Flutter - Avoiding Jank

Last Updated : 15 Jul, 2025

Parsing large JSON files can lead to poor performance of apps and shuttering animations. If parsing and computing a large JSON file take more than 16 milliseconds, the users will experience Jank. Dart by default uses a single thread to perform these tasks, though being simple and fast for less expensive computation, it fails when the computation in large.

To avoid these janks, isolates can be used to perform all computations in a different thread in the background. In this article, we will explore the process of parsing JSON in the background.

Steps to implement Avoiding Jank in Flutter

Step 1 : Create a new flutter application

Create a new Flutter application using command Prompt. For creating a new app, write flutter create YOUR_APP_NAME and run this command.

flutter create jank_flutter

To know more about it refer this article: Creating a Simple Application in Flutter

Step 2 : Adding the Dependency

To add the dependency to the pubspec.yaml file, add shimmer as a dependency in the dependencies part of the pubspec.yaml file as shown below:

dependencies:
      http: ^1.3.0

Now run the below command in terminal.

flutter pub get

Step 3 : Importing the Dependency

Use the below line of code in the main.dart file, to import the shimmer dependency :

import 'package:http/http.dart' as http;

Step 4 : Requesting Data

We can use the http.get() method to fetch the sample data of 5000 images from JSONPlaceholder REST API as shown below:

Dart

Future<List<Photo>> fetchPhotos(http.Client client) async {
  final response = await client.get(
    Uri.parse('https://jsonplaceholder.typicode.com/albums/1/photos'),
  );
  return response;
}

Step 5 : Parsing and Converting the Data

We now have 50 photos in JSON form received from the http.response and these need to be parsed and converted into a list of Dart objects. To do show First create a Photo class as shown below.

Here we will create a Photo class that contains the JSON data as shown below:

Dart

class Photo {
  final int photo_id;
  final String photo_title;
  final String photo_thumbnailUrl;

  Photo({
    required this.photo_id,
    required this.photo_title,
    required this.photo_thumbnailUrl,
  });

  factory Photo.fromJson(Map<String, dynamic> json) {
    return Photo(
      photo_id: json['id'] as int,
      photo_title: json['title'] as String,
      // Option 1: Use the original thumbnailUrl (may not load as expected)
      // photo_thumbnailUrl: json['thumbnailUrl'] as String,

      // Option 2: Generate a new URL from Picsum using the photo id as a seed.
      photo_thumbnailUrl: 'https://picsum.photos/seed/$%7Bjson%5B'id']}/150/150',
    );
  }
}

Now update the getPhoto() function to returns a Future<List<Photo>> through the below steps:

Make a Photos_parser() function to convert the response body into a List<Photo>.
Use the Photos_parser() function in the getPhotos() function.

Dart

List<Photo> Photos_parser(String responseBody) {
  final parsed = jsonDecode(responseBody).cast<Map<String, dynamic>>();
  return parsed.map<Photo>((json) => Photo.fromJson(json)).toList();
}

Future<List<Photo>> getPhotos(http.Client client) async {
  final response = await client.get(
    Uri.parse('https://jsonplaceholder.typicode.com/albums/1/photos'),
  );

  // Run Photos_parser in a separate isolate.
  return compute(Photos_parser, response.body);
}

Step 6 : Moving work to a different Isolate.

The compute() function can be used to move the work to a separate isolate where it will be parsed and converted in the background. It runs expensive functions in a background isolate and returns the result.

Dart

Future<List<Photo>> getPhotos(http.Client client) async {
  final response = await client.get(
    Uri.parse('https://jsonplaceholder.typicode.com/albums/1/photos'),
  );

  // Run Photos_parser in a separate isolate.
  return compute(Photos_parser, response.body);
}

Complete Source Code

main.dart:

Dart

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>> getPhotos(http.Client client) async {
  final response = await client.get(
    Uri.parse('https://jsonplaceholder.typicode.com/albums/1/photos'),
  );

  // Run Photos_parser in a separate isolate.
  return compute(Photos_parser, response.body);
}

List<Photo> Photos_parser(String responseBody) {
  final parsed = jsonDecode(responseBody).cast<Map<String, dynamic>>();
  return parsed.map<Photo>((json) => Photo.fromJson(json)).toList();
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final appTitle = 'geeksforgeeks';

    return MaterialApp(
      title: appTitle,
      debugShowCheckedModeBanner: false,
      home: HomePage(title: appTitle),
    );
  }
}

class HomePage extends StatelessWidget {
  final String title;

  HomePage({super.key, required this.title});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
       backgroundColor: Colors.green,
       foregroundColor: Colors.white, 
        title: Text(title),
      ),
      body: FutureBuilder<List<Photo>>(
        future: getPhotos(http.Client()),
        builder: (context, snapshot) {
          if (snapshot.hasError) print(snapshot.error);
          return snapshot.hasData
              ? PhotosList(photos: snapshot.data!)
              : Center(child: CircularProgressIndicator());
        },
      ),
    );
  }
}

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

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

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

class Photo {
  final int photo_id;
  final String photo_title;
  final String photo_thumbnailUrl;

  Photo({
    required this.photo_id,
    required this.photo_title,
    required this.photo_thumbnailUrl,
  });

  factory Photo.fromJson(Map<String, dynamic> json) {
    return Photo(
      photo_id: json['id'] as int,
      photo_title: json['title'] as String,
      // Option 1: Use the original thumbnailUrl (may not load as expected)
      // photo_thumbnailUrl: json['thumbnailUrl'] as String,

      // Option 2: Generate a new URL from Picsum using the photo id as a seed.
      photo_thumbnailUrl: 'https://picsum.photos/seed/$%7Bjson%5B'id']}/150/150',
    );
  }
}

To know more about GridView in flutter refer this article: Flutter – GridView

Output:

Flutter | An introduction to the open source SDK by Google

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Article Tags :

Flutter - Avoiding Jank

Steps to implement Avoiding Jank in Flutter

Step 1 : Create a new flutter application

Step 2 : Adding the Dependency

Step 3 : Importing the Dependency

Step 4 : Requesting Data

Step 5 : Parsing and Converting the Data

Step 6 : Moving work to a different Isolate.

Complete Source Code

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