Flutter：使用复选框进行下拉多选

本文向您展示了在 Flutter 中使用复选框实现下拉多选的两种不同方法。在第一种方法中，我们将从头开始构建多选。在第二种方法中，我们将使用第三方包快速完成工作。

从头开始使用复选框创建多选

应用预览

我们将要构建的应用程序有一个专业的、功能齐全的多选小部件，可显示选项列表。可以通过选中/取消选中与其关联的复选框来选择或取消选择一个选项。

当按下升高的按钮时，将显示多选对话框。它让用户在编程中选择他们最喜欢的主题。所选主题将作为筹码显示在屏幕上。

以下是我们的应用程序的运行方式：

构建自定义多选小部件

创建一个名为MultiSelect的可重用小部件，它可以获取选项列表（您可以对这些项目进行硬编码或从数据库/API 中获取它们）：

// Multi Select widget
// This widget is reusable
class MultiSelect extends StatefulWidget {
  final List<String> items;
  const MultiSelect({Key? key, required this.items}) : super(key: key);

  @override
  State<StatefulWidget> createState() => _MultiSelectState();
}

class _MultiSelectState extends State<MultiSelect> {
  // this variable holds the selected items
  final List<String> _selectedItems = [];

// This function is triggered when a checkbox is checked or unchecked
  void _itemChange(String itemValue, bool isSelected) {
    setState(() {
      if (isSelected) {
        _selectedItems.add(itemValue);
      } else {
        _selectedItems.remove(itemValue);
      }
    });
  }

  // this function is called when the Cancel button is pressed
  void _cancel() {
    Navigator.pop(context);
  }

// this function is called when the Submit button is tapped
  void _submit() {
    Navigator.pop(context, _selectedItems);
  }

  @override
  Widget build(BuildContext context) {
    return AlertDialog(
      title: const Text('Select Topics'),
      content: SingleChildScrollView(
        child: ListBody(
          children: widget.items
              .map((item) => CheckboxListTile(
                    value: _selectedItems.contains(item),
                    title: Text(item),
                    controlAffinity: ListTileControlAffinity.leading,
                    onChanged: (isChecked) => _itemChange(item, isChecked!),
                  ))
              .toList(),
        ),
      ),
      actions: [
        TextButton(
          child: const Text('Cancel'),
          onPressed: _cancel,
        ),
        ElevatedButton(
          child: const Text('Submit'),
          onPressed: _submit,
        ),
      ],
    );
  }
}

完整的代码

这是您在上面的演示中看到的应用程序的代码：

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    const String title = "坚果";
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: title,
      theme: ThemeData(
        primarySwatch: Colors.indigo,
      ),
      home: const HomePage(),
    );
  }
}

// Multi Select widget
// This widget is reusable
class MultiSelect extends StatefulWidget {
  final List<String> items;
  const MultiSelect({Key? key, required this.items}) : super(key: key);

  @override
  State<StatefulWidget> createState() => _MultiSelectState();
}

class _MultiSelectState extends State<MultiSelect> {
  // this variable holds the selected items
  final List<String> _selectedItems = [];

// This function is triggered when a checkbox is checked or unchecked
  void _itemChange(String itemValue, bool isSelected) {
    setState(() {
      if (isSelected) {
        _selectedItems.add(itemValue);
      } else {
        _selectedItems.remove(itemValue);
      }
    });
  }

  // this function is called when the Cancel button is pressed
  void _cancel() {
    Navigator.pop(context);
  }

// this function is called when the Submit button is tapped
  void _submit() {
    Navigator.pop(context, _selectedItems);
  }

  @override
  Widget build(BuildContext context) {
    return AlertDialog(
      title: const Text('Select Topics'),
      content: SingleChildScrollView(
        child: ListBody(
          children: widget.items
              .map((item) => CheckboxListTile(
                    value: _selectedItems.contains(item),
                    title: Text(item),
                    controlAffinity: ListTileControlAffinity.leading,
                    onChanged: (isChecked) => _itemChange(item, isChecked!),
                  ))
              .toList(),
        ),
      ),
      actions: [
        TextButton(
          child: const Text('取消'),
          onPressed: _cancel,
        ),
        ElevatedButton(
          child: const Text('确定'),
          onPressed: _submit,
        ),
      ],
    );
  }
}

// Implement a multi select on the Home screen
class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

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

class _HomePageState extends State<HomePage> {
  List<String> _selectedItems = [];
  String _title = "坚果";
  void _showMultiSelect() async {
    // a list of selectable items
    // these items can be hard-coded or dynamically fetched from a database/API

    final List<String> _items = [
      'Flutter',
      'Node.js',
      'React Native',
      'Java',
      'Docker',
      'MySQL'
    ];

    final List<String>? results = await showDialog(
      context: context,
      builder: (BuildContext context) {
        return MultiSelect(items: _items);
      },
    );

    // Update UI
    if (results != null) {
      setState(() {
        _selectedItems = results;
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(_title),
      ),
      body: Padding(
        padding: const EdgeInsets.all(20),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            // use this button to open the multi-select dialog
            ElevatedButton(
              child: const Text('选出你最喜欢的技术'),
              onPressed: _showMultiSelect,
            ),
            const Divider(
              height: 30,
            ),
            // display selected items
            Wrap(
              children: _selectedItems
                  .map((e) => Chip(
                        label: Text(e),
                      ))
                  .toList(),
            )
          ],
        ),
      ),
    );
  }
}