Flutter - Converting Json map of a map to List<Object> in flutter

Question

I am trying to de-serialize json in a parent class. One of my items is a Map of a Map, which I'd like to de-serialize into a Types class. I am struggling with parsing this because of the nested map in a map and I'm unclear on the proper syntax. At the end of the day I want a List in the parent class. The items within the types json block are dynamic, so there could be a type of critical, notice, etc. with varying descriptions.

Json sample:

{
   "types":{
      "alert":{
         "description":"Action item."
      },
      "question":{
         "description":"Select an applicable response."
      },
      "info":{
         "description":"This is an information message, no action required."
      }
   }
}

Types class:

class Types {
  final String name;
  final String description;

  Types({
    required this.name,
    required this.description,
  });

  factory Types.fromJson(String id, Map<String, dynamic> json) {
    return Types(
      name: id,
      description: json['description'] == null ? '' : json['description'],
    );
  }
}

Parent class:

class Parent {
  final String id;
  final String name;
  final String description;
  final Features features;
  final List<Types> types;
  final List<String> users;

  Parent({
    required this.id,
    required this.name,
    required this.description,
    required this.features,
    required this.types,
    required this.users,
  });

  factory Parent.fromJson( Map<String, dynamic> json) {
    return Parent(
      id: json['id'] == null ? '' : json['id'],
      name: json['name'] == null ? '' : json['name'],
      description: json['description'] == null ? '' : json['description'],
      features: json['features'] == null
          ? Features()
          : Features.fromJson(json['features']),
      types: json['types'] == null ? [] : // ??? How to deserialize Map{Map{}} ,
      users: json['users'] == null ? [] : List<String>.from(json['users']),
    );
  }
}

Any and all help is appreciated. Likewise if there is a better way to store this data I am open to that. The types class allows me to add future fields to it if necessary.

Thank you.

mmcdon20 · Accepted Answer · 2021-06-08 22:12:38Z

2

The general idea is to loop over each key/value pair in json['types'] and create an instance of Types for each one.

import 'dart:convert';

void main(List<String> args) {
  final json = jsonDecode('''
  {
    "id": "test id",
    "name": "test name",
    "description": "test description",
    "types":{
      "alert":{
         "description":"Action item."
      },
      "question":{
         "description":"Select an applicable response."
      },
      "info":{
         "description":"This is an information message, no action required."
      }
    }
  }
  ''');
  print(Parent.fromJson(json));
}

class Parent {
  final String id;
  final String name;
  final String description;
  final List<Types> types;
  // ignoring features and users fields

  Parent({
    required this.id,
    required this.name,
    required this.description,
    required this.types,
  });

  // prefer initializer lists to factory constructors
  // when you are only creating instances of the same class
  //
  // also prefer: 
  //     json['id'] ?? ''
  // to:
  //     json['id'] == null ? '' : json['id']
  // 
  Parent.fromJson(Map<String, dynamic> json)
      : id = json['id'] ?? '',
        name = json['name'] ?? '',
        description = json['description'] ?? '',
        // json['types'] is a map so you will have to loop
        // over each of its entries (key/value pairs) and
        // instantiate a new Types class for each entry.
        types = [
          for (final entry in (json['types'] ?? {}).entries)
            Types.fromJson(entry.key, entry.value),
        ];

  @override
  String toString() =>
      'Parent(id: $id, name: $name, description: $description, types: $types)';
}

class Types {
  final String name;
  final String description;

  Types({
    required this.name,
    required this.description,
  });

  Types.fromJson(String id, Map<String, dynamic> json)
      : name = id,
        description = json['description'] ?? '';

  @override
  String toString() => 'Types(name: $name, description: $description)';
}

answered Jun 8, 2021 at 22:12

mmcdon20

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2 Comments

Booyah Over a year ago

This makes sense, Thank you! I was trying to do everything with .map and having issues parsing the deeper map.

mmcdon20 Over a year ago

It would certainly be possible to do it by calling .map() on the entries such as (json['types'] as Map<String, dynamic>?)?.entries.map((entry) => Types.fromJson(entry.key, entry.value)).toList() ?? [] but I find the above approach more straightforward.

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