量规图表中的最后一段在水龙头回调中引发异常

Question

使用gauge.dart文件from the example here，我刚刚添加了一个changedListener回调来获取用户单击了哪个段。

仅对4个分段中的3个按预期方式起作用，但是如果单击最后一个分段，则在其区域末尾触发时会触发异常（取决于示例中给出的随机值，如果相对小段，然后整个段会给出相同的错误）。

static List<charts.Series<GaugeSegment, String>> _createRandomData() {
  final random = new Random();

  final data = [
    new GaugeSegment('Low', random.nextInt(100)),
    new GaugeSegment('Acceptable', random.nextInt(100)),
    new GaugeSegment('High', random.nextInt(100)),
    new GaugeSegment('Highly Unusual', random.nextInt(100)),
  ];

  return [
    new charts.Series<GaugeSegment, String>(
      id: 'Segments',
      domainFn: (GaugeSegment segment, _) => segment.segment,
      measureFn: (GaugeSegment segment, _) => segment.size,
      data: data,
    )
  ];
}

@override
Widget build(BuildContext context) {
  return new charts.PieChart(seriesList,
      animate: animate,

      // added selectionModels to pass in _onSelectionChanged callback
      selectionModels: [
        new charts.SelectionModelConfig(
          type: charts.SelectionModelType.info,
          changedListener: _onSelectionChanged,
        )
      ],
      defaultRenderer: new charts.ArcRendererConfig(
          arcWidth: 30, startAngle: 4 / 5 * pi, arcLength: 7 / 5 * pi));
}

_onSelectionChanged(charts.SelectionModel model) {
  final selectedDatum = model.selectedDatum;
  print("DATUM: ${selectedDatum.first.datum}");

  if (selectedDatum.isNotEmpty) {
    print('IS_NOT_EMPTY');

  }
}

会给这个例外

The following StateError was thrown while handling a gesture:
Bad state: No element

When the exception was thrown, this was the stack
#0      List.first  (dart:core-patch/array.dart:202:5)
#1      GaugeChart._onSelectionChanged 
package:example/pie_chart/gauge.dart:89
#2      MutableSelectionModel.updateSelection.<anonymous closure> 
package:charts_common/…/selection_model/selection_model.dart:178
#3      List.forEach  (dart:core-patch/growable_array.dart:283:8)
#4      MutableSelectionModel.updateSelection 
package:charts_common/…/selection_model/selection_model.dart:178
...
Handler: "onTapUp"
Recognizer: TapGestureRecognizer#1d1a0
    debugOwner: GestureDetector
    state: possible
    won arena
    finalPosition: Offset(286.5, 261.0)
    finalLocalPosition: Offset(278.5, 153.0)
    sent tap down