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当我使用一组无状态小部件制作 ListView 时，我没有收到任何错误，一切正常。但是，当我将有状态的小部件放入数组中并通过滚动它来重新渲染小部件使其在视图之外结束时，我会收到错误消息。

我在 ListView 和 ListView.builder 上试过了，似乎没有什么区别。我认为问题出在文本上，所以我尝试删除所有 Text 构造函数，但没有区别。小部件会初始化和处理，但不会重新初始化。

class CustomSettingsScreen extends StatefulWidget {
    static const String routeName = "/settings";

    @override
    CustomSettingsScreenState createState() =>CustomSettingsScreenState();
}

class CustomSettingsScreenState extends State<CustomSettingsScreen> {
    @override
    void initState() {
        super.initState();
    }

final listElements = {
    Divider(height: 500),
    VibrateStartRecordingEnablePreference(
        preferenceKey: PreferenceKeys.keyVibrateStartRecordingEnable,
    ),
    Divider(height: 500),
    Divider(height: 500),
    Divider(height: 500),
    Divider(height: 500),
    };

};

@override
Widget build(BuildContext context) {
    return new Scaffold(
        appBar: new AppBar(
            title: new Text("Settings"),
        ),
        body: ListView.builder(
        itemCount: listElements.length,
        itemBuilder: (BuildContext context, int index) {
            return listElements.elementAt(index);
        },
    ),);
    }
}

class VibrateStartRecordingEnablePreference extends StatefulWidget {
    VibrateStartRecordingEnablePreference({
    Key key,
    @required this.preferenceKey,
});

final preferenceKey;

final _VibrateStartRecordingEnablePreferenceState
  _vibrateStartRecordingEnablePreferenceState =
  new _VibrateStartRecordingEnablePreferenceState();

@override
_VibrateStartRecordingEnablePreferenceState createState() =>
    _vibrateStartRecordingEnablePreferenceState;
}

class _VibrateStartRecordingEnablePreferenceState extends State<VibrateStartRecordingEnablePreference> {
    String _title = "Vibrate at the start of recording";
    String _subtitleOn = "Vibrates once upon start of recording";
    String _subtitleOff = "Will not vibrate at start";
    bool _value = true;

String _getSubtitle() {
    if (_value)
        return _subtitleOn;
    else
      return _subtitleOff;
}

void _updateValue(bool value) {
    Settings().save(widget.preferenceKey, value);
    setState(() {
      _value = value;
    });
}

@override
void initState() {
    //Update the values and text
    Settings().getBool(widget.preferenceKey, true).then((value) {
        setState(() {
        _value = value;
      });
    });

    super.initState();
}

@override
Widget build(BuildContext context) {
    return new SwitchListTile(
        secondary: const Icon(Icons.vibration),
        title: Text("$_title"),
        subtitle: Text(_getSubtitle()),
        value: _value,
        onChanged: (value) {
            _updateValue(value);
        },
    );
    }
}

调试器中的错误

The following assertion was thrown building         NotificationListener<KeepAliveNotification>:     'package:flutter/src/widgets/framework.dart': Failed assertion: line 4006 pos         
12: '_state._widget == null': is not true.

这是正在发生的事情的视频。 https://webm.red/6ZTH

4

1 回答 1

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_vibrateStartRecordingEnablePreferenceState存储在VibrateStartRecordingEnablePreference里面是错误的。框架尝试将状态实例附加到小部件，但状态已经附加到一个。所以会导致异常。

每次createState()在 StatefulWidget 中调用时，您都必须返回一个新的状态实例

于 2019-07-09T09:53:45.830 回答