Question

From my observation the android CountDownTimer countDownInterval between ticks happens to be not accurate, the countDownInterval is regularly a few milliseconds longer than specified. The countDownInterval in my specific app is 1000ms, just counting down a certain amount of time with one second steps.

Due to this prolonged ticks I end up having less ticks then wanted when the the countdowntimer runs long enough which screws up the displayed countdown of the time (a 2 second step happens on the UI level when enough additional ms have summed up)

Looking into the source of CountDownTimer it seems possible to twist it so it corrects this unwanted inaccuracy yet I was wondering if there is already a better CountDownTimer available in the java/android world.

Thanks buddies for any pointer …

Answer 1

Rewrite

As you said, you also noticed that the next time in onTick() is calculated from the time the previous onTick() ran, which introduces a tiny error on every tick. I changed the CountDownTimer source code to call each onTick() at the specified intervals from the start time.

I build this upon the CountDownTimer framework, so cut & paste the source code into your project and give the class a unique name. (I called mine MoreAccurateTimer.) Now make a few changes:

1. Add a new class variable:

   private long mNextTime;
   

2. Change start():

   public synchronized final MoreAccurateTimer start() {
       if (mMillisInFuture <= 0) {
           onFinish();
           return this;
       }
   
       mNextTime = SystemClock.uptimeMillis();
       mStopTimeInFuture = mNextTime + mMillisInFuture;
   
       mNextTime += mCountdownInterval;
       mHandler.sendMessageAtTime(mHandler.obtainMessage(MSG), mNextTime);
       return this;
   }
   

3. Change the Handler’s handlerMessage():

   @Override
   public void handleMessage(Message msg) {
       synchronized (MoreAccurateTimer.this) {
           final long millisLeft = mStopTimeInFuture - SystemClock.uptimeMillis();
   
           if (millisLeft <= 0) {
               onFinish();
           } else {
               onTick(millisLeft);
   
               // Calculate next tick by adding the countdown interval from the original start time
               // If user's onTick() took too long, skip the intervals that were already missed
               long currentTime = SystemClock.uptimeMillis();
               do {
                   mNextTime += mCountdownInterval;
               } while (currentTime > mNextTime);
   
               // Make sure this interval doesn't exceed the stop time
               if(mNextTime < mStopTimeInFuture)
                   sendMessageAtTime(obtainMessage(MSG), mNextTime);
               else
                   sendMessageAtTime(obtainMessage(MSG), mStopTimeInFuture);
           }
       }
   }