flutter - flutter :断言失败下拉列表

Question

我正在尝试创建包含三个下拉按钮的行，但是此错误我无法理解为什么会出现

There should be exactly one item with [DropdownButton]'s value: jan. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 805 pos 15: 'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1'

有我的初始值和我在下拉按钮的值中使用的列表

var selected="Arabic";
  var select="pyramid";
  var sel="jan";
  List<String> languageList = ['Arabic','English','French','Germen'];
  List<String> placeList = ['pyramid','park','zoo','garden'];
  List<String> dateList = ['jan','feb','march','april'];

这是我要创建的行

Row(
                                    children: <Widget>[
                                       DropdownButton<String>(
                                              value: selected ,
                                              items: languageList.map((language){
                                                return DropdownMenuItem(
                                                  value: language ,
                                                  child: Text(language),
                                                );
                                              }).toList(),
                                              onChanged: (vr){
                                                setState(() {
                                                  selected=vr;
                                                });
                                              },
                                            ),

                                       DropdownButton<String>(
                                              value: select ,
                                              items: placeList.map((place){
                                                return DropdownMenuItem(
                                                  value: place ,
                                                  child: Text(place),
                                                );
                                              }).toList(),
                                              onChanged: (ve){
                                                setState(() {
                                                  select=ve;
                                                });
                                              },
                                            ),

                                       DropdownButton<String>(
                                              value: sel  ,
                                              items: languageList.map((dat){
                                                return DropdownMenuItem(
                                                  value: dat ,
                                                  child: Text(dat),
                                                );
                                              }).toList(),
                                              onChanged: (vm){
                                                setState(() {
                                                  sel=vm;
                                                });
                                              },
                                            ),
                                    ],
                                  ),

有人可以帮我吗？

Answer 1

您正在使用“相同语言”列表两次，并且将所选对象作为日期的值“jan”

第三个下拉输入列表是语言列表，但是您将所选项目作为“Jan”提供，但不在列表中。.将输入列表更改为 dateList

Row(
    children: <Widget>[
       DropdownButton<String>(
              value: selected ,
              items: languageList.map((language){
                return DropdownMenuItem(
                  value: language ,
                  child: Text(language),
                );
              }).toList(),
              onChanged: (vr){
                setState(() {
                  selected=vr;
                });
              },
            ),

       DropdownButton<String>(
              value: select ,
              items: placeList.map((place){
                return DropdownMenuItem(
                  value: place ,
                  child: Text(place),
                );
              }).toList(),
              onChanged: (ve){
                setState(() {
                  select=ve;
                });
              },
            ),

       DropdownButton<String>(
              value: sel  ,
              items: dateList.map((dat){ //changes need to done
                return DropdownMenuItem(
                  value: dat ,
                  child: Text(dat),
                );
              }).toList(),
              onChanged: (vm){
                setState(() {
                  sel=vm;
                });
              },
            ),
      ],
    ),