Saving a class within a class - Flutter

Question

I have class "DilemmData" which I need to save:

class DilemmData {
     double percent = 0;
     final String title;
     final String date;
     List<DilemmItemData> plus = [];
     List<DilemmItemData> minus = [];

     DilemmData(
       plus,
       minus, {
       this.percent = 0,
       this.title = '',
       this.date = '',
     });

     static Map<String, dynamic> toJson(DilemmData dilemm) {
       return {
         'percent': dilemm.percent,
         'title': dilemm.title,
         'date': dilemm.date,
         'plus': dilemm.plus.map((e) => e.toJson()).toList(),
         'minus': dilemm.minus.map((e) => e.toJson()).toList(),
       };
     }

     factory DilemmData.fromJson(Map<String, dynamic> json) {
       return DilemmData(
         json['plus'],
         json['minus'],
         percent: json['percent'],
         title: json['title'],
         date: json['date'],
       );
     }

     static String encode(List<DilemmData> pressure) => json.encode(
           pressure
              .map<Map<String, dynamic>>((dil) => DilemmData.toJson(dil))
              .toList(),
     );

     static List<DilemmData> decode(String dilemm) =>
         ((json.decode(dilemm) ?? []) as List<dynamic>)
              .map<DilemmData>((dil) => DilemmData.fromJson(dil))
              .toList();
      }

    class DilemmItemData {
        final int importance;
        final String argument;

        DilemmItemData({this.importance = 0, this.argument = ''});

        Map<String, dynamic> toJson() {
          return {
             'importance': importance,
             'argument': argument,
          };
        }

       factory DilemmItemData.fromJson(Map<String, dynamic> json) {
            return DilemmItemData(
              importance: json['importance'], argument: json['argument']);
       }
   }

There is save function:

        DilemmData dilemm = DilemmData(
               percent: 0,
               title: controller.text,
               date: clockString);

        SharedPreferences sharedPreferences =
          await SharedPreferences.getInstance();

          String data = jsonEncode(dilemm);
          sharedPreferences.setString('dilemms', data);

But when i try to save i get this error: JsonUnsupportedObjectError (Converting object to an encodable object failed: Instance of 'DilemmData'). Does anyone know how to save?

StackOverflow says I have a lot of code, but I don't know what else to write, I'll write random letters for this: shdjfhjjsdfjhwehfwouiehuwefuwefuwheugweghuweghuiweghuwueghuweweugihwueighuwhguwhgu

Here is the function to get the data:

  Future loadDillems() async {
      SharedPreferences sharedPreferences = await 
          SharedPreferences.getInstance();

      if (sharedPreferences.containsKey('dilemms')) {
            Map<String, dynamic> data =
            jsonDecode(sharedPreferences.getString('dilemms')!);
            MyApp.dilemmList = DilemmData.fromJson(data) as List<DilemmData>;
      }
   }

Robert Sandberg · Accepted Answer · 2022-08-22T11:37:03.463

1

When converting the "top" class using its toJson() method, make sure that the subsequent classes' toJson() methods is also called. As such:

'plus': dilemm.plus.map((e) => e.toJson()).toList(),

And you of course have to implement toJson() and fromJson() in DilemmItemData as well.

Edit:

Okey, so a couple of alternatives. First of all, you had to create the new toJson() (and soon also fromJson()) methods. That is good.

As @Ivo wrote, if you pass the object to jsonEncode as you do now, then the methods cannot be static. But you could also pass DilemmData.toJson(dilemm) as:

String data = jsonEncode(DilemmData.toJson(dilemm));

But I'd recommend as @Ivo wrote to make it non-static, as you did with the new toJson method, and of course keep the toJson in DilemmItemData, and I suggest that you write the fromJson as well...

Second edit:

Your fromJson have to be a bit sharper. Something like this:

DilemmData(
  (json['plus'] as List<dynamic>)
          .map((e) => DilemmItemData.fromJson(e as Map<String, dynamic>))
          .toList(),

Edit 3:

You are saving one DilemmData, so you have to read it as one (1) DilemmData. Remove the as List<DilemmData>; It is not all of a sudden a List when it is one object that you save (String data = jsonEncode(dilemm);)

So do as follows:

final oneDilemmData = DilemmData.fromJson(data);

edited Aug 22 '22 at 11:37

answered Aug 22 '22 at 10:28

Robert Sandberg

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still the same error :( – Alexandr Aug 22 '22 at 10:38
Then you'd have to update your code with the new toJson and fromJson, and show us what `dilemm` is declared as in `String data = jsonEncode(dilemm);` – Robert Sandberg Aug 22 '22 at 10:43
I have fromJson in two classes – Alexandr Aug 22 '22 at 11:12
Sorry, missed it! – Robert Sandberg Aug 22 '22 at 11:14
but now i got a new error when i want to get the data: https://imgur.com/a/UmJvwl3 – Alexandr Aug 22 '22 at 11:19
Updated answer again. – Robert Sandberg Aug 22 '22 at 11:25
And how to make it so that it can be saved and retrieved as a list? – Alexandr Aug 22 '22 at 13:08
First things first. Have you successfully stored and read the data as one object? – Robert Sandberg Aug 22 '22 at 13:17
@ RobertSandberg, yes – Alexandr Aug 22 '22 at 13:18
Okey. Then I'd like you to accept this answer (and give a vote up) to close this question. And then create a new question. That way things are kept apart and will not be an endless neverending question and answer stream :) This question was "saving a class within a class". Not "saving a list of classes with classes within". – Robert Sandberg Aug 22 '22 at 13:20
Ps. Maybe you can just search SO without creating a new question. Perhaps look at this: https://stackoverflow.com/questions/61316208/how-to-save-listobject-to-sharedpreferences-in-flutter – Robert Sandberg Aug 22 '22 at 13:23
I did everything as in the selected example and I get this exception: _CastError (type '_InternalLinkedHashMap' is not a subtype of type 'List' in the cast) – Alexandr Aug 22 '22 at 13:38
As I said, create a new specific question for that specific error. Only add relevant code to the new question. – Robert Sandberg Aug 22 '22 at 13:39

score 0 · Answer 2 · answered Aug 22 '22 at 10:51

I think you need to make your toJson() non static like this:

Map<String, dynamic> toJson() {
    return {
        'percent': percent,
        'title': title,
        'date': date,
        'plus': plus,
        'minus': minus,
    };
}

your encode could be like this then:

static String encode(List<DilemmData> pressure) => json.encode(
    pressure
        .map<Map<String, dynamic>>((dil) => dil.toJson())
        .toList(),
);

Saving a class within a class - Flutter

2 Answers2

Edit:

Second edit:

Edit 3: