Flutter开发报Missing concrete implementation of 'State.build'错误求助

错误原因 #

你的_MyHomePageState类的build方法被错误写在了_deleteTransaction方法的setState代码块内部，State类无法识别到独立的build方法实现，才触发了「Missing concrete implementation of 'State.build'」报错。
将类改为抽象类的解决方向完全错误，抽象类本身不能被实例化，自然会触发第二个报错，无需修改类的属性，仅调整方法的嵌套结构即可。

修正后的完整_MyHomePageState代码 #

class _MyHomePageState extends State<MyHomePage> {
  final List<Transaction> _userTransactions = [];

  List<Transaction> get _recentTransactions {
    return _userTransactions.where((tx) {
      return tx.date.isAfter(
        DateTime.now().subtract(
          Duration(days: 7),
        ),
      );
    }).toList();
  }

  void _addNewTransaction(
      String txTitle, double txAmount, DateTime chosenDate) {
    final newTx = Transaction(
      title: txTitle,
      amount: txAmount,
      date: chosenDate,
      id: DateTime.now().toString(),
    );

    setState(() {
      _userTransactions.add(newTx);
    });
  }

  void _startAddNewTransaction(BuildContext ctx) {
    showModalBottomSheet(
      context: ctx,
      builder: (_) {
        return GestureDetector(
          onTap: () {},
          child: NewTransaction(_addNewTransaction),
          behavior: HitTestBehavior.opaque,
        );
      },
    );
  }

  void _deleteTransaction(String id) {
    setState(() {
      _userTransactions.removeWhere((tx) => tx.id == id);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(
          'Expense Planner',
          style: TextStyle(
            fontFamily: 'OpenSans',
            fontSize: 22,
            fontWeight: FontWeight.bold,
          ),
        ),
        actions: <Widget>[
          IconButton(
            icon: Icon(Icons.add),
            onPressed: () => _startAddNewTransaction(context),
          ),
        ],
      ),
      body: SingleChildScrollView(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            Chart(_recentTransactions),
            TransactionList(_userTransactions, _deleteTransaction),
          ],
        ),
      ),
      floatingActionButtonLocation:
          FloatingActionButtonLocation.centerFloat,
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () => _startAddNewTransaction(context),
      ),
    );
  }
}

修改点说明 #

• 补全了_deleteTransaction方法中setState的闭合括号，删掉了冗余的空代码块
• 将build方法从_deleteTransaction内部移出，作为_MyHomePageState的直接成员方法，符合State类的结构要求

内容的提问来源于stack exchange，提问作者Harshit Chitkara