Using a plugin with a Flutter web app

1. Introduction

Flutter is Google's UI toolkit for building beautiful, natively compiled applications for mobile, web, and desktop from a single codebase. In this codelab, you'll finish an app that reports the number of stars on a GitHub repository. You'll use Dart DevTools to do some simple debugging. You'll learn how to host your app on Firebase. Finally, you'll use a Flutter plugin to launch the app and open the hosted privacy policy.

What you'll learn

• How to use a Flutter plugin in a web app
• The difference between a package and a plugin
• How to debug a web app using Dart DevTools
• How to host an app on Firebase

Prerequisites: This codelab assumes that you have some basic Flutter knowledge. If you are new to Flutter, you might want to first start with Write your first Flutter app on the web.

What would you like to learn from this codelab?

I'm new to the topic, and I want a good overview. I know something about this topic, but I want a refresher. I'm looking for example code to use in my project. I'm looking for an explanation of something specific.

2. What is a Flutter plugin?

A plugin (also called a plugin package) is a specialized Dart package that contains an API written in Dart code combined with one or more platform-specific implementations. Plugin packages can be written for Android (using Kotlin or Java), iOS (using Swift or Objective-C), web (using Dart), macOS (using Dart), or any combination thereof. (In fact, Flutter supports federated plugins, which allow support for different platforms to be split across packages.)

A package is a Dart library that you can use to extend or simplify your app's functionality. As previously mentioned, a plugin is a type of a package. For more information about packages and plugins, see Flutter Plugin or Dart Package?

3. Set up your Flutter environment

You need three pieces of software to complete this lab: the Flutter SDK, an editor, and the Chrome browser. You can use your preferred editor, such as Android Studio or IntelliJ with the Flutter and Dart plugins installed, or Visual Studio Code with the Dart Code and Flutter extensions. You will debug your code using Dart DevTools on Chrome.

Note: The code for this codelab uses a plugin, so you can't use DartPad.

4. Create the starter app

For this codelab, we provide much of the starting code so that you can quickly get to the interesting bits.

Create a simple, templated Flutter app.

Create a Flutter project called star_counter as follows.

$ flutter create star_counter
$ cd star_counter

Update project dependences. Remove the cupertino_icons dependency, as this project isn't targeting iOS.

$ flutter pub remove cupertino_icons
Resolving dependencies...
  async 2.8.1 (2.8.2 available)
  characters 1.1.0 (1.2.0 available)
  matcher 0.12.10 (0.12.11 available)
  test_api 0.4.2 (0.4.5 available)
  vector_math 2.1.0 (2.1.1 available)
These packages are no longer being depended on:
- cupertino_icons 1.0.3
Changed 1 dependency!

Add the flutter_markdown dependency.

$ flutter pub add flutter_markdown
Resolving dependencies...
+ args 2.3.0
  async 2.8.1 (2.8.2 available)
  characters 1.1.0 (1.2.0 available)
+ flutter_markdown 0.6.6
+ markdown 4.0.0
  matcher 0.12.10 (0.12.11 available)
  test_api 0.4.2 (0.4.5 available)
  vector_math 2.1.0 (2.1.1 available)
Changed 3 dependencies!

Add the github dependency.

$ flutter pub add github
Resolving dependencies...
  async 2.8.1 (2.8.2 available)
  characters 1.1.0 (1.2.0 available)
+ github 8.1.3
+ http 0.13.4
+ http_parser 4.0.0
+ json_annotation 4.1.0
  matcher 0.12.10 (0.12.11 available)
  test_api 0.4.2 (0.4.5 available)
  vector_math 2.1.0 (2.1.1 available)
Changed 4 dependencies!

Finally, add the intl dependency.

$ flutter pub add intl
Resolving dependencies...
  async 2.8.1 (2.8.2 available)
  characters 1.1.0 (1.2.0 available)
+ intl 0.17.0
  matcher 0.12.10 (0.12.11 available)
  test_api 0.4.2 (0.4.5 available)
  vector_math 2.1.0 (2.1.1 available)
Changed 1 dependency!

Fetch the updated dependencies. Click the Pub get button in your IDE or, at the command line, run flutter pub get from the top of the project.

Replace the contents of lib/main.dart. Delete all of the code from lib/main.dart, which creates a Material-themed, count-the-number-of-button-presses app. Add the following code, which sets up a not-yet-complete, count-the-number-of-stars-on-a-GitHub-repo app:

lib/main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(StarCounterApp());
}

class StarCounterApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        brightness: Brightness.light,
      ),
      routes: {
        '/': (context) => HomePage(),
      },
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  String _repositoryName = "";

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: ConstrainedBox(
          constraints: const BoxConstraints(maxWidth: 400),
          child: Card(
            child: Padding(
              padding: const EdgeInsets.all(16.0),
              child: Column(
                mainAxisSize: MainAxisSize.min,
                crossAxisAlignment: CrossAxisAlignment.center,
                children: [
                  Text(
                    'GitHub Star Counter',
                    style: Theme.of(context).textTheme.headline4,
                  ),
                  TextField(
                    decoration: const InputDecoration(
                      labelText: 'Enter a GitHub repository',
                      hintText: 'flutter/flutter',
                    ),
                    onSubmitted: (text) {
                      setState(() {
                        _repositoryName = text;
                      });
                    },
                  ),
                  Padding(
                    padding: const EdgeInsets.only(top: 32.0),
                    child: Text(
                      _repositoryName,
                    ),
                  ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}

Run the app. Run the app on Chrome. If you're using an IDE, then first select Chrome from the device pulldown. If you're using the command line, then from the top of the package, run flutter run -d chrome. (If flutter devices shows that you have web configured but no other connected devices, then the flutter run command defaults to Chrome.)

Chrome launches, and you should see something like the following:

Enter some text into the text field followed by pressing Return. The text you typed is displayed at the bottom of the window.

5. Add the GitHub star counter

Next, instead of displaying the text that was entered in the form, " google/flutter.widgets", you modify the app to show the number of stars for that repo.

Create a new file in lib called star_counter.dart:

lib/star_counter.dart

import 'package:flutter/material.dart';
import 'package:github/github.dart';
import 'package:intl/intl.dart' as intl;

class GitHubStarCounter extends StatefulWidget {
  /// The full repository name, e.g. torvalds/linux
  final String repositoryName;

  const GitHubStarCounter({
    required this.repositoryName,
  });

  @override
  _GitHubStarCounterState createState() => _GitHubStarCounterState();
}

class _GitHubStarCounterState extends State<GitHubStarCounter> {
  // The GitHub API client
  late GitHub github;

  // The repository information
  Repository? repository;

  // A human-readable error when the repository isn't found.
  String? errorMessage;

  @override
  void initState() {
    super.initState();
    github = GitHub();

    fetchRepository();
  }

  @override
  void didUpdateWidget(GitHubStarCounter oldWidget) {
    super.didUpdateWidget(oldWidget);

    // When this widget's [repositoryName] changes,
    // load the Repository information.
    if (widget.repositoryName == oldWidget.repositoryName) {
      return;
    }

    fetchRepository();
  }

  Future<void> fetchRepository() async {
    setState(() {
      repository = null;
      errorMessage = null;
    });

    if (widget.repositoryName.isNotEmpty) {
      var repo = await github.repositories
          .getRepository(RepositorySlug.full(widget.repositoryName));
      setState(() {
        repository = repo;
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    final textTheme = Theme.of(context).textTheme;
    final textStyle = textTheme.headline4?.apply(color: Colors.green);
    final errorStyle = textTheme.bodyText1?.apply(color: Colors.red);
    final numberFormat = intl.NumberFormat.decimalPattern();

    if (errorMessage != null) {
      return Text(errorMessage!, style: errorStyle);
    }

    if (widget.repositoryName.isNotEmpty && repository == null) {
      return const Text('loading...');
    }

    if (repository == null) {
      // If no repository is entered, return an empty widget.
      return const SizedBox();
    }

    return Text(
      numberFormat.format(repository!.stargazersCount),
      style: textStyle,
    );
  }
}

Observations

• The star counter uses the github Dart package to query GitHub for the number of stars a repo earned.
• You can find packages and plugins on pub.dev.
• You can also browse and search packages for a particular platform. If you select FLUTTER from the landing page, then on the next page, select WEB. This brings up all of the packages that run on the web. Either browse through the pages of packages, or use the search bar to narrow your results.
• The Flutter community contributes packages and plugins to pub.dev. If you look at the page for the github package, you'll see that it works for pretty much any Dart or Flutter app, including WEB.
• You might pay particular attention to packages that are marked as Flutter Favorites. The Flutter Favorites program identifies packages that meet specific criteria, such as feature completeness and good runtime behavior.
• Later, you add a plugin from pub.dev to this example.

Add the following import to main.dart:

lib/main.dart

import 'star_counter.dart';

Use the new GitHubStarCounter widget.

In main.dart, replace the Text widget (lines 60-62) with the 3 new lines that define the GitHubStarCounterWidget:

lib/main.dart

Padding(
  padding: const EdgeInsets.only(top: 32.0),
  child: GitHubStarCounter(              // New
    repositoryName: _repositoryName,     // New
  ),                                     // New
),

Run the app.

Hot restart the app by clicking the Run button again in the IDE (without first stopping the app), clicking the hot restart button in the IDE , or by typing r in the console. This updates the app without refreshing the browser.

Note: Hot reload doesn't yet work on the web. When running a web app from the command line, r performs a hot restart. For more information, see Hot reload on flutter.dev.

The window looks the same as before. Enter an existing repo, such as the one suggested: flutter/flutter. The number of stars is reported below the text field, for example:

6. Debugging

Are you ready for a debugging exercise? In the running app, enter a non-existent repo, such as foo/bar. The widget is stuck saying "Loading...". You fix that now.

Launch Dart DevTools.

You may be familiar with Chrome DevTools, but to debug a Flutter app, you'll want to use Dart DevTools. Dart DevTools was designed to debug and profile Dart and Flutter apps. There are a number of ways to launch Dart DevTools, depending on your workflow. The following pages have instructions about how to install and launch DevTools:

• Launch DevTools from Android Studio or IntelliJ.
• Launch DevTools from VS Code.
• Launch DevTools from the command line.

Bring up the debugger.

The initial browser page you see when Dart DevTools launches can be different, depending on how it was launched. Click the Debugger tab , to bring up the debugger.

Bring up the star_counter.dart source code.

In the Libraries text field, in the lower left, enter star_counter" Double-click the package:star_counter/star_counter.dart entry from the results list, to open it in the File view.

Set a breakpoint.

Find the following line in the source: var repo = await github.repositories. It should be on line 52. Click to the left of the line number, and a circle appears, indicating that you set a breakpoint. The breakpoint also appears in the Breakpoints list on the left. On the upper right, select the Break on exceptions checkbox. The UI should look like the following:

Run the app.

Enter a non-existent repository and press Return. In the error pane, below the code pane, you'll see that the github package threw a "repository not found" exception:

Error: GitHub Error: Repository Not Found: /
    at Object.throw_ [as throw] (http://localhost:52956/dart_sdk.js:4463:11)
    at http://localhost:52956/packages/github/src/common/xplat_common.dart.lib.js:1351:25
    at github.GitHub.new.request (http://localhost:52956/packages/github/src/common/xplat_common.dart.lib.js:10679:13)
    at request.next (<anonymous>)
    at http://localhost:52956/dart_sdk.js:37175:33
    at _RootZone.runUnary (http://localhost:52956/dart_sdk.js:37029:58)
    at _FutureListener.thenAwait.handleValue (http://localhost:52956/dart_sdk.js:32116:29)
    at handleValueCallback (http://localhost:52956/dart_sdk.js:32663:49)
    at Function._propagateToListeners (http://localhost:52956/dart_sdk.js:32701:17)
    at _Future.new.[_completeWithValue] (http://localhost:52956/dart_sdk.js:32544:23)
    at async._AsyncCallbackEntry.new.callback (http://localhost:52956/dart_sdk.js:32566:35)
    at Object._microtaskLoop (http://localhost:52956/dart_sdk.js:37290:13)
    at _startMicrotaskLoop (http://localhost:52956/dart_sdk.js:37296:13)
    at http://localhost:52956/dart_sdk.js:32918:9

Catch the error.

In star_counter.dart, find the following code (lines 52-58):

if (widget.repositoryName.isNotEmpty) {
  var repo = await github.repositories
      .getRepository(RepositorySlug.full(widget.repositoryName));
  setState(() {
    repository = repo;
  });
}

Replace that code with code that uses a try-catch block, to behave more gracefully by catching the error and printing a message:

lib/star_counter.dart

if (widget.repositoryName.isNotEmpty) {
  try {
    var repo = await github.repositories
        .getRepository(RepositorySlug.full(widget.repositoryName));
    setState(() {
      repository = repo;
    });
  } on RepositoryNotFound {
    setState(() {
      repository = null;
      errorMessage = '${widget.repositoryName} not found.';
    });
  }
}

Hot restart the app.

In DevTools, the source code is updated to reflect the changes. Once again, enter a non-existent repo. You should see the following:

7. Add a privacy policy

In this step you'll add a privacy policy page to your app. At first, you will embed the privacy policy text in your Dart code.

Add a lib/privacy_policy.dart file. In the lib directory, add a privacy_policy.dart file to your project:

lib/privacy_policy.dart

import 'package:flutter/widgets.dart';
import 'package:flutter_markdown/flutter_markdown.dart';

class PrivacyPolicy extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Markdown(
      data: _privacyPolicyText,
    );
  }
}

// The source for this privacy policy was generated by
// https://app-privacy-policy-generator.firebaseapp.com/
var _privacyPolicyText = '''
## Privacy Policy

Flutter Example Company built the Star Counter app as an Open Source app. This SERVICE is provided by Flutter Example Company at no cost and is intended for use as is.

This page is used to inform visitors regarding our policies with the collection, use, and disclosure of Personal Information if anyone decided to use our Service.

If you choose to use our Service, then you agree to the collection and use of information in relation to this policy. The Personal Information that we collect is used for providing and improving the Service. We will not use or share your information with anyone except as described in this Privacy Policy.

The terms used in this Privacy Policy have the same meanings as in our Terms and Conditions, which is accessible at Star Counter unless otherwise defined in this Privacy Policy.
''';

Add the following import to main.dart:

lib/main.dart

import 'privacy_policy.dart';

Add a new route (page) for the privacy policy.

After line 17, add the route for the privacy policy page:

lib/main.dart

routes: {
  '/': (context) => HomePage(),
  '/privacypolicy': (context) => PrivacyPolicy(),  // NEW
},

Add a button to display the privacy policy.

In the _HomePageState's build() method, add a TextButton to the bottom of the Column, after line 65:

lib/main.dart

TextButton(
  style: ButtonStyle(
    foregroundColor: MaterialStateProperty.all(Colors.blue),
    overlayColor: MaterialStateProperty.all(Colors.transparent),
  ),
  onPressed: () => Navigator.of(context).pushNamed('/privacypolicy'),
  child: const Text('Privacy Policy'),
),

Run the app.

Hot restart the app. It now has a Privacy Policy link at the bottom of the screen:

Click the Privacy Policy button.

Note that the privacy policy displays, and the URL changes to /privacypolicy.

Go back.

Use the browser's Back button to return to the first page. You get this behavior for free.

Tip: Because Flutter has only one Navigator object, you don't have to write any code to make the browser's Back button work. The Navigator manages Flutter's routes (sometimes referred to as screens or pages) inside a stack. Flutter's navigation is integrated into the browser's navigation, and the Back button just works.

8. Host with Firebase

The advantage of a hosted page is that you can change that page, without releasing a new version of your app.

From the command line, at the root of the project, use the following instructions:

Install the Firebase CLI.

Log in to Firebase to authenticate using firebase login.

Initialize a Firebase project using firebase init.

Use the following values:

• Which Firebase features? Hosting
• Project setup: Create a new project
• What project name? [yourname]-my-flutter-app (must be unique)
• What to call your project? Press Return to accept the default (which is the same as the name used in the previous question).
• What public directory? build/web (This is important.)
• Configure as a single page app? Yes
• Set up automatic builds and deploys with GitHub? No

At the command line, you'll see something like the following after you finish running firebase init:

At the completion of the init command, the following files are added to your project:

• firebase.json, the configuration file
• .firebaserc, containing your project data

Make sure that the public field in your firebase.json specifies build/web, for example:

firebase.json

{
  "hosting": {
    "public": "build/web",    # This is important!
    "ignore": [
      "firebase.json",
      "**/.*",
      "**/node_modules/**"
    ],
    "rewrites": [
      {
        "source": "**",
        "destination": "/index.html"
      }
    ]
  }
}

Build a release version of your app.

Configure your IDE to build a release version of your app using one of the following approaches:

• In Android Studio or IntelliJ, specify --release in the Additional arguments field of the Run > Edit Configuration dialog. Then, run your app.
• At the command line, run flutter build web --release.

Confirm that this step worked by examining the build/web directory of your project. The directory should contain a number of files, including index.html.

Deploy your app.

At the command line, run firebase deploy from the top of the project to deploy the contents of the public build/web directory. This shows the URL where it's hosted, https://project-id>.web.app.

In the browser, go to https://<project-id>.web.app or https://<project-id>.web.app/#/privacypolicy to see the running version of your privacy policy.

9. Use an HTML file for the privacy policy

Next, instead of embedding the privacy policy in the Dart code, you'll host it as an HTML page using Firebase.

Delete privacy_policy.dart.

Remove the file from the lib directory of your project.

Update main.dart.

In lib/main.dart, remove the import statement import privacy_policy.dart and the /privacypolicy route from StarCounterApp.

Add privacy_policy.html.

Place this file in the web directory of your project.

web/privacy_policy.html

<!doctype html>

<html lang="en">
<head>
  <meta charset="utf-8">

  <title>Privacy Policy</title>
</head>

<body>
<h2 id="privacy-policy">Privacy Policy</h2>
<p>Flutter Example Company built the Star Counter app as an Open Source app. This SERVICE is provided by Flutter Example Company at no cost and is intended for use as is.</p>
<p>This page is used to inform visitors regarding our policies with the collection, use, and disclosure of Personal Information if anyone decided to use our Service.</p>
<p>If you choose to use our Service, then you agree to the collection and use of information in relation to this policy. The Personal Information that we collect is used for providing and improving the Service. We will not use or share your information with anyone except as described in this Privacy Policy.</p>
<p>The terms used in this Privacy Policy have the same meanings as in our Terms and Conditions, which is accessible at Star Counter unless otherwise defined in this Privacy Policy.</p>
</body>
</html>

10. Use the url_launcher package

Next, you use the url_launcher plugin to open the privacy policy in a new tab.

Flutter web apps are Single Page Apps (SPAs). This is why using the standard routing mechanism in our web example opens the privacy policy in the same web page. The URL launcher plugin, on the other hand, opens a new tab in the browser, launches another copy of the app, and routes the app to the hosted page.

Add a dependency.

Add the url_launcher dependency:

$ flutter pub add url_launcher
Resolving dependencies...
  async 2.8.1 (2.8.2 available)
  characters 1.1.0 (1.2.0 available)
+ flutter_web_plugins 0.0.0 from sdk flutter
+ js 0.6.3
  matcher 0.12.10 (0.12.11 available)
+ plugin_platform_interface 2.0.2
  test_api 0.4.2 (0.4.5 available)
+ url_launcher 6.0.12
+ url_launcher_linux 2.0.2
+ url_launcher_macos 2.0.2
+ url_launcher_platform_interface 2.0.4
+ url_launcher_web 2.0.4
+ url_launcher_windows 2.0.2
  vector_math 2.1.0 (2.1.1 available)
Changed 9 dependencies!

Fetch the new dependency.

Stop the app, because adding a dependency requires a full restart of the app. Click the Pub get button in your IDE or, at the command line, run flutter pub get from the top of the project.

Add the following import to main.dart:

lib/main.dart

import 'package:url_launcher/url_launcher.dart';

Update the TextButton's handler.

Also in main.dart, replace the code that is called when the user presses the Privacy Policy button. The original code (on line 71 and 72) uses Flutter's normal routing mechanism:

onPressed: () =>
    Navigator.of(context).pushNamed('/privacypolicy'),

The new code for onPressed calls the url_launcher package:

lib/main.dart

onPressed: () => launch(
  '/privacy_policy.html',
  enableJavaScript: true,
  enableDomStorage: true,
),

Run the app.

Click the Privacy Policy button to open the file in a new tab. The primary advantage of using the url_launcher package is that (after the privacy policy page is hosted), it works on web and mobile platforms. An additional benefit is that you can modify the hosted privacy policy page without recompiling your app.

11. Clean up

After your finish with your project, remember to clean up:

Delete your firebase project.

12. Congratulations!

Congratulations, you successfully completed the GitHub Star Counter app! You also got a small taste of Dart DevTools, which you can use to debug and profile all Dart and Flutter apps, not just web apps.

What's next?

• Try one of the other Flutter codelabs.
• Learn more about the power of Dart Devtools.

Continue learning about Flutter:

• flutter.dev: The documentation site for the Flutter project
• The Flutter Cookbook
• The Flutter API reference documentation
• Additional Flutter sample apps, with source code