static final RangeThumbSelector _defaultRangeThumbSelector = (
      TextDirection textDirection,
      RangeValues values,
      double tapValue,
      Size thumbSize,
      Size trackSize,
      double dx, // drag displacement
    ) {
    final double touchRadius = math.max(thumbSize.width, RangeSlider._minTouchTargetWidth) / 2;

    final bool inStartTouchTarget = (tapValue - values.start).abs() * trackSize.width < touchRadius;

    final bool inEndTouchTarget = (tapValue - values.end).abs() * trackSize.width < touchRadius;


    if (inStartTouchTarget && inEndTouchTarget) {
      bool towardsStart;
      bool towardsEnd;
      switch (textDirection) {
        case TextDirection.ltr:
          towardsStart = dx < 0;
          towardsEnd = dx > 0;
          break;
        case TextDirection.rtl:
          towardsStart = dx > 0;
          towardsEnd = dx < 0;
          break;
      }
      if (towardsStart)
        return Thumb.start;
      if (towardsEnd)
        return Thumb.end;
    } else {
      if (tapValue < values.start || inStartTouchTarget)
        return Thumb.start;
      if (tapValue > values.end || inEndTouchTarget)
        return Thumb.end;
    }
    return null;
  };