How to Deserialize a list of objects from json in flutter

I am using the dart package json_serializable for json serialization. Looking at the flutter documentation it shows how to deserialize a single object as follow:

Future<Post> fetchPost() async {
  final response =
  await http.get('https://jsonplaceholder.typicode.com/posts/1');

  if (response.statusCode == 200) {
  // If the call to the server was successful, parse the JSON
  return Post.fromJson(json.decode(response.body));
  } else {
    // If that call was not successful, throw an error.
    throw Exception('Failed to load post');
  }
}

However, I am not familiar enough with dart to figure out how to do the same for a list of items instead of a single instance.

Well, your service would handle either the response body being a map, or a list of maps accordingly. Based on the code you have, you are accounting for 1 item.

If the response body is iterable, then you need to parse and walk accordingly, if I am understanding your question correctly.

Example:

Iterable l = json.decode(response.body);
List<Post> posts = List<Post>.from(l.map((model)=> Post.fromJson(model)));

where the post is a LIST of posts.

EDIT: I wanted to add a note of clarity here. The purpose here is that you decode the response returned. The next step, is to turn that iterable of JSON objects into an instance of your object. This is done by creating fromJson methods in your class to properly take JSON and implement it accordingly. Below is a sample implementation.

class Post {
  // Other functions and properties relevant to the class
  // ......
  /// Json is a Map<dynamic,dynamic> if i recall correctly.
  static fromJson(json): Post {
    Post p = new Post()
    p.name = ...
    return p
  }
}

I am a bit abstracted from Dart these days in favor of a better utility for the tasks needing to be accomplished. So my syntax is likely off just a little, but this is Pseudocode.

I always use this way with no problem;

List<MyModel> myModels;
var response = await http.get("myUrl");

myModels=(json.decode(response.body) as List).map((i) =>
              MyModel.fromJson(i)).toList();

You can also Do it like

  List<dynamic> parsedListJson = jsonDecode("your json string");
  List<Item> itemsList = List<Item>.from(parsedListJson.map((i) => Item.fromJson(i)));

where Item is your custom class, where you implemented toJson and fromJson.

Just another example on JSON Parsing for further clarification.

Let say we want to parse items array in our JSON Object.

factory YoutubeResponse.fromJSON(Map<String, dynamic> YoutubeResponseJson) 
 {

// Below 2 line code is parsing JSON Array of items in our JSON Object (YouttubeResponse)


var list = YoutubeResponseJson['items'] as List;
List<Item> itemsList = list.map((i) => Item.fromJSON(i)).toList();

return new YoutubeResponse(
    kind: YoutubeResponseJson['kind'],
    etag: YoutubeResponseJson['etag'],
    nextPageToken: YoutubeResponseJson['nextPageToken'],
    regionCode: YoutubeResponseJson['regionCode'],
    mPageInfo: pageInfo.fromJSON(YoutubeResponseJson['pageInfo']),

    // Here we are returning parsed JSON Array.

    items: itemsList);

  }

First, create a class that matches your json data, in my case, I create (generate) class named Img:

import 'dart:convert';

Img imgFromJson(String str) => Img.fromJson(json.decode(str));
String imgToJson(Img data) => json.encode(data.toJson());

class Img {
    String id;
    String img;
    dynamic decreption;

    Img({
        this.id,
        this.img,
        this.decreption,
    });

    factory Img.fromJson(Map<String, dynamic> json) => Img(
        id: json["id"],
        img: json["img"],
        decreption: json["decreption"],
    );

    Map<String, dynamic> toJson() => {
        "id": id,
        "img": img,
        "decreption": decreption,
    };
}

PS. You can use app.quicktype.io to generate your json data class in dart. then send your post/get to your server:

Future<List<Img>> _getimages() async {
    var response = await http.get("http://192.168.115.2/flutter/get_images.php");
    var rb = response.body;

    // store json data into list
    var list = json.decode(rb) as List;

    // iterate over the list and map each object in list to Img by calling Img.fromJson
    List<Img> imgs = list.map((i)=>Img.fromJson(i)).toList();

    print(imgs.runtimeType); //returns List<Img>
    print(imgs[0].runtimeType); //returns Img

    return imgs;
}

for more info see Parsing complex JSON in Flutter

Follow these steps:

1. Create a model class (named as LoginResponse): click here to convert json to dart .
   

2. LoginResponce loginResponce=LoginResponce.fromJson(json.decode(response.body));

3. Now you get your data in instence of model (as loginResponce ).

This is my Model class -

  class SuggestedMovie {
  String title;
  String genres;
  int movieId;
  SuggestedMovie({this.title, this.genres, this.movieId});
  factory SuggestedMovie.fromJson(Map<dynamic, dynamic> parsedJson) {
    return SuggestedMovie(
      movieId: parsedJson['movieId'],
      title: parsedJson['title'] as String,
      genres: parsedJson['genres'] as String,
    );
  }
}

The one below is the code for Deserializing the JSON response into List

 suggestedMovie = (json.decode(jsonResponse.data) as List)
      .map((i) => SuggestedMovie.fromJson(i))
      .toList();

With strong-mode enabled none of the above solutions will actually compile as type information is missing.

This compiles as of dart 2.14 with strong mode enabled:

analysis_options.yaml

include: package:lints/recommended.yaml

analyzer:
  strong-mode:
    implicit-casts: false
    implicit-dynamic: false

    import 'dart:convert';

    // read the json file. This example use dcli but you just
    // need [source] to contain the json string.
    var source = dcli.read(_failedTrackerFilename).toParagraph();

    var l = json.decode(source) as Iterable;
    var failures = List<UnitTest>.from(l.map<UnitTest>(
          (dynamic i) => UnitTest.fromJson(i as Map<String, dynamic>)));

Full example

Here a full example of JSON serialization and deserialization for Object, List<Object>. Here we have a Main class which is composed by a nested Sub class and a List<Sub>.

Json sample

{
    "title": "something",
    "sub": {"name": "a", "num": 0},
    "sub_list": [
      {"name": "b", "num": 1},
      {"name": "c", "num": 2}
    ]
}

Main class

class Main {
  String title;
  Sub sub;
  List<Sub> subList;

  Main(this.title, this.sub, this.subList);

  Main.fromJson(Map<String, dynamic> json)
      : title = json['title'],
        sub = Sub.fromJson(json['sub']),
        subList = List<dynamic>.from(json['sub_list'])
            .map((i) => Sub.fromJson(i))
            .toList();

  Map<String, dynamic> toJson() => {
        'title': title,
        'sub': sub.toJson(),
        'sub_list': subList.map((item) => item.toJson()).toList(),
      };
}

Sub class

class Sub {
  String name;
  int n;

  Sub(this.name, this.n);

  Sub.fromJson(Map<String, dynamic> json)
      : name = json['name'],
        n = json['n'];

  Map<String, dynamic> toJson() => {
        'name': name,
        'n': n,
      };
}

usage

void main(List<String> args) {
  var sub = Sub("a", 0);
  print(sub.name); // a

  Map<String, dynamic> jsonSub = {"name": "a", "n": 0};
  var subFromJson = Sub.fromJson(jsonSub);
  print(subFromJson.n); // 0

  var main = Main("something", Sub("a", 0), [Sub("b", 1)]);
  print(main.title); // something
  print(main.sub.name); // a
  print(main.subList[0].name); // b

  var jsonMain = {
    "title": "something",
    "sub": {"name": "a", "n": 0},
    "sub_list": [
      {"name": "b", "n": 1},
      {"name": "c", "n": 2}
    ]
  };
  var mainFromJson = Main.fromJson(jsonMain);
  print(mainFromJson.title); // something
  print(mainFromJson.sub.name); // a
  print(mainFromJson.subList[0].name); // b
  print(mainFromJson.subList[1].name); // c
}

>> How to Deserialize a list of objects from json in flutter

import 'package:json_helpers/json_helpers.dart';

void main() {
  // String request.body
  final body = '[{"name": "Jack"}, {"name": "John"} ]';
  final personList = body.jsonList((e) => Person.fromJson(e));
  assert(personList[1].name == 'John');
  print(personList[1].name);
}

class Person {
  final String name;

  Person({required this.name});

  factory Person.fromJson(Map<String, dynamic> json) {
    return Person(
      name: json['name'] as String,
    );
  }
}