Add a new page on "Concurrency and isolates" (flutter#9777)

Adds documentation about using isolates in Flutter apps.

**Staged:**
https://flutter-docs-prod--pr9777-isolate-doc-update-ru4mbku4.web.app/perf/isolates

This will address
[6564](flutter#6564), albeit in a
different way than suggested.

---------

Co-authored-by: Brett Morgan <[email protected]>
Co-authored-by: Parker Lougheed <[email protected]>
Co-authored-by: Shams Zakhour (ignore Sfshaza) <[email protected]>

examples/development/concurrency/isolates/analysis_options.yaml

@@ -0,0 +1,10 @@
+# Take our settings from the example_utils analysis_options.yaml file.
+# If necessary for a particular example, this file can also include
+# overrides for individual lints.
+
+include: package:example_utils/analysis.yaml
+
+linter:
+  rules:
+    avoid_print: false
+    prefer_const_constructors: false

examples/development/concurrency/isolates/lib/isolate_binary_messenger.dart

@@ -0,0 +1,20 @@
+import 'dart:isolate';
+
+import 'package:flutter/services.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+void main() {
+  // Identify the root isolate to pass to the background isolate.
+  RootIsolateToken rootIsolateToken = RootIsolateToken.instance!;
+  Isolate.spawn(_isolateMain, rootIsolateToken);
+}
+
+Future<void> _isolateMain(RootIsolateToken rootIsolateToken) async {
+  // Register the background isolate with the root isolate.
+  BackgroundIsolateBinaryMessenger.ensureInitialized(rootIsolateToken);
+
+  // You can now use the shared_preferences plugin.
+  SharedPreferences sharedPreferences = await SharedPreferences.getInstance();
+
+  print(sharedPreferences.getBool('isDebug'));
+}

examples/development/concurrency/isolates/lib/main.dart

@@ -0,0 +1,71 @@
+// Copyright 2021 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:convert';
+import 'dart:isolate';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+void main() => runApp(const MyApp());
+
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Isolates demo',
+      home: Scaffold(
+        appBar: AppBar(
+          title: const Text('Isolates demo'),
+        ),
+        body: Center(
+          child: ElevatedButton(
+            onPressed: () {
+              getPhotos();
+            },
+            child: Text('Fetch photos'),
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+// #docregion isolate-run
+// Produces a list of 211,640 photo objects.
+// (The JSON file is ~20MB.)
+Future<List<Photo>> getPhotos() async {
+  final String jsonString = await rootBundle.loadString('assets/photos.json');
+  final List<Photo> photos = await Isolate.run<List<Photo>>(() {
+    final List<Object?> photoData = jsonDecode(jsonString) as List<Object?>;
+    return photoData.cast<Map<String, Object?>>().map(Photo.fromJson).toList();
+  });
+  return photos;
+}
+// #enddocregion isolate-run
+
+class Photo {
+  final int albumId;
+  final int id;
+  final String title;
+  final String thumbnailUrl;
+
+  Photo({
+    required this.albumId,
+    required this.id,
+    required this.title,
+    required this.thumbnailUrl,
+  });
+
+  factory Photo.fromJson(Map<String, dynamic> data) {
+    return Photo(
+      albumId: data['albumId'] as int,
+      id: data['id'] as int,
+      title: data['title'] as String,
+      thumbnailUrl: data['thumbnailUrl'] as String,
+    );
+  }
+}

examples/development/concurrency/isolates/pubspec.yaml

@@ -0,0 +1,17 @@
+name: isolates
+description: Sample isolate code
+
+version: 1.0.0+1
+
+environment:
+  sdk: ^3.1.0
+
+dependencies:
+  flutter:
+    sdk: flutter
+  shared_preferences: any
+
+dev_dependencies:
+  example_utils:
+    path: ../../../example_utils
+

src/_data/sidenav.yml

@@ -512,6 +512,8 @@
       permalink: /perf/shader
     - title: Performance metrics
       permalink: /perf/metrics
+    - title: Concurrency and isolates
+      permalink: /perf/isolates
     - title: Performance FAQ
       permalink: /perf/faq
     - title: Appendix

src/cookbook/networking/background-parsing.md

@@ -289,7 +289,7 @@ class PhotosList extends StatelessWidget {
 
 ![Isolate demo]({{site.url}}/assets/images/docs/cookbook/isolate.gif){:.site-mobile-screenshot}
 
-[`compute()`]: {{site.api}}/flutter/foundation/compute-constant.html
+[`compute()`]: {{site.api}}/flutter/foundation/compute.html
 [Fetch data from the internet]: {{site.url}}/cookbook/networking/fetch-data
 [`http`]: {{site.pub-pkg}}/http
 [`http.get()`]: {{site.pub-api}}/http/latest/http/get.html