onCompute method

@override void onCompute(Size size)

获取画布宽度和高度 计算当前屏内相对转换后的X轴位置 根据X轴位置，计算屏内起始和结束展示数据的索引 计算最大值和最小值

Implementation

@override
void onCompute(Size size) {
  super.onCompute(size);
  if (getValues().isNotEmpty) {
    List<double> originLows = values.map((e) => e.low).toList();
    List<double> lows = originLows.sublist(startIndex, endIndex);
    double lowsMin = lows.reduce(math.min);
    double lowsMax = lows.reduce(math.max);
    if (lowsMax > maxValue) {
      maxValue = lowsMax;
      _drawMaxIndex = originLows.indexOf(maxValue);
    } else {
      _drawMaxIndex = getValues().indexOf(maxValue);
    }
    if (lowsMin < minValue) {
      minValue = lowsMin;
      _drawMinIndex = originLows.indexOf(minValue);
    } else {
      _drawMinIndex = getValues().indexOf(minValue);
    }
    _drawMaxValue = maxValue;
    _drawMinValue = minValue;

    double abs = (maxValue - minValue).abs() * 1.2;
    double mx = minValue + abs;
    double mi = maxValue - abs;
    maxValue = mx;
    minValue = mi;
    _radio = contentHeight / (maxValue - minValue);
  }
}