Unhandled Exception: type 'List<dynamic>' is not a subtype of type 'Map<String, dynamic>'

Question

UPDATE: Thanks to @Rjulcaa answer My map is now a List, however the List wasn't displayed on the UI. I turns that I needed to handle the future with a FutureBuilder, so in case anyone faces this problem remember to add a FutureBuilder to the body of your UI and on the builder return the widget you want to be displayed.

I'm making a todo list tutorial, I'm trying to add the feature of encoding on a JSON my todoList the save it on a sharedPreferences, and when I start the app retrieve it on the screen

This is my TODO class

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
};
}

This is my screen

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,
       ),
   );
  }


  }

when I run the app i get this error

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

I tried to change the List to a Map but didn't succeed

I have succeeded on everything except returning the widget to the screen. Help me, please :c

try this var todo = Todo.fromJson(o);

Rubens Melo
– Rubens Melo

2019-05-08 19:07:23 +00:00
Commented May 8, 2019 at 19:07 — Rubens Melo
– Rubens Melo, Commented May 8, 2019 at 19:07

Rjulcaa · Accepted Answer · 2019-05-11 01:50:07Z

0

I faced this problem before because I wanted to save an array object in shared preferences. You can try this.

//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));
 }
}

Transform the JSON into an array of Strings

 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 [];
    }
 }
}

if you want to get and transform the data in a List of Todo objects

void loadData() async {

 SharedPreferences sharedPreferences = await SharedPreferences.getInstance();
 final todoArray = _decodeTodoData(sharedPreferences.getStringList("savedTodo"))
 //This is your array :) -> todoArray    
}
List<Todo> _decodeTodoData(List<String> todos) {
    try {
      //Transforming List<String> to Json
      var result = todos.map((v) => json.decode(v)).toList();
      //Transforming the Json into Array<Todo>
      var todObjects = result.map((v) => Todo.fromJson(v)).toList();
      return todObjects;
    } catch (error) {
      return [];
    }
  }
}

This worked for me, I really struggled with this problem.

I Hope this works for you too.

edited May 11, 2019 at 1:50

answered May 8, 2019 at 20:35

Rjulcaa

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1 Comment

Alexis Sanchez Over a year ago

Hi! thanks for the quick and detailed response, now I'm handling a different problem, I can't use de saveTodo method it tells me ` [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: type 'Future<dynamic>' is not a subtype of type 'List<String>' _TodoListScreenState._saveTodo (package:flutter_todo/UI/todo_list_screen.dart:60:50) and the codeline is this one _mapTodoData(todo)); }`

Nadeem Siddique · Accepted Answer · 2019-05-09 15:13:35Z

0

Here you're trying to iterate over a Map which is not possible because only possible Iterables objects are Set,List and Queue.

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

answered May 9, 2019 at 15:13

Nadeem Siddique

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Comments

Sachin Mishra · Accepted Answer · 2020-04-18 13:56:19Z

0

I faced the same issue during app development in dart. The issue with json decoding techniques or might be json conversion. I have done work around for creating the method as passing the parameter as

getCardData(data[0]["cardContent"][0]["cardData"] as List),

  List<String> getCardData(data) {
    List<String> _cardInfo = [];
    for (int i = 0; i < data.length; i++) {
      _cardInfo.add(data[i] as String);
    }
    return _cardInfo;
  }

answered Apr 18, 2020 at 13:56

Sachin Mishra

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