Question

I have a very simple UI and i need to constantly run a check process, so I am trying to use a Thread with a while loop.
When I run the loop with nothing but a Thread.sleep(1000) command, it works fine, but as soon as I put in a display.setText(), the program runs for a second on the emulator then quits. I cannot even see the error message since it exits so fast.

I then took the display.setText() command outside the thread and just put it directly inside onCreate, and it works fine (so there is no problem with the actual command).

here is my code, and help will be greatly appreciated.
Thank you!

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);



    on=(Button) findViewById(R.id.bon);
    off=(Button) findViewById(R.id.boff);
    display=(TextView) findViewById(R.id.tvdisplay);
    display2=(TextView) findViewById(R.id.tvdisplay2);
    display3=(TextView) findViewById(R.id.tvdisplay3);
    stopper=(Button) findViewById(R.id.stops);


    stopper.setOnClickListener(new View.OnClickListener() {


        @Override

        public void onClick(View v) {
            // TODO Auto-generated method stub
            if(boo=true)
            {
                boo=false;
                display3.setText("System Off");
            }
            else{
                boo=true;
            }
            }
    });





    Thread x = new Thread() {
        public void run() {
            while (boo) {
                 display3.setText("System On");

                try {
                    // do something here
                    //display3.setText("System On");

                    Log.d(TAG, "local Thread sleeping");
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    Log.e(TAG, "local Thread error", e);
                }

            }
        }
    };


    display3.setText("System On");
    display3.setText("System On");


    x.start();
}

Answer 1

You can’t update the UI from a non-UI thread. Use a Handler. Something like this could work:

// inside onCreate:
final Handler handler = new Handler();
final Runnable updater = new Runnable() {
    public void run() {
        display3.setText("System On");
    }
};

Thread x = new Thread() {
    public void run() {
        while (boo) {
            handler.invokeLater(updater);

            try {
                // do something here
                //display3.setText("System On");

                Log.d(TAG, "local Thread sleeping");
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                Log.e(TAG, "local Thread error", e);
            }

        }
    }
};

You could also avoid a Handler for this simple case and just use

        while (boo) {
            runOnUiThread(updater);
            // ...

Alternatively, you could use an AsyncTask instead of your own Thread class and override the onProgressUpdate method.