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  9. pubspec.yaml

Dart & Flutter DevTools Extensions

Note: this package is under active development; more thorough documentation coming soon.

Extend Dart & Flutter‘s developer tool suite, Dart DevTools, with a custom tool for your package. DevTools’ extension framework allows you to build a tool for your Dart package that can leverage existing frameworks and utilities from DevTools (VM service connection, theming, shared widgets, utilities, etc.). When an app is connected to DevTools that depends on your package, your extension will show up in its own DevTools tab:

Example devtools extension

Follow the instructions below to get started, and use the end-to-end example for reference.

Setup your package to provide a DevTools extension

DevTools extensions must be written as Flutter web apps. This is because DevTools embeds extensions in an iFrame to display them dynamically in DevTools.

To add an extension to your Dart package, add a top-level extension directory:


Under this directory, create the following structure:


The config.yaml file contains metadata that DevTools needs in order to load the extension. Copy the config.yaml file below and fill in the approproate value for each key. The material_icon_code_point field should correspond to the codepoint value of an icon from material/icons.dart.

name: foo_package
issue_tracker: <>
version: 0.0.1
material_icon_code_point: '0xe0b1'

Now it is time to build your extension.

Build a DevTools extension

Where to put your source code

Only the pre-compiled output of your extension needs to be shipped with your pub package in order for DevTools to load it. To keep the size of your pub package small, we recommend that you develop your DevTools extension outside of your pub package. Here is the recommended package structure:

foo_package/  # formerly the repository root of your pub package
    foo_package/  # your pub package
            ...  # pre-compiled output of foo_package_devtools_extension
    foo_package_devtools_extension/  # source code for your extension

Create the extension web app

From the directory where you want your extension source code to live, run the following command, replacing foo_package_devtools_extension with `<your_package_name>_devtools_extension``:

flutter create --template app --platforms web foo_package_devtools_extension

In foo_package_devtools_extension/pubspec.yaml, add a dependency on devtools_extensions:

devtools_extensions: ^1.0.0

In lib/main.dart, place a DevToolsExtension widget at the root of your app:

import 'package:devtools_extensions/devtools_extensions.dart';

void main() {
  runApp(const FooPackageDevToolsExtension());

class FooPackageDevToolsExtension extends StatelessWidget {
  const FooPackageDevToolsExtension({super.key});

  Widget build(BuildContext context) {
    return const DevToolsExtension(
      child: FooDevToolsExtension(),

The DevToolsExtension widget automatically performs all extension initialization required to interact with DevTools. From anywhere your extension web app, you can access the globals extensionManager and serviceManager to send messages and interact with the connected app.

Utilize helper packages

Use package:devtools_app_shared for access to service managers, common widgets, DevTools theming, utilities, and more. See devtools_app_shared/example for sample usages.

Debug the extension web app

Use the Simulated DevTools Environment (recommended for development)

For debugging purposes, you will likely want to use the “simulated DevTools environment”. This is a simulated environment that allows you to build your extension without having to develop it as an embedded iFrame in DevTools. The simulated environment is enabled by an environment parameter use_simulated_environment.

Simulated devtools environment

To run your extension web app with this flag enabled, add a configuration to your launch.json file in VS code:

    "configurations": [
            "name": "foo_devtools_extension + simulated environment",
            "program": "foo_package/extension/foo_devtools_extension/lib/main.dart",
            "request": "launch",
            "type": "dart",
            "args": [

or launch your app from the command line with the added flag:

flutter run -d chrome --dart-define=use_simulated_environment=true

Use a real DevTools Environment

To use a real DevTools environment, you will need to perform a series of setup steps:

  1. Develop your extension to a point where you are ready to test your changes in a real DevTools environment. Build your flutter web app and copy the built assets from your_extension_web_app/build to your pub package's extension/devtools/build directory.

Use the build_extension command from package:devtools_extensions to help with this step.

cd your_extension_web_app &&
flutter pub get &&
dart run devtools_extensions build_and_copy \
  --source=path/to/your_extension_web_app \
  1. Prepare and run a test application that depends on your pub package. You'll need to change the pubspec.yaml dependency to be a path dependency that points to your local pub package source code. Once you have done this, run pub get, and run the application.

  2. Start DevTools:

    • If you need local or unreleased changes from DevTools, you'll need to build and run DevTools from source. See the DevTools for a guide on how to do this.

      Note: you'll need to build DevTools with the server and the front end to test extensions - see instructions.

    • If not, and if your local Dart or Flutter SDK version is >= <TODO: insert version>, you can launch the DevTools instance that was just started by running your app (either from a url printed to command line or from the IDE where you ran your test app). You can also run dart devtools from the command line.
  3. Connect your test app to DevTools if it is not connected already, and you should see a tab in the DevTools app bar for your extension. The enabled or disabled state of your extension is managed by DevTools, which is exposed from an “Extensions” menu in DevTools, available from the action buttons in the upper right corner of the screen.