Flutter: The instance member'stars' can't be accessed in an initializer. Error

问题

https://flutter.dev/docs/development/ui/layout#nesting-rows-and-columns

When I was looking at the linked page above

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart' show debugPaintSizeEnabled;

void main() {
  debugPaintSizeEnabled = true; // Remove to suppress visual layout
  runApp(MyApp());
}

class MyApp extends StatelessWidget {

  Widget stars=Row(
    mainAxisSize: MainAxisSize.min,
    children: [
      Icon(Icons.star, color: Colors.green[500]),
      Icon(Icons.star, color: Colors.green[500]),
      Icon(Icons.star, color: Colors.black),
      Icon(Icons.star, color: Colors.black),
      Icon(Icons.star, color: Colors.black),
    ],
  );

  Widget ratings = Container(
    padding: EdgeInsets.all(20),
    child: Row(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: [
        stars,
        Text(
          '130 Reviews',
          style: TextStyle(
            color: Colors.black,
            fontWeight: FontWeight.w800,
            fontFamily: 'Roboto',
            letterSpacing: 0.5,
            fontSize: 20,
          ),
        ),
      ],
    ),
  );

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter layout demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter layout demo'),
        ),
        // Change to buildColumn() for the other column example
        body: ratings,
      ),
    );
  }
}

When I run the above code

The instance member 'stars' can't be accessed in an initializer.

Try replacing the reference to the instance member with a different expression

I get the above error.

For the time being

==> Make ratings and stars a function.

==> Make the ratings and stars variables local variables of the build () method.

When I changed it as above, the error disappeared, but I'm not sure why the above code gives an error. What's the reason?

回答1:

move variables into build method, or you can defined theirs as getter

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart' show debugPaintSizeEnabled;

void main() {
  debugPaintSizeEnabled = true; // Remove to suppress visual layout
  runApp(MyApp());
}

class MyApp extends StatelessWidget {

  Widget get stars => Row(
    mainAxisSize: MainAxisSize.min,
    children: [
      Icon(Icons.star, color: Colors.green[500]),
      Icon(Icons.star, color: Colors.green[500]),
      Icon(Icons.star, color: Colors.black),
      Icon(Icons.star, color: Colors.black),
      Icon(Icons.star, color: Colors.black),
    ],
  );

  Widget get ratings => Container(
    padding: EdgeInsets.all(20),
    child: Row(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: [
        stars,
        Text(
          '130 Reviews',
          style: TextStyle(
            color: Colors.black,
            fontWeight: FontWeight.w800,
            fontFamily: 'Roboto',
            letterSpacing: 0.5,
            fontSize: 20,
          ),
        ),
      ],
    ),
  );

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter layout demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter layout demo'),
        ),
        // Change to buildColumn() for the other column example
        body: ratings,
      ),
    );
  }
}

reason : in dart you can't create a class level variable depends with another variable

another way is define stars variable as static variable :

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart' show debugPaintSizeEnabled;

void main() {
  debugPaintSizeEnabled = true; // Remove to suppress visual layout
  runApp(MyApp());
}

class MyApp extends StatelessWidget {

  static Widget stars=Row(
    mainAxisSize: MainAxisSize.min,
    children: [
      Icon(Icons.star, color: Colors.green[500]),
      Icon(Icons.star, color: Colors.green[500]),
      Icon(Icons.star, color: Colors.black),
      Icon(Icons.star, color: Colors.black),
      Icon(Icons.star, color: Colors.black),
    ],
  );

  Widget ratings = Container(
    padding: EdgeInsets.all(20),
    child: Row(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: [
        stars,
        Text(
          '130 Reviews',
          style: TextStyle(
            color: Colors.black,
            fontWeight: FontWeight.w800,
            fontFamily: 'Roboto',
            letterSpacing: 0.5,
            fontSize: 20,
          ),
        ),
      ],
    ),
  );

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter layout demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter layout demo'),
        ),
        // Change to buildColumn() for the other column example
        body: ratings,
      ),
    );
  }
}

回答2:

You can't accessed method where it define. To use add your method below the build (i.e. above the return function)

Widget stars=Row(
    mainAxisSize: MainAxisSize.min,
    children: [
      Icon(Icons.star, color: Colors.green[500]),
      Icon(Icons.star, color: Colors.green[500]),
      Icon(Icons.star, color: Colors.black),
      Icon(Icons.star, color: Colors.black),
      Icon(Icons.star, color: Colors.black),
    ],
  );

  Widget ratings = Container(
    padding: EdgeInsets.all(20),
    child: Row(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: [
        stars,
        Text(
          '130 Reviews',
          style: TextStyle(
            color: Colors.black,
            fontWeight: FontWeight.w800,
            fontFamily: 'Roboto',
            letterSpacing: 0.5,
            fontSize: 20,
          ),
        ),
      ],
    ),
  );

来源：https://stackoverflow.com/questions/64455240/flutter-the-instance-memberstars-cant-be-accessed-in-an-initializer-error