flutter - Flutter:句柄值不存在于筛选器ListViewBuilder中

Question

我尝试从link在ListviewBuilder引用中进行简单搜索。问题是如果搜索值为null或不存在，我将无法处理。

源代码

class _TestingFilterState extends State<TestingFilter> {
  TextEditingController searchController = TextEditingController();
  final List<String> _listViewData = [
    "Inducesmile.com",
    "Flutter Dev",
    "Android Dev",
    "iOS Dev!",
    "React Native Dev!",
    "React Dev!",
    "express Dev!",
    "Laravel Dev!",
    "Angular Dev!",
  ];
  List<String> _newData = [];
  @override
  void dispose() {
    searchController.dispose();
    super.dispose();
  }

  _onChanged(String value) {
    setState(() {
      _newData = _listViewData
          .where((string) => string.toLowerCase().contains(value.toLowerCase()))
          .toList();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        Padding(padding: const EdgeInsets.only(top: 20.0)),
        TextField(controller: searchController, onChanged: _onChanged),
        ListView.builder(
          itemCount: _newData.length,
          shrinkWrap: true,
          itemBuilder: (BuildContext context, int index) {
            if (_newData == null || _newData.length == 0) {
              return Text('Your Search Is Null');
            } else {
              return Text(_newData[index]);
            }
          },
        ),
      ],
    );
  }
}

我敢肯定已经用此代码处理了这种情况，但是我看不到 Your Search Is Null出现在屏幕上。我做错了吗？

if (_newData == null || _newData.length == 0) {
              return Text('Your Search Is Null');
            } else {
              return Text(_newData[index]);
            }

Answer 1

您可以在下面复制粘贴运行完整代码
您可以将条件上移

(_newData == null || _newData.length == 0) ? Text('Your Search Is Null') : ListView.builder(

工作演示



程式码片段

return Column(
      children: <Widget>[
        Padding(padding: const EdgeInsets.only(top: 20.0)),
        TextField(controller: searchController, onChanged: _onChanged),
        (_newData == null || _newData.length == 0)
            ? Text('Your Search Is Null')
            : ListView.builder(
                itemCount: _newData.length,
                shrinkWrap: true,
                itemBuilder: (BuildContext context, int index) {
                  return Text(_newData[index]);
                },
              ),
      ],
    );

完整的代码

import 'package:flutter/material.dart';

class TestingFilter extends StatefulWidget {
  @override
  _TestingFilterState createState() => _TestingFilterState();
}

class _TestingFilterState extends State<TestingFilter> {
  TextEditingController searchController = TextEditingController();
  final List<String> _listViewData = [
    "Inducesmile.com",
    "Flutter Dev",
    "Android Dev",
    "iOS Dev!",
    "React Native Dev!",
    "React Dev!",
    "express Dev!",
    "Laravel Dev!",
    "Angular Dev!",
  ];
  List<String> _newData = [];
  @override
  void dispose() {
    searchController.dispose();
    super.dispose();
  }

  _onChanged(String value) {
    setState(() {
      _newData = _listViewData
          .where((string) => string.toLowerCase().contains(value.toLowerCase()))
          .toList();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        Padding(padding: const EdgeInsets.only(top: 20.0)),
        TextField(controller: searchController, onChanged: _onChanged),
        (_newData == null || _newData.length == 0)
            ? Text('Your Search Is Null')
            : ListView.builder(
                itemCount: _newData.length,
                shrinkWrap: true,
                itemBuilder: (BuildContext context, int index) {
                  return Text(_newData[index]);
                },
              ),
      ],
    );
  }
}

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Expanded(child: TestingFilter()),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}