How to parse the json data and display it in Listview in Flutter?

Question

I want to parse the below data in flutter and show it in listview. i have tried lots of way but got error of _InternalLinkedHashMap' has no instance method 'map' with matching arguments like this.

How can i do this? Please Help

Json Data

{
    "success": true,
    "data": {
        "categoryList": [{
            "category_id": 4,
            "category_name": "Super Hero",
            "category_type": "Free",
            "order_number": 3,
            "category_img": "https://avatars0.githubusercontent.com/u/1?v=4",
            "thumb_img": "https://avatars0.githubusercontent.com/u/1?v=4",
            "description": "Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium, totam rem aperiam, eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae dicta sunt explicabo. Nemo enim ipsam voluptatem quia voluptas sit aspernatur aut odit aut fugit",
            "website_url": "www.superhero.com",
            "created_date": "2018-05-14 12:15:38",
            "number_of_images": "21",
            "categoryImageList": [{
                "category_name": "Super Hero",
                "images_id": 35,
                "category_id": 4,
                "image_large": "https://avatars0.githubusercontent.com/u/2?v=4",
                "thumb_img": "https://avatars0.githubusercontent.com/u/2?v=4",
                "status": "Active",
                "created_date": "2018-05-14 12:50:56"
            }]
        }],
        "ListData": [{
            "wallpaper_id": 30,
            "wallpaper_img": "https://avatars0.githubusercontent.com/u/6?v=4",
            "thumb_img": "https://avatars0.githubusercontent.com/u/6?v=4",
            "website_url": "www.Yahoo.com",
            "created_date": "2018-05-14T12:56:35.000Z"
        }]
    }
}

---

import 'dart:async';
import 'dart:convert';

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;


Future<List<Photo>> fetchPhotos(http.Client client) async {
  final response =     
  await client.get('jsondataurl');

  // Use the compute function to run parsePhotos in a separate isolate
  return compute(parsePhotos, response.body);
}

// A function that will convert a response body into a List<Photo>
List<Photo> parsePhotos(String responseBody) {
  final parsed = json.decode(responseBody);

  return parsed.map<Photo>((json) => new Photo.fromJson(json)).toList();
}

class Photo {
  final int albumId;
  final int id;
  final String title;
  final String url;
  final String thumbnailUrl;

  Photo({this.albumId, this.id, this.title, this.url, this.thumbnailUrl});

  factory Photo.fromJson(Map<String, dynamic> json) {
    return new Photo(
      albumId: json['category_id'] as int,
      id: json['order_number'] as int,
      title: json['category_name'] as String,
      url: json['category_img'] as String,
      thumbnailUrl: json['thumb_img'] as String,
    );
  }
}

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final String title;
  MyHomePage({Key key, this.title}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new FutureBuilder<List<Photo>>(
        future: fetchPhotos(new http.Client()),
        builder: (context, snapshot) {
          if (snapshot.hasError) print(snapshot.error);

          return snapshot.hasData
              ? new PhotosList(photos: snapshot.data)
              : new Center(child: new CircularProgressIndicator());
        },
      ),
    );
  }
}

class PhotosList extends StatelessWidget {
  final List<Photo> photos;

  PhotosList({Key key, this.photos}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return new GridView.builder(
      gridDelegate: new SliverGridDelegateWithFixedCrossAxisCount(
        crossAxisCount: 2,
      ),
      itemCount: photos.length,
      itemBuilder: (context, index) {
        return new Image.network(photos[index].thumbnailUrl);
      },
    );
  }
}

Answer 1

The problem is with your parsePhotos function. You have assumed that the JSON file that you are receiving is only a list of photos, but it also has other items inside it. Changing it like this will fix the issue:

List<Photo> parsePhotos(String responseBody) {
  final parsed = json.decode(responseBody);

  return (parsed["data"]["categoryList"] as List).map<Photo>((json) => 
       new Photo.fromJson(json)).toList();
}