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id: json['id'] as int,
title: json['title'] as String,
url: json['url'] as String,
thumbnailUrl: json['thumbnailUrl'] as String,
);
}
}<code_end>
<topic_end>
<topic_start>
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>>:
<code_start>// A function that converts a response body into a List<Photo>.
List<Photo> parsePhotos(String responseBody) {
final parsed =
(jsonDecode(responseBody) as List).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(Uri.parse('https://jsonplaceholder.typicode.com/photos'));
// Synchronously run parsePhotos in the main isolate.
return parsePhotos(response.body);
}<code_end>
<topic_end>
<topic_start>
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 get 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.
<code_start>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);
}<code_end>
<topic_end>
<topic_start>
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.As an alternate solution, check out the worker_manager or
workmanager packages for background processing.<topic_end>
<topic_start>
Complete example
<code_start>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) as List).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({super.key});
@override
Widget build(BuildContext context) {
const appTitle = 'Isolate Demo';
return const MaterialApp(
title: appTitle,
home: MyHomePage(title: appTitle),