Flutter，从凸起按钮的 onPressed 调用 FutureBuilder 不会调用 builder 属性

Question

我正在尝试学习 Dart/Flutter，并且正在研究一个示例，其中应用程序上有一个显示“获取数据”的按钮，当我触摸它时，我想从一个安静的服务中检索 JSON 数据。

我看到在 fetchPost 中调用了 Web 服务，但未调用 FutureBuilder 的 builder 属性。

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'ResultsList.dart';
import 'dart:convert';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Restul Test',
      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>[
            RaisedButton(
              onPressed: (){
                FutureBuilder<ResultsList>(
                  future: fetchPost(),
                  builder: (context, snapshot){
                    print('In Builder');
                  }
                );
              },
              child: Text('Get data'),
            )
          ],
        ),
      )
    );
  }
}

Future<ResultsList> fetchPost() async {
    final response =  await http.get('http://mywebserviceurl');

    if (response.statusCode == 200){
      print('Received data');
      return ResultsList.fromJson(json.decode(response.body));
    }
    else {
      throw Exception('Failed to load data');
    }
}

但有趣的是，如果我将 FutureBuilder 从按钮的 onPressed 移到 Center 的子级，我确实看到 builder 属性被调用。

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'ResultsList.dart';
import 'dart:convert';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Restul Test',
      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: FutureBuilder<ResultsList>(
                future: fetchPost(),
                builder: (context, snapshot){
                  print ('In Builder');
                  return Container();
                }
              )
      )
    );
  }
}

Future<ResultsList> fetchPost() async {
    final response =  await http.get('http://mywebserviceurl');

    if (response.statusCode == 200){
      print('Received data');
      return ResultsList.fromJson(json.decode(response.body));
    }
    else {
      throw Exception('Failed to load data');
    }
}

显然我错过了什么，但知道我做错了什么吗？

Answer 1

如果您想从请求中获取一些数据 - 您不需要 FutureBuilder。你可以这样做：

RaisedButton(
          onPressed: (){
            fetchPost().then((result) {
                print('In Builder');
            })
          },
          child: Text('Get data'),
        )

或

RaisedButton(
          onPressed: () async {
            var result = await fetchPost()
            print('In Builder');
          },
          child: Text('Get data'),
        )

Answer 2

这个RaisedButton 中的onPressed 方法实际上没有做任何事情。它只是创建了一个新的FutureBuilder，除了现有的之外什么都不做^^这就像你只是调用1+1;，它只是创建一个值，但该值不用于做任何事情。

RaisedButton(
  onPressed: (){
    FutureBuilder<ResultsList>(
      future: fetchPost(),
      builder: (context, snapshot){
        print('In Builder');
      }
    );
  },
  child: Text('Get data'),
)

您可以将body 分配给Widget（可以直接称为body 或您想要的任何名称^^），然后您可以在setState((){body = FutureBuilder(/*...*/}); 调用中对其进行更改。

Answer 3

对我来说，FutureBuilder 不能在 onPresses 中工作...... 我是这样用的：

我在 state 中定义了一个变量：

    bool visiblity = false;

我在构建中使用了这段代码：

  @override
  Widget build(BuildContext context) {
    return Scaffold(
     appBar: AppBar(
      title: Text(widget.title),
    ),
    body: Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          RaisedButton(
            onPressed: () {
              visiblity=true;
              fetchPost();
            },
            child: Text('Get data'),
          ),
          FutureBuilder<ResultsList>(
              future: ("Your View Model that return from call back"),
              builder: (context, snapshot) {
                if (visiblity) {
                  print('In Builder');
                  visiblity=false;
                } else
                  return Container();
              }
          ),
        ],
      ),
    )
   );
  }

我没有将 FutureBuilder 放入 onPressed。我把它放在正文中，并在返回结果后改变了可见性。