json - 带有 HTTP 数据库的 Flutter 下拉菜单
问题描述
我正在尝试从我的数据库中检索 JSON 代码映射，并尝试将该映射的一部分放入下拉菜单中。我对这一切都很陌生，感谢任何帮助。我在这个网站上找到了不同的解决方案，但无法开始工作。我得到不同的错误，我现在得到的错误如下。先感谢您！
I/flutter (30929): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (30929): The following NoSuchMethodError was thrown building AddSubCategory(dirty, dependencies:
I/flutter (30929): [_ModalScopeStatus], state: _AddSubCategoryState#a24b3):
I/flutter (30929): The method 'map' was called on null.
I/flutter (30929): Receiver: null
I/flutter (30929): Tried calling: map<DropdownMenuItem<String>>(Closure: (dynamic) => DropdownMenuItem<String>)
I/flutter (30929):
I/flutter (30929): The relevant error-causing widget was:
I/flutter (30929): AddSubCategory file:///D:/apps/reboo/lib/main.dart:45:46
I/flutter (30929):
I/flutter (30929): When the exception was thrown, this was the stack:
I/flutter (30929): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
I/flutter (30929): #1 _AddSubCategoryState.build (package:reboo/screens/admin_screens/add_subcategory_screen.dart:272:37)
I/flutter (30929): #2 StatefulElement.build (package:flutter/src/widgets/framework.dart:4744:28)
I/flutter (30929): #3 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4627:15)
I/flutter (30929): #4 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4800:11)
I/flutter (30929): #5 Element.rebuild (package:flutter/src/widgets/framework.dart:4343:5)
I/flutter (30929): #6 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4606:5)
I/flutter (30929): #7 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4791:11)
I/flutter (30929): #8 ComponentElement.mount (package:flutter/src/widgets/framework.dart:4601:5)
I/flutter (30929): ... Normal element mounting (132 frames)
I/flutter (30929): #140 Element.inflateWidget (package:flutter/src/widgets/framework.dart:3569:14)
I/flutter (30929): #141 Element.updateChild (package:flutter/src/widgets/framework.dart:3327:18)
I/flutter (30929): #142 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:5705:32)
I/flutter (30929): #143 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6246:17)
I/flutter (30929): #144 Element.updateChild (package:flutter/src/widgets/framework.dart:3314:15)
I/flutter (30929): #145 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4652:16)
I/flutter (30929): #146 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4800:11)
I/flutter (30929): #147 Element.rebuild (package:flutter/src/widgets/framework.dart:4343:5)
I/flutter (30929): #148 StatefulElement.update (package:flutter/src/widgets/framework.dart:4832:5)
I/flutter (30929): #149 Element.updateChild (package:flutter/src/widgets/framework.dart:3314:15)
I/flutter (30929): #150 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4652:16)
I/flutter (30929): #151 Element.rebuild (package:flutter/src/widgets/framework.dart:4343:5)
I/flutter (30929): #152 ProxyElement.update (package:flutter/src/widgets/framework.dart:4987:5)
I/flutter (30929): #153 _InheritedNotifierElement.update (package:flutter/src/widgets/inherited_notifier.dart:183:11)
I/flutter (30929): #154 Element.updateChild (package:flutter/src/widgets/framework.dart:3314:15)
I/flutter (30929): #155 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6125:14)
I/flutter (30929): #156 Element.updateChild (package:flutter/src/widgets/framework.dart:3314:15)
I/flutter (30929): #157 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4652:16)
I/flutter (30929): #158 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4800:11)
I/flutter (30929): #159 Element.rebuild (package:flutter/src/widgets/framework.dart:4343:5)
I/flutter (30929): #160 StatefulElement.update (package:flutter/src/widgets/framework.dart:4832:5)
I/flutter (30929): #161 Element.updateChild (package:flutter/src/widgets/framework.dart:3314:15)
I/flutter (30929): #162 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6125:14)
I/flutter (30929): #163 Element.updateChild (package:flutter/src/widgets/framework.dart:3314:15)
I/flutter (30929): #164 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6125:14)
I/flutter (30929): #165 Element.updateChild (package:flutter/src/widgets/framework.dart:3314:15)
I/flutter (30929): #166 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4652:16)
I/flutter (30929): #167 Element.rebuild (package:flutter/src/widgets/framework.dart:4343:5)
I/flutter (30929): #168 StatelessElement.update (package:flutter/src/widgets/framework.dart:4708:5)
I/flutter (30929): #169 Element.updateChild (package:flutter/src/widgets/framework.dart:3314:15)
I/flutter (30929): #170 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4652:16)
I/flutter (30929): #171 Element.rebuild (package:flutter/src/widgets/framework.dart:4343:5)
I/flutter (30929): #172 ProxyElement.update (package:flutter/src/widgets/framework.dart:4987:5)
I/flutter (30929): #173 Element.updateChild (package:flutter/src/widgets/framework.dart:3314:15)
I/flutter (30929): #174 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4652:16)
I/flutter (30929): #175 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4800:11)
I/flutter (30929): #176 Element.rebuild (package:flutter/src/widgets/framework.dart:4343:5)
I/flutter (30929): #177 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2730:33)
I/flutter (30929): #178 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:913:20)
I/flutter (30929): #179 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:302:5)
I/flutter (30929): #180 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1117:15)
I/flutter (30929): #181 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1055:9)
I/flutter (30929): #182 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:971:5)
I/flutter (30929): #186 _invoke (dart:ui/hooks.dart:251:10)
I/flutter (30929): #187 _drawFrame (dart:ui/hooks.dart:209:3)
I/flutter (30929): (elided 3 frames from dart:async)
I/flutter (30929):
I/flutter (30929): ════════════════════════════════════════════════════════════════════════════════════════════════════
I/flutter (30929): {-MJld0iwmyBsLs3wwp17: {icon: https://some link, title: Beauty}, -MK9sJCCMTbvdzzUQZFl: {icon: https://https://some link, title: Fun & Adventure}}
I/flutter (30929): {-MJld0iwmyBsLs3wwp17: {icon: https://, title: Beauty}, -MK9sJCCMTbvdzzUQZFl: {icon: https://some link, title: Fun & Adventure}}
I/flutter (30929): Another exception was thrown: NoSuchMethodError: The method 'map' was called on null.
打印（resBody）；
I/flutter (30929): {-MJld0iwmyBsLs3wwp17: {icon: somelink.png, title: Beauty}}
我来自数据库的 JSON 代码
{
"category" : {
"-MJld0iwmyBsLs3wwp17" : {
"icon" : "https:https://Some link",
"title" : "Beauty"
},
"-MK9sJCCMTbvdzzUQZFl" : {
"icon" : "https://Some link",
"title" : "Fun & Adventure"
}
}
}
第一部分：
class AddSubCategory extends StatefulWidget {
static const routeName = '/addsubcategory';
@override
_AddSubCategoryState createState() => _AddSubCategoryState();
}
class _AddSubCategoryState extends State<AddSubCategory> {
final _form = GlobalKey<FormState>();
String _mySelection;
Map<String, dynamic> map;
List<dynamic> data;
Future<String> getSWData() async {
final url = "some link";
var response = await http.get(url);
var resBody = json.decode(response.body) as Map<String, dynamic>;
print(resBody);
setState(() {
map = resBody;
data = map["id"];
});
print(map);
return "Sucess";
}
@override
void initState() {
this.getSWData();
super.initState();
}
下拉部分
DropdownButton(
isDense: true,
hint: new Text("Select Category"),
items: data.map((item) {
return new DropdownMenuItem(
child: new Text(item['title']),
value: item['id'].toString(),
);
}).toList(),
onChanged: (newVal) {
setState(() {
_mySelection = newVal;
});
},
value: _mySelection,
),
解决方案
该data变量不包含 的条目'id'。
这就是为什么data设置为null，这会导致错误。