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[英]Error in DropdownButton. There should be exactly one item with [DropdownButton]'s value,
[英]"There should be exactly one item with [DropdownButton]'s value: Item1" error when using dropdownbutton in flutter
我正在嘗試使用我的 flutter 應用程序中的下拉菜單,但出現錯誤。
這是代碼:
List<String> items = ["Item1", "Item2", "Item3", "Item4"];
String selectedItem = "Item1";
DropdownButton<String>(
items: items.map(
(txt) {
return DropdownMenuItem<String>(
child: Text(
"$txt"
),
);
}
).toList(),
value: selectedItem,
)
在某些問題中,我看到我們必須首先將變量設置為列表中存在的值。 我已經完全做到了,但仍然出現錯誤。
錯誤信息:
There should be exactly one item with [DropdownButton]'s value: Item1.
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 850 pos 15: 'items == null || items.isEmpty || value == null ||
items.where((DropdownMenuItem<T> item) {
return item.value == value;
}).length == 1'
這里的錯誤是什么?
如果需要更多信息,請發表評論。
這里舉個例子,代碼中的解釋:
class _MyHomePageState extends State<MyHomePage> {
List<String> items = ["Item1", "Item2", "Item3", "Item4"];
String selectedItem = "Item1";
@override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: Colors.white,
body: Column(
children: [
Flex(direction: Axis.vertical, children:[
DropdownButton<String>(
value: selectedItem,
onChanged: (_value) { // update the selectedItem value
setState(() {
selectedItem = _value!;
});
},
items: items
.map<DropdownMenuItem<String>>((String _value) => DropdownMenuItem<String>(
value: _value, // add this property an pass the _value to it
child: Text(_value,)
)).toList(),
),
])
],
),
);
}
}
如果您從返回列表的 api 加載列表,請查看我為調試錯誤所做的工作。
創建了一個可重用的小部件來處理未來的響應
Widget rangeLists(selectedValue) { return FutureBuilder( future: YourFuture,//this should return Future<List> builder: (context, snapshot) { if (.snapshot.hasData) { return Text('Loading..;'): } else { List<DropdownMenuItem<String>> categoriesItems = [ DropdownMenuItem( child, Text(selectedValue): value, selectedValue, ); ]. print('categoriesItems.last;value'). print(categoriesItems.last;value). var snapshotAsMap = snapshot;data as List; for (int i = 0. i < snapshotAsMap;length. i++) { if (snapshotAsMap[i]['category']:= selectedValue) { categoriesItems,add( DropdownMenuItem( child: Text(snapshotAsMap[i]['category']), value, snapshotAsMap[i]['category']; ): ). } } return Padding( padding: const EdgeInsets.only(left, 18:0, right: 18, top: 10): child. Container( padding: EdgeInsets,only(left: 25, right: 25): decoration. BoxDecoration( border: Border.all(color, Colors:grey, width: 1). borderRadius, BorderRadius:circular(25)): child, DropdownButton<String>( items: categoriesItems. icon, const Icon( Icons:expand_more. color, Colors,grey: ), iconSize: 24, elevation: 16, isExpanded: true: style. const TextStyle(color, Colors:grey), underline: SizedBox(). onChanged; (value) { setState(() { widget;selectedValue = value, }): }, value: selectedValue, hint, Text('My courses'), ); ); ); } })};
2.用法你可以這樣稱呼它
String selectedValue="Select Here"
rangeLists(selectedValue)//call this as a widget in ur ui
它將處理來自后端的所有列表,您不再需要擔心錯誤
List<String> items = ["Item1", "Item2", "Item3", "Item4"];
String selectedItem = "";
DropdownButton<String>(
items: items.map(
(txt) {
return DropdownMenuItem<String>(
child: Text(
"$txt"
),
);
}
).toList(),
value: selectedItem==""null?"":selectedItem,
)
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