飞镖/颤振：更改值时，DropdownButton导致异常 [英] Dart / flutter: DropdownButton causes exception when value is changed

问题描述

我使用 DropdownButton 和 TextField 小部件编写了相当广泛的表格。这个概念是，我有一个 StatefulWidget ，其中 State< StatefulWidget> 的类包含2个返回小部件的方法我要建造。这样，我可以轻松地访问和使用输入的数据，并将其传递给函数以将其组成电子邮件。

I have written a pretty extensive form using DropdownButton and TextField widgets. The concept is that I have a StatefulWidget, where the class of State<StatefulWidget> contains 2 methods that return the widget I want to build. This way I can easily access and use the entered data and pass it along a function to compose an e-mail out of them.

但是，当我从中选择一项时选项，则框架在重建期间会引发异常。我放入了一些日志函数，它表明 setState（）方法成功将值保存到 selectedValue 变量中。

However, when I select an item from the options, the framework throws an exception during the rebuild. I put in some log functions, and it shows that the setState() method successfully saves the value to selectedValue variable.

Widget buildMultiChoiceInputRow(var label, List<String> values) {
    final List<String> options = values.toList();
    selection = options.first;

    final dropDownMenuOptions = options.map((String value) {
      return new DropdownMenuItem<String>(
        value: value,
        child: new Text(value),
      );
    }).toList();

    return new Column(
      children: <Widget>[
        new Row(
          children: <Widget>[
            new Expanded(
              child: new Container(
                  padding:
                      const EdgeInsets.only(left: 5.0, top: 2.0, right: 5.0),
                  child: new Text(label, style: commonInfoCardInfoTextBlack16Bold)),
            ),
          ],
        ),
        new Row(
          children: <Widget>[
            new Expanded(
              child: new Container(
                padding: const EdgeInsets.only(left: 5.0, right: 5.0),
                child: new DropdownButton(
                value: selectedValue,
                items: dropDownMenuOptions,
                onChanged: (selection) {
                  setState(() {
                    selectedValue = selection;
                    switch (label) {
                      case labelVirtualAdoption:
                        tempAdoptionType =
                            composeMultiChoiceAnswer(label, selection);
                            print(selection);
                            print(selectedValue);
                        break;
                          case labelAskedAboutSpecies:
                            tempAskedAboutSpecies =
                                composeMultiChoiceAnswer(label, selection);
                            break;
                          case labelHouseOrFlat:
                            tempHouseOrFlat =
                                composeMultiChoiceAnswer(label, selection);
                            break;
                            ....
                          default:
                            break;
                        }
                      });
                    }),
              ),
            )
          ],
        ),
        new Divider(color: Colors.transparent)
      ],
    );
  }

这里是个例外：

I/flutter (20998): The following assertion was thrown building AdoptionInput(dirty, state: AdoptionInputState#3cc80):
I/flutter (20998): 'package:flutter/src/material/dropdown.dart': Failed assertion: line 481 pos 15: 'value == null ||
I/flutter (20998): items.where((DropdownMenuItem<T> item) => item.value == value).length == 1': is not true.

这是堆栈，显示在重建过程中引发了异常：

And here is the stack, showing that the exception is thrown during the rebuild:

I/flutter (20998): #2      new DropdownButton (package:flutter/src/material/dropdown.dart)
I/flutter (20998): #3      AdoptionInputState.buildMultiChoiceInputRow (package:osszefogasaszanhuzokert/adoptionPageUtilities.dart:443:28)
I/flutter (20998): #4      AdoptionInputState.build (package:osszefogasaszanhuzokert/adoptionPageUtilities.dart:639:11)
I/flutter (20998): #5      StatefulElement.build (package:flutter/src/widgets/framework.dart:3730:27)
I/flutter (20998): #6      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3642:15)
I/flutter (20998): #7      Element.rebuild (package:flutter/src/widgets/framework.dart:3495:5)
I/flutter (20998): #8      BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2242:33)

问题似乎真的很相似o 以前是Flutter的错误，但是如果我尝试初始化 initState（）中选择> selection 和 selectedValue ，将抛出与

The problem seems really similar to a former bug in flutter, but if I try to initialize the selection and selectedValue in initState(), the same exception will be thrown right as the form is built for the first time.

我在这里缺少什么？

推荐答案

您的DropdownButton的值应设置为 null或为值列表中的一个。

Your "value" for DropdownButton should be set to 'null' or or be one from the values list.

DropdownButton(
      value: null,
      isDense: true,
      onChanged: (String newValue) {
        // somehow set here selected 'value' above whith 
       // newValue
       // via setState or reactive.
      },
      items: ['yellow', 'brown', 'silver'].map((String value) {
        return DropdownMenuItem(
          value: value,
          child: Text(value),
        );
      }).toList(),
    ),

因此，在我的示例中，DropdownButton值应设置为null或为黄色，棕色或银色。

So for my example DropdownButton value should be set to null or be 'yellow' or 'brown' or 'silver'.

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