Flutter如何通过setState刷新Dialog的内容？

假设您有一个带有一些Widget的Dialog，例如RadioListTile，DropdowButton ......或者当对话框仍然可见时可能需要更新的任何内容，该怎么做？
难度：初学者

背景

最近我必须显示一个Dialog，让用户从列表中选择一个项目，我想显示一个RadioListTile列表。

通过以下源代码显示对话框并显示列表没有问题：

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';

class Sample extends StatefulWidget {
  @override
  _SampleState createState() => new _SampleState();
}

class _SampleState extends State<Sample> {
  List<String> countries = <String>['Belgium','France','Italy','Germany','Spain','Portugal'];
  int _selectedCountryIndex = 0;

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_){_showDialog();});
  }

  _buildList(){
    if (countries.length == 0){
      return new Container();
    }

    return new Column(
      children: new List<RadioListTile<int>>.generate(
        countries.length,
        (int index){
          return new RadioListTile<int>(
            value: index,
            groupValue: _selectedCountryIndex,
            title: new Text(countries[index]),
            onChanged: (int value) {
              setState((){
                _selectedCountryIndex = value;
              });
            },
          );
        }
      )
    );
  }

  _showDialog() async{
    await showDialog<String>(
      context: context,
      builder: (BuildContext context){
        return new CupertinoAlertDialog(
          title: new Text('Please select'),
          actions: <Widget>[
            new CupertinoDialogAction(
              isDestructiveAction: true,
              onPressed: (){Navigator.of(context).pop('Cancel');},
              child: new Text('Cancel'),
            ),
            new CupertinoDialogAction(
              isDestructiveAction: true,
              onPressed: (){Navigator.of(context).pop('Accept');},
              child: new Text('Accept'),
            ),
          ],
          content: new SingleChildScrollView(
            child: new Material(
              child: _buildList(),
            ),
          ),
        );
      },
      barrierDismissible: false,
    );
  }

  @override
  Widget build(BuildContext context) {
    return new Container();
  }
}

我很惊讶地看到，尽管第34-36行中的setState，当用户点击其中一个项目时，所选的RadioListTile没有刷新。

说明

经过一些调查，我意识到setState（）引用了调用setState的有状态小部件。在此示例中，对setState（）的任何调用都会重建Sample Widget 的视图，而不是对话框的内容之一。所以怎么办？

解

一个非常简单的解决方案是创建另一个呈现对话框内容的有状态窗口小部件。然后，任何setState的调用都将重建对话框的内容。

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';

class Sample extends StatefulWidget {
  @override
  _SampleState createState() => new _SampleState();
}

class _SampleState extends State<Sample> {
  List<String> countries = <String>['Belgium','France','Italy','Germany','Spain','Portugal'];

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_){_showDialog();});
  }

  _showDialog() async{
    await showDialog<String>(
      context: context,
      builder: (BuildContext context){
        return new CupertinoAlertDialog(
          title: new Text('Please select'),
          actions: <Widget>[
            new CupertinoDialogAction(
              isDestructiveAction: true,
              onPressed: (){Navigator.of(context).pop('Cancel');},
              child: new Text('Cancel'),
            ),
            new CupertinoDialogAction(
              isDestructiveAction: true,
              onPressed: (){Navigator.of(context).pop('Accept');},
              child: new Text('Accept'),
            ),
          ],
          content: new SingleChildScrollView(
            child: new Material(
              child: new MyDialogContent(countries: countries),
            ),
          ),
        );
      },
      barrierDismissible: false,
    );
  }

  @override
  Widget build(BuildContext context) {
    return new Container();
  }
}

class MyDialogContent extends StatefulWidget {
  MyDialogContent({
    Key key,
    this.countries,
  }): super(key: key);

  final List<String> countries;

  @override
  _MyDialogContentState createState() => new _MyDialogContentState();
}

class _MyDialogContentState extends State<MyDialogContent> {
  int _selectedIndex = 0;

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

  _getContent(){
    if (widget.countries.length == 0){
      return new Container();
    }

    return new Column(
      children: new List<RadioListTile<int>>.generate(
        widget.countries.length,
        (int index){
          return new RadioListTile<int>(
            value: index,
            groupValue: _selectedIndex,
            title: new Text(widget.countries[index]),
            onChanged: (int value) {
              setState((){
                _selectedIndex = value;
              });
            },
          );
        }
      )
    );
  }

  @override
  Widget build(BuildContext context) {
    return _getContent();
  }
}

结论

有时一些基本概念很棘手，而setState就是其中之一。由于官方文档尚未解释这一点，我想与您分享。

翻译不易，大家且看且珍惜
原文