Runnable() {
            public void run() {
                if(data==0)
                {
                    dataLabel.setText("Value not known");
                }
                else
                {
                    System.out.println("In update");
                    dataLabel.setText("Value="+data);

                }
            }
        });
    }
}

Answer 1

Your “run” method in dataUpdater has a while(true) loop. You never check for any exit condition so it will never stop. What you could do is include a volatile flag in DataUpdater (initially set to true) and have stopConnection set it to false. Your while loop in run can check that flag instead of just looping infinitely.

To make sure your DataUpdater is running whenever your activity is running:

Move the call to startDataUpdater(); from onCreate to onResume (Android always calls onResume after onCreate so this will ensure that you are always calling startDataUpdater on creation as well as on resuming after pause).

Change your run method in DataUpdater to look like this:

public void run(){
   isRunning = true; // isRunning should be declared as a "private volatile boolean"
    try {
        while(isRunning){
            System.out.println("In run");
            data = data + 1;        
            updateData();               
            Thread.sleep(dataUpdateRate);
        }
    }