被调用的构造函数不是 const 构造函数

Question

I am getting an error for creating a Column Widget saying 'The constructor being called isn't a const constructor.' Having a tough time creating a Column Widget itself,

    import 'package:flutter/material.dart';

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

    class MyApp extends StatelessWidget {
      const MyApp({Key? key}) : super(key: key);
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
            debugShowCheckedModeBanner: false,
            title: 'My First Flutter App',
            theme: ThemeData(
              scaffoldBackgroundColor: Colors.white,
            ),
            home: const WelcomeScreen());
      }
    }

    class WelcomeScreen extends StatelessWidget {
      const WelcomeScreen({Key? key}) : super(key: key);

      @override
      Widget build(BuildContext context) {
        return const Scaffold(

          body: Column(
              children: <Widget>[
                Text("Login"),
              ]
            )
          );
        }}

Answer 1

Just remove const from Scaffold widget

Answer 2

Column is a non-const constructor

 Column({
    Key? key,

You can use const where everything is available as compile time constant. In this case, Column breaks the condition. You can use const before children, it is possible with Text for now

class WelcomeScreen extends StatelessWidget {
  const WelcomeScreen({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(children: const <Widget>[
        Text("Login"),
      ]),
    );
  }
}

I will encourage you to lean more about const , you can check this question and on guides .

Answer 3

The error is at the level of the scaffold widget.

Just remove the const before scaffold. Then add const keyword before the children of your column widget.

That is how your code is supposed to be

 import 'package:flutter/material.dart'; void main() { runApp(const MyApp()); } class MyApp extends StatelessWidget { const MyApp({Key? key}): super(key: key); @override Widget build(BuildContext context) { return MaterialApp( debugShowCheckedModeBanner: false, title: 'My First Flutter App', theme: ThemeData( scaffoldBackgroundColor: Colors.white, ), home: const WelcomeScreen()); } } class WelcomeScreen extends StatelessWidget { const WelcomeScreen({Key? key}): super(key: key); @override Widget build(BuildContext context) { return Scaffold( body: Column( children: const <Widget>[ Text("Login"), ... ] ) ); }}

Answer 4

Remove the const from the Scaffold widget and it would work fine. But there will be another warning telling that the Column widget does not have const .

To fix that you can either put const in front of <Widget> or put const in front of the children of columns