Question

更新：感谢@Rjulcaa回答我的地图现在是一个列表，但是该列表未显示在UI上。我转而说我需要使用FutureBuilder处理未来，因此如果有人遇到此问题，请记住将FutureBuilder添加到UI主体，并在构建器上返回要显示的小部件。

我正在制作待办事项列表教程，我正在尝试在todoList上添加JSON编码功能，然后将其保存在sharedPreferences中，并在我启动应用程序时在屏幕上检索它

这是我的TODO课

class Todo  {
Todo ({this.title,this.isDone = false});
String title;
bool isDone;


//Decode method to convert a Json String into a Dynamic object
Todo.fromJson(Map <String, dynamic> json)
: title = json ["title"],
  isDone  = json ["isDone"];
Map <String,dynamic> toJson() =>
{
       "title" : title,
       "isDone" : isDone
};
}

这是我的屏幕

class _TodoListScreenState extends State<TodoListScreen> {
List<Todo> todos = [];
@override
void initState() {
super.initState();
 _getTodoFromSharedPreferences();

}



//updates the state of the checkbox and reflects it on the UI
_toggleTodo(Todo todo, bool isChecked) {
setState(() {
  todo.isDone = isChecked;
});
}

_addTodo() async {
final todo = await showDialog<Todo>(
  context: context,
builder:(BuildContext context) { // <- Here you draw the Dialog
  return NewTodoDialog();
},
);
if (todo != null) {
  setState(() {
    todos.add(todo);
    _saveTodo(todos);
    print(todos.length);

    });
   }
   }

   _saveTodo(List<Todo> todo) async{
   final String newTodo = jsonEncode(todo);
   setSharedPreferences(newTodo);
   }



    _getTodoFromSharedPreferences () async {
    final prefs = await SharedPreferences.getInstance();
    final savedTodo = prefs.getString("savedTodo");
    if (savedTodo == null) {return null;} else {
    Map MapofTodos = jsonDecode(savedTodo);
  for (var i = 0; i < MapofTodos.length; ++i) {
    var o = MapofTodos[i];
    setState(() {
      Todo todo = Todo.fromJson(o);
      todos.add(todo);
    });


  }


}
}
 setSharedPreferences (String newTodo ) async{
 final prefs = await SharedPreferences.getInstance();
 await prefs.setString("savedTodo", newTodo );
 print(newTodo);
 }

 @override
  Widget build(BuildContext context) {
   return Scaffold(
            appBar: AppBar(backgroundColor:Colors.deepPurple[900] ,
               title: Text('Todo List')),
       body: TodoList(
         todos: todos,
         onTodoToggle: _toggleTodo,
       ),
       floatingActionButton: FloatingActionButton(
         backgroundColor: Colors.purpleAccent[700],
         child: Icon(Icons.add),
         onPressed: _addTodo,
       ),
   );
  }


  }

当我运行应用程序时，出现此错误

E/flutter ( 7476): [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] 
Unhandled Exception: type 'List<dynamic>' is not a subtype of type 
'Map<dynamic, dynamic>'
E/flutter ( 7476): #0      
_TodoListScreenState._getTodoFromSharedPreferences 
(package:flutter_todo/UI/todo_list_screen.dart:70:11)
E/flutter ( 7476): <asynchronous suspension>
E/flutter ( 7476): #1      _TodoListScreenState.initState 
(package:flutter_todo/UI/todo_list_screen.dart:29:6)
E/flutter ( 7476): #2      StatefulElement._firstBuild 
(package:flutter/src/widgets/framework.dart:3851:58)
E/flutter ( 7476): #3      ComponentElement.mount 
(package:flutter/src/widgets/framework.dart:3716:5)
E/flutter ( 7476): #4      Element.inflateWidget 
(package:flutter/src/widgets/framework.dart:2960:14)
E/flutter ( 7476): #5      Element.updateChild 
(package:flutter/src/widgets/framework.dart:2763:12)
E/flutter ( 7476): #6      SingleChildRenderObjectElement.mount 
(package:flutter/src/widgets/framework.dart:4881:14)
E/flutter ( 7476): #7      Element.inflateWidget 
(package:flutter/src/widgets/framework.dart:2960:14)
E/flutter ( 7476): #8      Element.updateChild 
(package:flutter/src/widgets/framework.dart:2763:12)
E/flutter ( 7476): #9      ComponentElement.performRebuild 
(package:flutter/src/widgets/framework.dart:3752:16)
E/flutter ( 7476): #10     Element.rebuild 
(package:flutter/src/widgets/framework.dart:3564:5)
E/flutter ( 7476): #11     ComponentElement._firstBuild 
(package:flutter/src/widgets/framework.dart:3721:5)
E/flutter ( 7476): #12     ComponentElement.mount 
(package:flutter/src/widgets/framework.dart:3716:5)
E/flutter ( 7476): #13     Element.inflateWidget 
(package:flutter/src/widgets/framework.dart:2960:14)
E/flutter ( 7476): #14     Element.updateChild 
(package:flutter/src/widgets/framework.dart:2763:12)
E/flutter ( 7476): #15     SingleChildRenderObjectElement.mount 
(package:flutter/src/widgets/framework.dart:4881:14)
E/flutter ( 7476): #16     Element.inflateWidget 
(package:flutter/src/widgets/framework.dart:2960:14)
E/flutter ( 7476): #17     Element.updateChild 
(package:flutter/src/widgets/framework.dart:2763:12)
E/flutter ( 7476): #18     SingleChildRenderObjectElement.mount 
(package:flutter/src/widgets/framework.dart:4881:14)
E/flutter ( 7476): #19     Element.inflateWidget 
(package:flutter/src/widgets/framework.dart:2960:14)
E/flutter ( 7476): #20     Element.updateChild 
(package:flutter/src/widgets/framework.dart:2763:12)
E/flutter ( 7476): #21     SingleChildRenderObjectElement.mount 
(package:flutter/src/widgets/framework.dart:4881:14)
E/flutter ( 7476): #22     Element.inflateWidget 
(package:flutter/src/widgets/framework.dart:2960:14)
E/flutter ( 7476): #23     Element.updateChild 
(package:flutter/src/widgets/framework.dart:2763:12)
E/flutter ( 7476): #24     SingleChildRenderObjectElement.mount 
(package:flutter/src/widgets/framework.dart:4881:14)
E/flutter ( 7476): #25     Element.inflateWidget 
(package:flutter/src/widgets/framework.dart:2960:14)
E/flutter ( 7476): #26     Element.updateChild 
(package:flutter/src/widgets/framework.dart:2763:12)
E/flutter ( 7476): #27     ComponentElement.performRebuild 
(package:flutter/src/widgets/framework.dart:3752:16)
E/flutter ( 7476): #28     Element.rebuild 
(package:flutter/src/widgets/framework.dart:3564:5)
E/flutter ( 7476): #29     ComponentElement._firstBuild 
(package:flutter/src/widgets/framework.dart:3721:5)
E/flutter ( 7476): #30     StatefulElement._firstBuild 
(package:flutter/src/widgets/framework.dart:3869:11)
E/flutter ( 7476): #31     ComponentElement.mount 
(package:flutter/src/widgets/framework.dart:3716:5)
E/flutter ( 7476): #32     Element.inflateWidget 
(package:flutter/src/widgets/framework.dart:2960:14)
E/flutter ( 7476): #33     Element.updateChild 
(package:flutter/src/widgets/framework.dart:2763:12)
E/flutter ( 7476): #34     ComponentElement.performRebuild 
(package:flutter/src/widgets/framework.dart:3752:16)
E/flutter ( 7476): #35     Element.rebuild 
(package:flutter/src/widgets/framework.dart:3564:5)
E/flutter ( 7476): #36     ComponentElement._firstBuild 
(package:flutter/src/widgets/framework.dart:3721:5)
E/flutter ( 7476): #37     ComponentElement.mount 
(package:flutter/src/widgets/framework.dart:3716:5)
E/flutter ( 7476): #38     Element.inflateWidget 
(package:flutter/src/widgets/framework.dart:2960:14)
E/flutter ( 7476): #39     Element.updateChild 
(package:flutter/src/widgets/framework.dart:2763:12)
E/flutter ( 7476): #40     ComponentElement.performRebuild 
(package:flutter/src/widgets/framework.dart:3752:16)
E/flutter ( 7476): #41     Element.rebuild 
(package:flutter/src/widgets/framework.dart:3564:5)
E/flutter ( 7476): #42     ComponentElement._firstBuild 
(package:flutter/src/widgets/framework.dart:3721:5)
E/flutter ( 7476): #43     StatefulElement._firstBuild 
(package:flutter/src/widgets/framework.dart:3869:11)
E/flutter ( 7476): #44     ComponentElement.mount 
(package:flutter/src/widgets/framework.dart:3716:5)
E/flutter ( 7476): #45     Ele

我试图将列表更改为地图，但未成功

除了将小部件返回屏幕之外，我在所有事情上都成功完成了。请帮帮我：c

Answer 1

我之前遇到过这个问题，因为我想将数组对象保存在共享首选项中。你可以试试看。

data have;
input Pt_ID interval_round DX $10.;
datalines;
4 1 HIV
4 1 Warts
3 1 HIV
5 2 Chlamydia
;
run;

proc sort data=have1; by Pt_Id; run;

proc transpose data=have1 out=want(drop=_NAME_);
by Pt_Id;
id Dx;
var interval_round;
run;

proc print data=want; run;

将JSON转换为字符串数组

//Save you array object as an array of Strings in Shared Preferences
 void _saveTodo(List<Todo> todo) async{
  SharedPreferences sharedPreferences = await SharedPreferences.getInstance();
  sharedPreferences.setStringList("savedTodo", _mapTodoData(todo));
 }
}

如果要获取和转换Todo对象列表中的数据

 List<String> _mapTodoData(List<dynamic> todos) async{
  try {
      var res = todos.map((v) => json.encode(v)).toList();
      return res;
    } catch (err) {
     // Just in case
      return [];
    }
 }
}

这对我有用，我真的为这个问题苦苦挣扎。

我希望这对您也有用。

Answer 2

这里您要遍历Map，这是不可能的，因为只有可能的Iterables对象是Set，List和Queue。

// Map MapofTodos = jsonDecode(savedTodo); 
//Try to debug it first first and see what type it's returning
print(jsonDecode(savedTodo).runtimeType);

Answer 3

在dart应用开发过程中，我遇到了同样的问题。 json解码技术的问题，或者可能是json转换。我已经完成了将参数传递为

的方法的创建工作

popup.showAtLocation(view, Gravity.TOP | Gravity.LEFT, p.right, p.top + 15);

!pip install --target=$'/content/gdrive/My Drive/PythonLibs/' --upgrade pycuda
!pip install --target=$'/content/gdrive/My Drive/PythonLibs/' --upgrade pydicom

未处理的异常：类型'List <dynamic>'不是类型'Map <string，dynamic =“”>'的子类型

3 个答案: