How to parse a nested JSON dictionary (map)

Question

I am trying to read a JSON code which is compatible with a Swift program into a flutter app. The structure is like this:

{
  "tagDict" : {
    "abc" : {
      "helpHdr" : "short text1",
      "helpText" : "long text1"
    },
    "def" : {
      "helpHdr" : "short text2",
      "helpText" : "long text2"
    }
  }
}

This creates in Swift a dictionary and shall create a map in Dart of the type {key : {helpHdr, helpText}}. A variable based on this should enable label = myVariable[tag].helpHdr, or staying with the example label = myVariable["abc"].helpHdr should assign "short text1" to label

To parse nested arrays I am using this, however, no clue how to transfer this to such a nested map.

class MyClass {
  List<MySubClass> myArray;
  MyClass({
    this.myArray,
  });

  factory MyClass.fromJson(Map<String, dynamic> parsedJson){
    var list = parsedJson['myArray'] as List;
    List<MySubClass> listObject = list.map((i) => MySubClass.fromJson(i)).toList();
    return new MyClass(
      myArray: listObject,
    );
  }
}

class MySubClass {
  int id;
  String text1;
  String text2;
  MySubClass({
    this.id,
    this.text1,
    this.text2,
  });

  factory MySubClass.fromJson(Map<String, dynamic> parsedJson){
    return new MySubClass(
      id: parsedJson['id'],
      text1: parsedJson['text1'],
      text2: parsedJson['text2'],
    );
  }
}

What is wrong with what you already have? Why do you need a nested map? But if you really need a nested map you can instead of using a subclass, use a method that does the same thing as the subclass.fromJson but returns a map. — Rob
– Rob, Commented Aug 19, 2020 at 15:51
Try this. You can define how the class will look, and let the build runner take care of everything else. flutter.dev/docs/development/data-and-backend/… — David Sedlář
– David Sedlář, Commented Aug 19, 2020 at 16:03

Tipu Sultan · Accepted Answer · 2020-08-19 16:10:15Z

1

If I'm correct you want to parse your json into Data class object. If that's right then you can try this

void main() {
  List<MyClass> myClassList = new List<MyClass>();
  Map map = {
    "tagDict": {
      "abc": {"helpHdr": "short text1", "helpText": "long text1"},
      "def": {"helpHdr": "short text2", "helpText": "long text2"}
    }
  };

  map['tagDict'].forEach((key, value) {
    value['id'] = key;
    myClassList.add(MyClass.fromJson(value));
  });

  myClassList.forEach((myClass) {
    print(myClass.id);
    print(myClass.helpHdr);
    print(myClass.helpText);
    print("--------------------\n");
  });
}

class MyClass {
  String id;
  String helpHdr;
  String helpText;

  MyClass({this.id, this.helpHdr, this.helpText});

  MyClass.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    helpHdr = json['helpHdr'];
    helpText = json['helpText'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['id'] = this.id;
    data['helpHdr'] = this.helpHdr;
    data['helpText'] = this.helpText;
    return data;
  }
}

This is the Output:

abc
short text1
long text1
--------------------

def
short text2
long text2
--------------------

answered Aug 19, 2020 at 16:10

Tipu Sultan

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2 Comments

w461 Over a year ago

not sure. need to run to a meeting so will check this later in more detail. However, I want to have a dictionary of the kind that I do not have to search where id = "abc" and then print the related helpHdr but to use myClassList["abc"].helpHdr instead.

w461 Over a year ago

Though this does not provide the intended data structure, it provided me the building blocks towards the solution. Thanks!

Johny Saini · Accepted Answer · 2020-08-19 16:12:19Z

class TagRes {
TagDict tagDict;

TagRes({this.tagDict});

TagRes.fromJson(Map<String, dynamic> json) {
tagDict =
    json['tagDict'] != null ? new TagDict.fromJson(json['tagDict']) : null;
}

Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
if (this.tagDict != null) {
  data['tagDict'] = this.tagDict.toJson();
}
return data;
}
 }

class TagDict {
 Abc abc;
Abc def;

TagDict({this.abc, this.def});

 TagDict.fromJson(Map<String, dynamic> json) {
  abc = json['abc'] != null ? new Abc.fromJson(json['abc']) : null;
  def = json['def'] != null ? new Abc.fromJson(json['def']) : null;
 }

 Map<String, dynamic> toJson() {
  final Map<String, dynamic> data = new Map<String, dynamic>();
  if (this.abc != null) {
  data['abc'] = this.abc.toJson();
  }
   if (this.def != null) {
  data['def'] = this.def.toJson();
 }
  return data;
 }
 }

class Abc {
String helpHdr;
  String helpText;

  Abc({this.helpHdr, this.helpText});

  Abc.fromJson(Map<String, dynamic> json) {
   helpHdr = json['helpHdr'];
   helpText = json['helpText'];
 }

 Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['helpHdr'] = this.helpHdr;
data['helpText'] = this.helpText;
return data;
 }
}

w461 · Accepted Answer · 2020-08-19 22:48:21Z

Based on Tipu's answer, I came up with the following code which creates the intended dictionary (or map in Dart - why couldn't they stick to standard terminology like arrays etc?!)

class TaskTags {
  var tagDict = Map<String, TaskTag>();
  TaskTags({
    this.tagDict,
  });

  factory TaskTags.fromJson(Map<String, dynamic> json){
    var innerMap = json['tagDict'];
    var tagMap = Map<String, TaskTag>();
    innerMap.forEach((key, value) {
      tagMap.addAll({key: TaskTag.fromJson(value)});
    });
    return new TaskTags(
      tagDict: tagMap,
    );
  }
}

class TaskTag {
  String helpHdr;
  String helpText;
  TaskTag({
    this.helpHdr,
    this.helpText,
});

  factory TaskTag.fromJson(Map<String, dynamic> json){
    return new TaskTag(
      helpHdr: json['helpHdr'],
      helpText: json['helpText'],
    );
  }
}

This creates the following map

{"abc“ : {helpHdr: "short text1", helpText: "long text1"}, "def“ : {helpHdr: "short text2", helpText: "long text2"}}

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