我希望你一切都好。我有一個小問題，我想根據另一個 DropdownButton 的值動態更改一個 DropdownButton 的值。
好吧，我將解釋我所處的位置以及問題所在：
首先，我有一個包含 3 個元素的StudyCycle串列；
final _studyCycles = const <StudyCycle>[
StudyCycle(cycle: Cycles.elementary, classrooms: <ClassroomsLevels>[
ClassroomsLevels.cp,
ClassroomsLevels.ce1,
ClassroomsLevels.ce2,
ClassroomsLevels.cm1,
ClassroomsLevels.cm2,
]),
StudyCycle(cycle: Cycles.primary, classrooms: <ClassroomsLevels>[
ClassroomsLevels.sixth,
ClassroomsLevels.fifth,
ClassroomsLevels.fourth,
ClassroomsLevels.third,
]),
StudyCycle(cycle: Cycles.secondary, classrooms: <ClassroomsLevels>[
ClassroomsLevels.second,
ClassroomsLevels.first,
ClassroomsLevels.terminal,
])
];
這個StudyCycle串列用于構建第一個 DropdownButton
DropdownButton<StudyCycle>(
value: _selectedStudyCycle,
items: List<DropdownMenuItem<StudyCycle>>.generate(
_studyCycles.length,
(index) => DropdownMenuItem<StudyCycle>(
value: _studyCycles[index],
child: Text(_studyCycles[index].cycle.toString()),
),
),
hint: const Text('Your Study Cycle'),
onChanged: (StudyCycle? studyCycle) {
if (studyCycle != _selectedStudyCycle) {
if (_selectedStudyCycle != null) setState(() => _selectedStudyCycle = null);
if (studyCycle != null) setState(() => _selectedStudyCycle = studyCycle);
}
},
)
并通過選擇StudyCycle從StudyCycles下拉，它允許生成包含此時另一個DropdownButton的值ClassroomsLevels所述的StudyCycle目前在所選StudyCycles下拉（到目前為止好）
DropdownButton<ClassroomsLevels>(
hint: const Text('Select your classroom'),
value: _selectedClassroom,
items: _selectedStudyCycle == null
? []
: List<DropdownMenuItem<ClassroomsLevels>>.generate(
_selectedStudyCycle!.classrooms.length,
(index) => DropdownMenuItem<ClassroomsLevels>(
value: _selectedStudyCycle!.classrooms[index],
child: Text(_selectedStudyCycle!.classrooms[index].toString()),
),
),
onChanged: (ClassroomsLevels? classroom) => setState(() => _selectedClassroom = classroom),
)
The problem occurs when a ClassroomLevel is selected and I change StudyCycle , I get this error:
There should be exactly one item with [DropdownButton]'s value: ClassroomsLevels.fourth.
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 915 pos 15: 'items == null || items.isEmpty || value == null ||
items.where((DropdownMenuItem<T> item) {
return item.value == value;
}).length == 1'
我盡力解決問題，但我不能（我真的不明白為什么），所以我依賴你。你能幫我理解錯誤是什么以及如何解決它嗎？
完整的代碼
import 'package:flutter/material.dart';
class MyHomePage2 extends StatefulWidget {
const MyHomePage2({Key? key, required this.title}) : super(key: key);
final String title;
@override
State<MyHomePage2> createState() => _MyHomePage2State();
}
class _MyHomePage2State extends State<MyHomePage2> {
final _studyCycles = const <StudyCycle>[
StudyCycle(cycle: Cycles.elementary, classrooms: <ClassroomsLevels>[
ClassroomsLevels.cp,
ClassroomsLevels.ce1,
ClassroomsLevels.ce2,
ClassroomsLevels.cm1,
ClassroomsLevels.cm2,
]),
StudyCycle(cycle: Cycles.primary, classrooms: <ClassroomsLevels>[
ClassroomsLevels.sixth,
ClassroomsLevels.fifth,
ClassroomsLevels.fourth,
ClassroomsLevels.third,
]),
StudyCycle(cycle: Cycles.secondary, classrooms: <ClassroomsLevels>[
ClassroomsLevels.second,
ClassroomsLevels.first,
ClassroomsLevels.terminal,
])
];
StudyCycle? _selectedStudyCycle;
ClassroomsLevels? _selectedClassroom;
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text(widget.title)),
body: SizedBox(
width: MediaQuery.of(context).size.width,
child: Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
DropdownButton<StudyCycle>(
value: _selectedStudyCycle,
items: List<DropdownMenuItem<StudyCycle>>.generate(
_studyCycles.length,
(index) => DropdownMenuItem<StudyCycle>(
value: _studyCycles[index],
child: Text(_studyCycles[index].cycle.toString()),
),
),
hint: const Text('Your Study Cycle'),
onChanged: (StudyCycle? studyCycle) {
if (studyCycle != _selectedStudyCycle) {
if (_selectedStudyCycle != null) setState(() => _selectedStudyCycle = null);
if (studyCycle != null) setState(() => _selectedStudyCycle = studyCycle);
}
},
),
//
DropdownButton<ClassroomsLevels>(
hint: const Text('Select your classroom'),
value: _selectedClassroom,
items: _selectedStudyCycle == null
? []
: List<DropdownMenuItem<ClassroomsLevels>>.generate(
_selectedStudyCycle!.classrooms.length,
(index) => DropdownMenuItem<ClassroomsLevels>(
value: _selectedStudyCycle!.classrooms[index],
child: Text(_selectedStudyCycle!.classrooms[index].toString()),
),
),
onChanged: (ClassroomsLevels? classroom) => setState(() => _selectedClassroom = classroom),
),
],
),
),
);
}
}
class StudyCycle {
final Cycles cycle;
final List<ClassroomsLevels> classrooms;
final String? description;
final int? iconCodePoint;
const StudyCycle({
required this.cycle,
required this.classrooms,
this.description,
this.iconCodePoint,
});
}
enum Cycles { elementary, primary, secondary, secondaryTechnique }
enum ClassroomsLevels {
cp,
ce1,
ce2,
cm1,
cm2,
sixth,
fifth,
fourth,
third,
second,
first,
terminal,
mechanical,
electronic,
electricity,
inEngineering,
secretariat,
accounting,
economy,
agr1,
agr2,
agr3,
}
謝謝你！
uj5u.com熱心網友回復：
錯誤的原因是，更改第一個下拉_selectedClassroom串列的值后，第二個下拉串列中的值不存在于新教室串列中_selectedStudyCycle!.classrooms。只需在setState方法中重置第二個下拉串列的值：
if (studyCycle != null) {
setState(() {
// Add this line
_selectedClassroom = null;
_selectedStudyCycle = studyCycle;
});
}
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