Flutter HTTP Client example with ListView – Fetch data and parse JSON in background

https://grokonez.com/flutter/flutter-http-client-example-listview-fetch-data-parse-json-background

Flutter HTTP Client example with ListView – Fetch data and parse JSON in background

In this tutorial, we're gonna build a Flutter App that use http package to fetch data from the internet, then parse JSON to a Dart List of Objects and display that List in ListView widget.

Related Post: Flutter ListView example with ListView.builder

Flutter App Overview

We will build a Flutter App that can display a List of Post objects gotten from JSONPlaceholder REST API. We are parsing JSON document that performs an expensive computation in the background.

HTTP Fetch Data in background and Display in ListView

Add http package

Add this package to the dependencies section of our pubspec.yaml:

dependencies:
  http: 

Create DataModel class

class Post {
  final int userId;
  final int id;
  final String title;
  final String body;

  Post({this.userId, this.id, this.title, this.body});

  factory Post.fromJson(Map json) {
    return Post(
      userId: json['userId'] as int,
      id: json['id'] as int,
      title: json['title'] as String,
      body: json['body'] as String,
    );
  }
}

Parse JSON into a List of Objects

We will convert our HTTP Response into a list of Dart objects.

List<Post> parsePosts(String responseBody) {
  final parsed = json.decode(responseBody).cast<Map<String, dynamic>>();

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

Fetch Data in a separate isolate

This is how we fetch data using the get method:

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

  // compute function to run parsePosts in a separate isolate
  return parsePosts(response.body);
}

If we run the fetchPosts() function on a slower phone, the App may freeze for a moment when it parses and converts the JSON.

So we are moving the parsing and conversion to a background isolate using Flutter compute() function. This function runs parsePosts() in a background isolate and return the result.

More at:

https://grokonez.com/flutter/flutter-http-client-example-listview-fetch-data-parse-json-background

Flutter HTTP Client example with ListView – Fetch data and parse JSON in background