Parse JSON in the background

By default, Dart apps do all of their work on a single thread. In many cases, this model simplifies coding and is fast enough that it does not result in poor app performance or stuttering animations, often called “jank.”

However, you might need to perform an expensive computation, such as parsing a very large JSON document. If this work takes more than 16 milliseconds, your users experience jank.

To avoid jank, you need to perform expensive computations like this in the background. On Android, this means scheduling work on a different thread. In Flutter, you can use a separate Isolate. This recipe uses the following steps:

  1. Add the http package.
  2. Make a network request using the http package.
  3. Convert the response into a list of photos.
  4. Move this work to a separate isolate.

1. Add the http package

First, add the http package to your project. The http package makes it easier to perform network requests, such as fetching data from a JSON endpoint.

dependencies:
  http: <latest_version>

2. Make a network request

This example covers how to fetch a large JSON document that contains a list of 5000 photo objects from the JSONPlaceholder REST API, using the http.get() method.

Future<http.Response> fetchPhotos(http.Client client) async {
  return client.get('https://jsonplaceholder.typicode.com/photos');
}

3. Parse and convert the JSON into a list of photos

Next, following the guidance from the Fetch data from the internet recipe, convert the http.Response into a list of Dart objects. This makes the data easier to work with.

Create a Photo class

First, create a Photo class that contains data about a photo. Include a fromJson() factory method to make it easy to create a Photo starting with a JSON object.

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

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

  factory Photo.fromJson(Map<String, dynamic> json) {
    return Photo(
      id: json['id'] as int,
      title: json['title'] as String,
      thumbnailUrl: json['thumbnailUrl'] as String,
    );
  }
}

Convert the response into a list of photos

Now, use the following instructions to update the fetchPhotos() function so that it returns a Future<List<Photo>>:

  1. Create a parsePhotos() function that converts the response body into a List<Photo>.
  2. Use the parsePhotos() function in the fetchPhotos() function.
// A function that converts a response body into a List<Photo>.
List<Photo> parsePhotos(String responseBody) {
  final parsed = jsonDecode(responseBody).cast<Map<String, dynamic>>();

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

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

  return parsePhotos(response.body);
}

4. Move this work to a separate isolate

If you run the fetchPhotos() function on a slower device, you might notice the app freezes for a brief moment as it parses and converts the JSON. This is jank, and you want to be rid of it.

You can remove the jank by moving the parsing and conversion to a background isolate using the compute() function provided by Flutter. The compute() function runs expensive functions in a background isolate and returns the result. In this case, run the parsePhotos() function in the background.

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

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

Notes on working with isolates

Isolates communicate by passing messages back and forth. These messages can be primitive values, such as null, num, bool, double, or String, or simple objects such as the List<Photo> in this example.

You might experience errors if you try to pass more complex objects, such as a Future or http.Response between isolates.

Complete example

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('https://jsonplaceholder.typicode.com/photos');

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

// A function that converts a response body into a List<Photo>.
List<Photo> parsePhotos(String responseBody) {
  final parsed = jsonDecode(responseBody).cast<Map<String, dynamic>>();

  return parsed.map<Photo>((json) => 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 Photo(
      albumId: json['albumId'] as int,
      id: json['id'] as int,
      title: json['title'] as String,
      url: json['url'] as String,
      thumbnailUrl: json['thumbnailUrl'] as String,
    );
  }
}

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

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

    return MaterialApp(
      title: appTitle,
      home: MyHomePage(title: appTitle),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final String title;

  MyHomePage({Key key, this.title}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: FutureBuilder<List<Photo>>(
        future: fetchPhotos(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({Key key, this.photos}) : super(key: key);

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

Isolate demo