flutter - 字符串未使用 changeNotifierProvider 更新

Question

模型类：

Venue with ChangeNotifier{
String id;
update(String venueId) {
    id = venueId;
    notifyListeners();
  }
}

尝试像这样获取 id 值：

Widget _buildContents(BuildContext context) {
    final venue = Provider.of<Venue>(context);
    final id = venue.id;
    print('@@@@@@@@@@');  //// trying to get the the id here.
    
  }

我的 id 应该在 dropDownButton 的 onTap 回调中更新：

DropdownButton(
....

items: venues.map((venue) {
                return DropdownMenuItem<String>(
                  onTap: () {
                    venue.update(venue.id);
                  },
                  value: venue.name,
                  child: Text(
                    venue.name,
                    style: TextStyle(fontSize: 28),
                  ),
                );
              }).toList(),
.....
)

Provider.. 位于小部件上方：

MultiProvider(providers: [
 ChangeNotifierProvider<Venue>(create: (_) => Venue()),
              ], child: HomePage());

出事了..救命！？

标签： flutter

Answer 1

您可以在下面复制粘贴运行完整代码您可以在 代码片段
中使用DropdownButton<Venue>和调用venue.updateonChanged

DropdownButton<Venue>(
      value: dropdownValue,
      icon: Icon(Icons.arrow_downward),
      iconSize: 24,
      elevation: 16,
      style: TextStyle(color: Colors.deepPurple),
      underline: Container(
        height: 2,
        color: Colors.deepPurpleAccent,
      ),
      onChanged: (Venue newValue) {
        setState(() {
          dropdownValue = newValue;
          venue.update(newValue.id);
        });
      },
      items: venues.map((venue) {
        return DropdownMenuItem<Venue>(
          value: venue,
          child: Text(
            venue.name,
            style: TextStyle(fontSize: 28),
          ),
        );
      }).toList()

工作演示输出

I/flutter (27988): @@@@@@@@@@ 1
I/flutter (27988): @@@@@@@@@@ 2

工作演示

完整代码

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

class Venue with ChangeNotifier {
  String id;
  String name;

  update(String venueId) {
    id = venueId;
    notifyListeners();
  }

  Venue({this.id, this.name});
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MultiProvider(providers: [
        ChangeNotifierProvider<Venue>(create: (_) => Venue()),
      ], child: HomePage()),
    );
  }
}

class HomePage extends StatefulWidget {
  HomePage({Key key}) : super(key: key);

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

class _HomePageState extends State<HomePage> {
  Venue dropdownValue;

  List<Venue> venues = [
    Venue(id: "1", name: "name1"),
    Venue(id: "2", name: "name2"),
    Venue(id: "3", name: "name3"),
  ];

  @override
  Widget build(BuildContext context) {
    final venue = Provider.of<Venue>(context);
    final id = venue.id;
    print('@@@@@@@@@@ $id');

    return Scaffold(
      appBar: AppBar(
        title: Text("demo"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            DropdownButton<Venue>(
              value: dropdownValue,
              icon: Icon(Icons.arrow_downward),
              iconSize: 24,
              elevation: 16,
              style: TextStyle(color: Colors.deepPurple),
              underline: Container(
                height: 2,
                color: Colors.deepPurpleAccent,
              ),
              onChanged: (Venue newValue) {
                setState(() {
                  dropdownValue = newValue;
                  venue.update(newValue.id);
                });
              },
              items: venues.map((venue) {
                return DropdownMenuItem<Venue>(
                  value: venue,
                  child: Text(
                    venue.name,
                    style: TextStyle(fontSize: 28),
                  ),
                );
              }).toList(),
            ),
          ],
        ),
      ),
    );
  }
}