Why do I get a "Can't create handler inside a thread that has not called Looper.prepare()" with android.os.CountDownTimer?

Question

When using this class I get this strange exception. It is used to print out a nice timer display for my game, and is created when my main GameView class is created. The error gets thrown at line 26: super(s*1000,1000);

package tommedley.android.game;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.drawable.Drawable;
import android.os.CountDownTimer;

public class Timer extends CountDownTimer{
    public static int MODE_COUNTING = 0;
    public static int MODE_PAUSED = 1;
    public static int MODE_FINISHED = 2;

    private Canvas canvas;
    private Context context;
    private float xPos;
    private float yPos;
    private static int DIGIT_WIDTH = 50;
    private static int DIGIT_HEIGHT = 70;
    private int numDigits = 3;
    private int seconds;
    Drawable[] digitImgs;
    private int mode;


    public Timer(Canvas c, Context con, float x, float y, int digits, int s){
        super(s*1000, 1000);
        mode = MODE_COUNTING;
        canvas = c;
        context = con;
        xPos = x;
        yPos = y;
        seconds = s;
        numDigits = digits;
        digitImgs = new Drawable[numDigits];
        this.start();
    }
    public void draw(){
        String reprNum = String.format("%0"+numDigits+"d", seconds);
        for(int i = 0;i<numDigits;i++){
            switch(reprNum.charAt(i)){
            case '0':
                digitImgs[i] = context.getResources().getDrawable(R.drawable.digit_0);
                break;
            case '1':
                digitImgs[i] = context.getResources().getDrawable(R.drawable.digit_1);
                break;
            case '2':
                digitImgs[i] = context.getResources().getDrawable(R.drawable.digit_2);
                break;
            case '3':
                digitImgs[i] = context.getResources().getDrawable(R.drawable.digit_3);
                break;
            case '4':
                digitImgs[i] = context.getResources().getDrawable(R.drawable.digit_4);
                break;
            case '5':
                digitImgs[i] = context.getResources().getDrawable(R.drawable.digit_5);
                break;
            case '6':
                digitImgs[i] = context.getResources().getDrawable(R.drawable.digit_6);
                break;
            case '7':
                digitImgs[i] = context.getResources().getDrawable(R.drawable.digit_7);
                break;
            case '8':
                digitImgs[i] = context.getResources().getDrawable(R.drawable.digit_8);
                break;
            case '9':
                digitImgs[i] = context.getResources().getDrawable(R.drawable.digit_9);
                break;
            }
            digitImgs[i].setBounds((int)xPos+DIGIT_WIDTH*i, (int)yPos, (int)xPos+DIGIT_WIDTH*(i+1), (int)yPos+DIGIT_HEIGHT);
            digitImgs[i].draw(canvas);
        }
    }
    @Override
    public void onFinish() {
        seconds = 0;
        mode = MODE_FINISHED;
    }
    @Override
    public void onTick(long millsLeft) {
        seconds = (int)millsLeft / 1000;
    }
    public int getMode(){
        return mode;
    }
}

Answer 1

That is a bit strange -- I think you must be constructing this object on a thread other than the main thread?

I don't know if you're familiar with Handlers and Loopers, but they are Android's way of doing asynchronous operations on a thread. If you want to execute asynchronous operations on a thread, you use a Handler to do it. But before you can use a Handler you must prep the thread by creating a MessageQueue, which is done by calling Looper.prepare().

If you look at the source for the CountDownTimer class you'll notice that it creates a private Handler instance. This member variable is created directly before the constructor executes, which is why the exception is being thrown at the call to super.

So, all you need to do is call Looper.prepare() before you construct the Timer. However, you do not need to do this if you construct the Timer on the main thread. The main thread of an Android process is automatically declared to be a looper (if you are already calling this on the main thread and are still getting this exception, then that is very strange indeed). As far as I can tell from the sparse javadocs, this class was meant to be used on the main thread, so that's what I would recommend doing. Your onTick method seems light enough that it's not worth the overhead of an extra thread.