dart - 当在全局变量中使用值时，由于 EdgeInsets.all，Flutter dart 编译失败

Question

错误:

Argument of a constant creation must be constant expressions.

代码:

import 'package:flutter/material.dart';

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

    class MyApp extends StatefulWidget{
    @override
    State<StatefulWidget> createState() {
        // TODO: implement createState
        return _MyAppState();
      }
    }

    class _MyAppState extends State<MyApp> {
      List<String> _products = ['Food Tester'];
      var _font_size = 20.0;

      build(context) {
        return MaterialApp(
            home: Scaffold(
          appBar: AppBar(
            title: Text('EasyList'),
          ),
          body: Column(
            children: [
              Container(
                margin:  EdgeInsets.all(_font_size),
                child: RaisedButton(
                onPressed: () {},
                child: Text('Add product'),
              ),
              ),
            Column(children: _products.map((element) =>Card(
                child: Column(
                  children: <Widget>[Image.asset("assets/food.jpg"), Text(element)],
                ),
              )).toList()),

            ],
          ),
        ));
      }
    }

注意:当参数为静态时，我的意思是 20.0 然后它工作完美，但如果我们将它作为变量，则 Dart 编译失败。这里变量在 EdgeInsets.all 方法中传递，编译失败。

Answer 1

@override
  Widget build(BuildContext context) {
    const double _font_size = 20;

    .....

}

这只是使用常量，所以我们需要将 const 声明为变量。