Access properties of nested JSON object in flutter

Question

This is a flutter/dart question, in case you want to ask.

I have the following JSONObject:

{
  "header": [
    2223,
    2224
  ],
  "2223": {
    "L": 3,
    "A": 6,
    "B": 5,
    "Mode": 15,
  },
  "2224": {
    "L": 9,
    "A": 6,
    "B": 1,
    "Mode": 16,
  }
}

First I load the 'table of contents' from the header object.

var tagsJson = jsonDecode(jsonContent)['header'];
List<String> timestamps = tagsJson != null ? List.from(tagsJson) : null;

And here is my Result Object I want to assign the values to

  class Result {


  double L;
  double A;
  double B;
  int mode;

  Result({this.L, this.A, this.B, this.mode});


  void dump()
  {
    print(this.L);
    print(this.A);
    print(this.B);
    print(this.mode);
  }
}

How can I get those values of the timestamp JSONObjects, and assign them the corresponding Result Object using flutter?

Abion47 · Accepted Answer · 2020-04-02 15:04:34Z

When you do

var tagsJson = jsonDecode(jsonContent)['header'];

You are dumping the rest of the JSON value out the window. Instead, cache the parsed JSON and refer to it separately:

final parsedJson = jsonDecode(jsonContent);
final tags = parsedJson['header'];
List<String> timestamps = tags != null ? List.from(tags) : null;

It will also be easier and cleaner if you create a fromJson factory constructor for Result:

class Result {
  ...

  factory Result.fromJson(Map<String, dynamic> jsonData) {
    double l = (jsonData['L'] ?? 0.0) as double;
    double a = (jsonData['A'] ?? 0.0) as double;
    double b = (jsonData['B'] ?? 0.0) as double;
    int mode = (jsonData['Mode'] ?? 0) as int;

    return Result(L: l, A: a, B: b, mode: mode);
  }

  ...
}

Now you can iterate through timestamps and create your Results:

List<Result> results = [];
for (var timestamp in timestamps) {
  final jsonData = parsedJson[timestamp];
  if (jsonData != null) {
    results.add(Result.fromJson(jsonData));
  }
}

Alexander Mazuruk · Accepted Answer · 2020-04-02 16:04:21Z

Quick answer is to use this library https://github.com/k-paxian/dart-json-mapper to map your json onto your Dart classes.

"B*" ? is this for a purpose, or typo? anyway here is the beautiful code for your case, no boilerplate, no pain, no loops, no logic. And a bonus you are able to dump your model back to json string in one line of code.

// Here is the result model classes

@jsonSerializable
class ResultItem {
  num L;
  num A;
  num B;
  int Mode;
}

@jsonSerializable
class Result {
  List<num> header = [];

  final Map<String, dynamic> _itemsMap = {};

  @jsonProperty
  void setItem(String name, dynamic value) {
    _itemsMap[name] = JsonMapper.fromMap<ResultItem>(value);
  }

  @jsonProperty
  Map<String, dynamic> getItemsMap() {
    return _itemsMap;
  }
}

// here is the input json

      final json = '''
      {
        "header": [2223, 2224],
        "2223": {
                  "L": 3,
                  "A": 6,
                  "B": 5,
                  "Mode": 15
                },
        "2224": {
                  "L": 9,
                  "A": 6,
                  "B": 1,
                  "Mode": 16
                }
      }
 ''';

  // Now you have an result model instance populated from json in one line of code
  final resultInstance = JsonMapper.deserialize<Result>(json);

  // Now you have a result model instance back to json in one line of code
  print(JsonMapper.serialize(resultInstance));

Console output will be

{
 "header": [
  2223,
  2224
 ],
 "2223": {
  "L": 3,
  "A": 6,
  "B": 5,
  "Mode": 15
 },
 "2224": {
  "L": 9,
  "A": 6,
  "B": 1,
  "Mode": 16
 }
}

yes, the object stores L* A* B*-Color values. I didn't want to use the asterisk in json, I must have missed it.

Vincent Guttmann · Accepted Answer · 2020-04-05 07:10:09Z

1

Create this function in your Result Object

    /// Creates a [Result] model
    /// from a valid JSON Object
    ///
    factory Result.fromJson(Map<String, dynamic> json) => Result(
        L: json['L'],
        M: json['M'],
        B: json['B'],
        mode: json['Mode],
    );

edited Apr 5, 2020 at 7:10

Vincent Guttmann

2936 silver badges23 bronze badges

answered Apr 2, 2020 at 15:03

NqbraL

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