@Override protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    super.onSizeChanged(w, h, oldw, oldh);
    halfWidth = w/2;
    int radius = Math.min(halfWidth, h);
    outerRadius = (int) (radius * OUTER_RADIUS_PERCENT);
    innerRadius = outerRadius - blockHeight;
  }

  @Override protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.rotate(2.5f, halfWidth, getHeight());
    for (int i=0; i<BLOCKS; i++) {
      canvas.drawRect(halfWidth - outerRadius, getHeight() - blockWidth,
          halfWidth - innerRadius, getHeight(),
          i > activeBlocks ? blockPaint : activeBlockPaint);
      canvas.rotate(ROTATE_ANGLE, halfWidth, getHeight());
    }
  }

  public void setAngle(float angle) {
    int oldActiveBlocks = activeBlocks;
    activeBlocks = (int) (angle / ROTATE_ANGLE);
    currentAngle = angle;
    if (oldActiveBlocks != activeBlocks) postInvalidate();
  }