flutter - 错误：参数类型“Future”不能分配给参数类型“void Function()”

Question

当我将值从主页传递到源页面时，它显示错误：无法将参数类型“Future”分配给参数类型“void Function()”。（argument_type_not_assignable 在 [ strong text ] lib\home.dart:15）

我在哪里做错了？？

主页 -

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

class Home extends StatefulWidget {
  int value;
  Home({this.value});
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: FlatButton(
          onPressed: Navigator.push(context, MaterialPageRoute(builder: (context)=>SourceScreen({value:value}))), child: null,
        ),
      ),
    );
  }
}

下面的页面是我想使用主页值的地方-

import 'package:flutter/material.dart';
import 'dart:convert';
import 'package:http/http.dart' as http;
import 'models/model.dart';
import 'models/card.dart';
import 'article.dart';

final API_KEY = '***';

Future<List<Source>> fetchNewsSource() async {
  final response = await http.get(
      'https://newsapi.org/v2/sources?language=en&country=in&apiKey=$API_KEY');

  if (response.statusCode == 200) {
    List sources = json.decode(response.body)['sources'];
    return sources.map((source) => new Source.formJson(source)).toList();
  } else {
    throw Exception('Fail to load data');
  }
}

class SourceScreen extends StatefulWidget {
  @override
  _SourceScreenState createState() => _SourceScreenState();
}

class _SourceScreenState extends State<SourceScreen> {
  var list_source;
  var refreshKey = GlobalKey<RefreshIndicatorState>();

  @override
  void initState() {
    super.initState();
    refreshListSource();
  }

  Future<Null> refreshListSource() async {
    refreshKey.currentState?.show(atTop: false);
    setState(() {
      list_source = fetchNewsSource();
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        backgroundColor: Color.fromRGBO(58, 66, 86, 1.0),
        appBar: AppBar(
          elevation: 1.0,
          backgroundColor: Color.fromRGBO(58, 66, 86, 1.0),
          title: Text('uTTerNews'),
        ),
        body: Center(
          child: RefreshIndicator(
              child: FutureBuilder<List<Source>>(
                future: list_source,
                builder: (context, snapshot) {
                  if (snapshot.hasError) {
                    Text('Error: ${snapshot.error}');
                  } else if (snapshot.hasData) {
                    List<Source> sources = snapshot.data;
                    return new ListView(
                        children: sources
                            .map((source) =>
                            GestureDetector(
                              onTap: () {
                                Navigator.push(context, MaterialPageRoute(
                                    builder: (context) =>
                                        articleScreen(source: source,)));
                              },
                              child: card(source),
                            ))
                            .toList());
                  }
                  return CircularProgressIndicator();
                },
              ),
              onRefresh: refreshListSource),
        ),
      ),
    );
  }
}

Answer 1

代替

onPressed: Navigator.push(...)

和

onPressed: () => Navigator.push(...)

---

如果你需要使用await关键字，你可以这样做

onPressed: () async {
  await Navigator.push(...);
  await anyOtherMethod();
}

Answer 2

这个在最新版本中对我有用

 onPressed: () {
          selectHandler();
 },

Answer 3

    @CopsOnRoad -- Here is the sample code that is working for me.

*Answer.dart file*
    
    import 'package:flutter/material.dart';
    
    class Answer extends StatelessWidget {
      final Function selectHandler;
    
      // ignore: use_key_in_widget_constructors
      const Answer(this.selectHandler);
    
      @override
      Widget build(BuildContext context) {
        return Container(
          width: double.infinity,
          child: RaisedButton(
            color: Colors.grey,
            textColor: Colors.black,
            child: Text('Answer 1'),
            **onPressed: () => selectHandler()**,
          ),
        );
      }
    }
    
    *Main.dart file snippet*
    
    @override
      Widget build(BuildContext context) {
        var questions = [
          'What\'s your favorite color?',
          'What\'s your favorite animal?'
        ];
        return MaterialApp(
          home: Scaffold(
            appBar: AppBar(
              title: Text(questions[_questionIndex]),
            ),
            body: Column(
              children: <Widget>[
                Question(questions[_questionIndex]),
                **Answer(_answerQuestion)**,
                Answer(_answerQuestion),
                Answer(_answerQuestion),
              ],
            ),
          ),
        );
      }
    }

Answer 4

只需将 final Function selectHandler 更改为 final VoidCallback selectHandler；