Parse JSON in the background
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(Uri.parse('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;
const Photo({
required this.albumId,
required this.id,
required this.title,
required this.url,
required 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(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
const appTitle = 'Isolate Demo';
return const MaterialApp(
title: appTitle,
home: MyHomePage(title: appTitle),
);
}
}
class MyHomePage extends StatelessWidget {
const MyHomePage({Key? key, required this.title}) : super(key: key);
final String title;
@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) {
return const Center(
child: Text('An error has occurred!'),
);
} else if (snapshot.hasData) {
return PhotosList(photos: snapshot.data!);
} else {
return const Center(
child: CircularProgressIndicator(),
);
}
},
),
);
}
}
class PhotosList extends StatelessWidget {
const PhotosList({Key? key, required this.photos}) : super(key: key);
final List<Photo> 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].thumbnailUrl);
},
);
}
}