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<Item>((dynamic 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

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
}

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>)));

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();

Sometimes, the problem arise for the structure of JSON and for your Pojo class.

My JSON is as follows:

{"records":[{"name":"noor ","email":"email","mobileNo":187,"feedback":"good"}]}

According to this JSON we have to create the pojo classes right way. When you paste your JSON it will give you the pojo class in this site: https://javiercbk.github.io/json_to_dart/

You have to give a class name there. With name of question I found the following two pojo classes according to the above JSON:

class Questions {
  List<Records>? records;

  Questions({this.records});

  Questions.fromJson(Map<String, dynamic> json) {
    if (json['records'] != null) {
      records = <Records>[];
      json['records'].forEach((v) {
        records!.add(new Records.fromJson(v));
      });
    }
  }

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

class Records {
  String? name;
  String? email;
  int? mobileNo;
  String? feedback;

  Records({this.name, this.email, this.mobileNo, this.feedback});

  Records.fromJson(Map<String, dynamic> json) {
    name = json['name'];
    email = json['email'];
    mobileNo = json['mobileNo'];
    feedback = json['feedback'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['name'] = this.name;
    data['email'] = this.email;
    data['mobileNo'] = this.mobileNo;
    data['feedback'] = this.feedback;
    return data;
  }
}

Now the important part: that convert that record to list. Here is the calling:

Future<List<Records>> getRecordList() async {
    return await http.get(Uri.parse(URL)).then((response) {
      print(response.body.toString());

      Questions questions = Questions.fromJson(json.decode(response.body));

     

      print(questions.records![0].feedback);

      List<Records> list_s = [];
      for (int i = 0; i < questions.records!.length; i++) {
        list_s.add(questions.records![i]);
      }

      print(list_s[0].name);

      return list_s;
    });
  }
}

And At last the UI part Is :

class _FeedbackListPageState extends State<FeedbackListPage> {
 
  List<Records> list_s = [];
  

  @override
  void initState() {
    super.initState();

    FormController().getRecordList().then((value) {
      setState(() {
        list_s = value;
      });
    });

    
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: darkGreen,
        elevation: 0.0,
        titleSpacing: 10.0,
        title: Text(widget.title),
        centerTitle: true,
        leading: InkWell(
          onTap: () {
            Navigator.pop(context);
          },
          child: const Icon(
            Icons.arrow_back_ios,
            color: Colors.black54,
          ),
        ),
      ),
      body: ListView.builder(
        itemCount: list_s.length,
        itemBuilder: (context, index) {
          return ListTile(
            title: Row(
              children: <Widget>[
                Icon(Icons.person),
                Expanded(
                  child: Text("${list_s[index].name} (${list_s[index].email})"),
                )
              ],
            ),
            subtitle: Row(
              children: <Widget>[
                Icon(Icons.message),
                Expanded(
                  child: Text(list_s[index].feedback.toString()),
                )
              ],
            ),
          );
        },
      ),
    );
  }
}

What I did to solve the problem was the following:

List<YourItemClass> items = [];

final data = jsonDecode(resp.body); //Here it goes your json array

for (dynamic item in data) {
    items.add(YourItemClass.fromMap(item));
}

return items;

Of course you need to have a fromMap() method in YourItemClass, if you do not, you can get it in quicktype.io pasting one single json instance of your class (and selecting dart language).

Simply you can typecast your date like the below.

class SubCategories {
  String? id;
  String? name;
  String? detailsIcon;
  List<SubCategoriesList>? subCategorieslist = [];

  SubCategories(this.id, this.name, this.detailsIcon, this.subCategorieslist);

  SubCategories.fromJson(Map json) {
    id = json["_id"];
    detailsIcon = json["detailsIcon"];
    name = json["name"];
    json["subCategories"]!.forEach((v) {
      subCategorieslist!.add(SubCategoriesList.fromJson(v));
    });
    subCategorieslist = subCategorieslist;
  }
}

Parse data class from json array:

import 'dart:convert';

...

final http.Response response = await http.get(uri, headers: headers);
if (response.statusCode == 200) {
      final List<dynamic> jsonArray = jsonDecode(response.body);
      final List<DataClass> data = jsonArray
                                   .map((jsonObject) => DataClass.fromJson(jsonObject))
                                   .toList(growable: false);
}