我想，这是一些基本的东西。我尝试根据本手册和以下说明创建本地化应用程序。一切都正常，直到我尝试实现一个带有提示消息的字段。
我尝试实现的表单（form.dart）：

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter/services.dart';
import 'package:flutter_gen/gen_l10n/app_localizations.dart';

class MyCustomForm extends StatelessWidget {
  const MyCustomForm({super.key});

  @override
  Widget build(BuildContext context) {
    final local_delegate = AppLocalizations.of(context);
    return Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      children: <Widget>[
        const Padding(
          padding: EdgeInsets.symmetric(horizontal: 8, vertical: 16),
          child: TextField(
            decoration: InputDecoration(
              border: OutlineInputBorder(),
              hintText: local_delegate?.search_term, // a line with an error
            ),
          ),
        ),
        Padding(
          padding: const EdgeInsets.symmetric(horizontal: 8, vertical: 16),
          child: new TextFormField(
            keyboardType: TextInputType.number,
            inputFormatters: [FilteringTextInputFormatter(new RegExp(r'[+-]?([0-9]*[.])?[0-9]+'), allow: true)],
            decoration: const InputDecoration(
                filled: true,
                fillColor: CupertinoColors.white,
                border: const OutlineInputBorder(),
                labelText: 'Amount invested',
                prefixText: '\R',
                suffixText: 'ZAR',
                suffixStyle:
                const TextStyle(color: Colors.green)),
            maxLines: 1,
            validator: (String? value) {
              if (value != null && value.isEmpty) {
                return "Amount is required";
              }

              return null;
            },
          ),
        ),
      ],
    );
  }
}

我已经尝试使用两个操作符assertion（！）和条件成员访问（？.）运算符，这将导致“无效常量值”错误。我也试着做类似String(local_delegate?.search_term)的事情，但是收到了这样的错误“The class 'String' doesn 't have an unnamed constant constructor.”尝试调用不同的构造函数。”我也尝试过使用这些解决方案：Flutter i18n shortcutFlutter: SearchBar hint text with internationalization。另一个solution建议从装饰线装饰中删除 const：const InputDecoration
但代码仍然无法编译。
在这种情况下，如何正确使用i18 n？
应用程序：

import 'package:flutter/material.dart';
import 'package:flutter_gen/gen_l10n/app_localizations.dart';
import 'package:flutter_localizations/flutter_localizations.dart';
import 'form.dart';

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      onGenerateTitle: (context) => AppLocalizations.of(context)!.title,
      theme: ThemeData(
        // This is the theme of your application.
        //
        // TRY THIS: Try running your application with "flutter run". You'll see
        // the application has a blue toolbar. Then, without quitting the app,
        // try changing the seedColor in the colorScheme below to Colors.green
        // and then invoke "hot reload" (save your changes or press the "hot
        // reload" button in a Flutter-supported IDE, or press "r" if you used
        // the command line to start the app).
        //
        // Notice that the counter didn't reset back to zero; the application
        // state is not lost during the reload. To reset the state, use hot
        // restart instead.
        //
        // This works for code too, not just values: Most code changes can be
        // tested with just a hot reload.
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      localizationsDelegates: [
        AppLocalizations.delegate,
        GlobalMaterialLocalizations.delegate,
        GlobalWidgetsLocalizations.delegate,
        GlobalCupertinoLocalizations.delegate,
      ],
      supportedLocales: [
        Locale('en'), // English
      ],

      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });}
    void _decrementCounter() {
      setState(() {
        // This call to setState tells the Flutter framework that something has
        // changed in this State, which causes it to rerun the build method below
        // so that the display can reflect the updated values. If we changed
        // _counter without calling setState(), then the build method would not be
        // called again, and so nothing would appear to happen.
        _counter--;
      });
  }

  @override
  Widget build(BuildContext context) {
    final local_delegate = AppLocalizations.of(context);
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // TRY THIS: Try changing the color here to a specific color (to
        // Colors.amber, perhaps?) and trigger a hot reload to see the AppBar
        // change color while the other colors stay the same.
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(AppLocalizations.of(context)!.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          //
          // TRY THIS: Invoke "debug painting" (choose the "Toggle Debug Paint"
          // action in the IDE, or press "p" in the console), to see the
          // wireframe for each widget.
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              // 'You have pushed the button this many times:',
                local_delegate!.push_text,
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
            MyCustomForm()],
        ),

      ),

      floatingActionButton: Column(
          mainAxisAlignment: MainAxisAlignment.end,
        children: [
            FloatingActionButton(
          onPressed: _incrementCounter,
          tooltip: 'Increment',
          child: const Icon(Icons.add)),
          FloatingActionButton(
            onPressed: _decrementCounter,
            tooltip: 'Decrement',
            child: const Icon(Icons.remove),
      ),
        ]
      ),

      // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

主 dart

import 'src/app.dart';
import 'package:flutter/material.dart';

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

本地化app_en.arb

{
  "title": "Test title",
  "push_text": "You have pushed the button this many times:",
  "search_term": "Enter a search term",
  "@search_term": {
    "description": "Enter a search term"
  }
}