private void updatePointersOnTap(MotionEvent event) {
    mPointerCount = 0;
    for (int i = 0; i < MAX_POINTERS; i++) {
      int index = getPressedPointerIndex(event, i);
      if (index == -1) {
        mId[i] = MotionEvent.INVALID_POINTER_ID;
      } else {
        mId[i] = event.getPointerId(index);
        mCurrentX[i] = mStartX[i] = event.getX(index);
        mCurrentY[i] = mStartY[i] = event.getY(index);
        mPointerCount++;
      }
    }
  }
Gets the number of pressed pointers (fingers down).
samples/gestures/src/main/java/com/facebook/samples/gestures/MultiPointerGestureDetector.java