Flutter - setState不使用新数据

Question

我有一个屏幕，我将数据传回到这样：

final myUpdatedObject = await Navigator.of(context).push(...);
setState({
  object = myUpdatedObject;
});

在我的小部件主体中使用我的对象的所有位置检查了一个简单的print后，新数据在被Navigator传回并且调用setState之后出现。

但是，当重建窗口小部件时，即使新数据显然存在，它也不会反映在UI更改中，而是显示旧数据。

这是调试模式下的某种缓存吗？是什么导致了这个问题？

Answer 1

下面的示例以Map开头，名为textMessageMap，带有message键，用'Home'填充Text Widget。点击FloatingActionButton，你将导航到SecondScreen。如果点击“返回！” SecondScreen中的按钮，message中的textMessageMap键将更新为“Second Screen”。如果你点击Scaffold的SecondScreen上的后退按钮，textMessageMap将被淘汰。调用setState会适当地更新UI。看看你的实现是否不同。

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  Map<String, String> textMessageMap = {'message': 'Home'};

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text(
              '${textMessageMap != null ? textMessageMap['message'] : 'map is null'}',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: () {
          _launchSecondScreen();
        },
        child: new Icon(Icons.add),
      ),
    );
  }

  _launchSecondScreen() async {
    final value = await Navigator.push(
        context,
        MaterialPageRoute<Map<String, String>>(
            builder: (BuildContext _) => SecondScreen()));
    setState(() {
      textMessageMap = value;
    });
  }
}

class SecondScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Second Screen"),
      ),
      body: Center(
        child: RaisedButton(
          onPressed: () {
            // Navigate back to the first screen by popping the current route
            // off the stack. The text 'Second Screen' will replace 'Home'.
            // If you hit the scaffold's back button, the return value will be
            // null instead.
            final map = {'message': 'Second Screen'};
            Navigator.pop(context, map);
          },
          child: Text('Go back!'),
        ),
      ),
    );
  }
}