How to run a Runnable thread in Android at defined

Question

I developed an application to display some text at defined intervals in the Android emulator screen. I am using the Handler class. Here is a snippet from my code:

handler = new Handler();
Runnable r = new Runnable() {
    public void run() {
        tv.append("Hello World");               
    }
};
handler.postDelayed(r, 1000);

When I run this application the text is displayed only once. Why?

Answer 1

I believe for this typical case, i.e. to run something with a fixed interval, Timer is more appropriate. Here is a simple example:

myTimer = new Timer();
myTimer.schedule(new TimerTask() {          
@Override
public void run() {
    // If you want to modify a view in your Activity
    MyActivity.this.runOnUiThread(new Runnable()
        public void run(){
            tv.append("Hello World");
        });
    }
}, 1000, 1000); // initial delay 1 second, interval 1 second

Using Timer has few advantages:

• Initial delay and the interval can be easily specified in the schedule function arguments
• The timer can be stopped by simply calling myTimer.cancel()
• If you want to have only one thread running, remember to call myTimer.cancel() before scheduling a new one (if myTimer is not null)

Answer 2

now in Kotlin you can run threads this way:

class SimpleRunnable: Runnable {
    public override fun run() {
        println("${Thread.currentThread()} has run.")
    }
}
fun main(args: Array<String>) {
    val thread = SimpleThread()
    thread.start() // Will output: Thread[Thread-0,5,main] has run.
    val runnable = SimpleRunnable()
    val thread1 = Thread(runnable)
    thread1.start() // Will output: Thread[Thread-1,5,main] has run
}

Answer 3

The simple fix to your example is :

handler = new Handler();

final Runnable r = new Runnable() {
    public void run() {
        tv.append("Hello World");
        handler.postDelayed(this, 1000);
    }
};

handler.postDelayed(r, 1000);

Or we can use normal thread for example (with original Runner) :

Thread thread = new Thread() {
    @Override
    public void run() {
        try {
            while(true) {
                sleep(1000);
                handler.post(this);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
};

thread.start();

You may consider your runnable object just as a command that can be sent to the message queue for execution, and handler as just a helper object used to send that command.

More details are here http://developer.android.com/reference/android/os/Handler.html

Answer 4

new Handler().postDelayed(new Runnable() {
    public void run() {
        // do something...              
    }
}, 100);

Answer 5

An interesting example is you can continuously see a counter/stop-watch running in separate thread. Also showing GPS-Location. While main activity User Interface Thread is already there.

Excerpt:

try {    
    cnt++; scnt++;
    now=System.currentTimeMillis();
    r=rand.nextInt(6); r++;    
    loc=lm.getLastKnownLocation(best);    

    if(loc!=null) { 
        lat=loc.getLatitude();
        lng=loc.getLongitude(); 
    }    

    Thread.sleep(100); 
    handler.sendMessage(handler.obtainMessage());
} catch (InterruptedException e) {   
    Toast.makeText(this, "Error="+e.toString(), Toast.LENGTH_LONG).show();
}

To look at code see here:

Thread example displaying GPS Location and Current Time runnable alongside main-activity's User Interface Thread

Answer 6

Kotlin

private lateinit var runnable: Runnable
override fun onCreate(savedInstanceState: Bundle?) {
    val handler = Handler()
    runnable = Runnable {
        handler.postDelayed(runnable, 2000)
    }
    handler.postDelayed(runnable, 2000)
}

Java

Runnable runnable;
Handler handler;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    handler = new Handler();
    runnable = new Runnable() {
        @Override
        public void run() {
            handler.postDelayed(this, 1000);
        }
    };
    handler.postDelayed(runnable, 1000);
}