Flutter Dropdown：“对象？”类型的值不能分配给“字符串”类型的变量。 - “对象”来自“飞镖：核心”

Question

我不断收到以下错误： lib/main.dart:45:37：错误：“对象”类型的值？不能分配给“字符串”类型的变量。

• “对象”来自“飞镖：核心”。 _startMeasure = 值；

这完全有道理，但我尝试将值更改为字符串，但这并不能解决问题。我试过“$value”和 _startMeasure = value as String。但是，这些都不起作用。

代码：

import 'package:flutter/material.dart';

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

class MyApp extends StatefulWidget {
  MyAppState createState() => MyAppState();
}//class

class MyAppState extends State<MyApp> {

  double _numberFrom = 0;
  String _startMeasure = "";
  final List<String> _measures = [
    'meters',
    'kilometers',
    'grams',
    'kilograms',
    'feet',
    'miles',
    'pounds (lbs)',
    'ounces',
  ];

  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(

        appBar: AppBar(
          title: Center(child: Text("Convert Units")),
          backgroundColor: Colors.deepOrange[300],
        ),
        
        body: Center(

          child: Column(
            children: [

              DropdownButton(
                items: _measures.map((String value) {
                  return DropdownMenuItem<String>(value: value, child: Text(value),);
                }).toList(),

                onChanged: (value) {
                  setState(() {
                    _startMeasure = value;
                  });
                },

              ),

              TextField(
                onChanged: (text) {
                  var rv = double.tryParse(text);
                  if (rv != null) {
                    setState(() {
                      _numberFrom = rv;
                    });
                  }

                  else {
                    setState(() {
                      _numberFrom = 0;
                    });
                  }

                },
              ),

              //Text((_numberFrom == null) ? '' : _numberFrom.toString()),
              Text("Number entered: $_numberFrom"),
            ],
          ),//Column

        ),

      ),
    );
  }//widget
}

Answer 1

您的下拉按钮没有类型，所以它认为 onChanged 中的值是 Object？ 相反，它应该是这样的：

             DropdownButton<String>(
                items: _measures.map((String value) {
                  return DropdownMenuItem<String>(value: value, child: Text(value),);
                }).toList(),

                onChanged: (String? value) {
                  setState(() {
                    // You can't assign String? to String that's why the ! tells dart it cant be null
                    _startMeasure = value!;
                  });
                },

              ),