Flutter报错：‘MyHomePage’未定义‘Transaction’，求文档及结构变更信息

跟随2022年Maximilian Schwarzmüller的Flutter教程开发，使用Transaction时出现上述错误，查阅官方文档无收获，疑惑是否是Flutter更新导致结构名称变更，附上代码如下：

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter App',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final List<Transaction> transactions = [
    Transaction(
      id: 't1',
      Title: 'Weekly Shoes',
      amount: 69.99,
      date: DateTime.now(),
    ),
    Transaction(
      id: 't2',
      Title: 'Weekly Groceries',
      amount: 16.53,
      date: DateTime.now(),
    ),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter App'),
      ),
      body: Column(
        children: <Widget>[
          Container(
            width: double.infinity,
            child: Card(
              color: Colors.blue,
              child: Text('Chart!'),
              elevation: 5,
            ),
          ),
          Card(
            child: Text('List of TX!'),
          ),
        ],
      ),
    );
  }
}

是否使用了旧结构？

这个错误和Flutter版本更新无关，Transaction不是Flutter框架自带的API，是教程里Max自定义的业务类，你漏写了这个类的定义，导致编译器无法识别它。

只需在代码中添加Transaction类的定义即可，同时修正原代码中属性名大小写不匹配的问题：

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

// 新增自定义Transaction类
class Transaction {
  final String id;
  final String title;
  final double amount;
  final DateTime date;

  Transaction({
    required this.id,
    required this.title,
    required this.amount,
    required this.date,
  });
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter App',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final List<Transaction> transactions = [
    Transaction(
      id: 't1',
      title: 'Weekly Shoes', // 修正属性名大小写，和类定义保持一致
      amount: 69.99,
      date: DateTime.now(),
    ),
    Transaction(
      id: 't2',
      title: 'Weekly Groceries', // 同步修正属性名
      amount: 16.53,
      date: DateTime.now(),
    ),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter App'),
      ),
      body: Column(
        children: <Widget>[
          Container(
            width: double.infinity,
            child: Card(
              color: Colors.blue,
              child: Text('Chart!'),
              elevation: 5,
            ),
          ),
          Card(
            child: Text('List of TX!'),
          ),
        ],
      ),
    );
  }
}

教程里肯定有这个自定义类的讲解部分，你可能遗漏了这一步，不存在Flutter结构名称变更的情况。

内容的提问来源于stack exchange，提问作者Rezam