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Adding Flutter to any web application

Contents

Flutter views and web content can be composed to produce a web application in different ways. Choose one of the following depending on your use-case:

Full page mode

#

In full page mode, the Flutter web application takes control of the whole browser window and covers its viewport completely when rendering.

This is the default embedding mode for new Flutter web projects, and no additional configuration is needed.

html
<!DOCTYPE html>
<html>
  <head>
  </head>
  <body>
    <script src="flutter_bootstrap.js" defer></script>
  </body>
</html>

When Flutter web is launched without referencing multiViewEnabled or a hostElement, it uses full page mode.

To learn more about the flutter_bootstrap.js file, check out Customize app initialization.

iframe embedding

#

Full page mode is recommended when embedding a Flutter web application through an iframe. The page that embeds the iframe can size and position it as needed, and Flutter will fill it completely.

html
<iframe src="https://url-to-your-flutter/index.html"></iframe>

To learn more about the pros and cons of an iframe, check out the Inline Frame element docs on MDN.

Embedded mode

#

Flutter web applications can also render content into an arbitrary number of elements (commonly divs) of another web application; this is called "embedded mode" (or "multi-view").

In this mode:

  • A Flutter web application can launch, but doesn't render until the first "view" is added, with addView.
  • The host application can add or remove views from the embedded Flutter web application.
  • The Flutter application is notified when views are added or removed, so it can adjust its widgets accordingly.

Enable multi-view mode

#

Enable multi-view mode setting multiViewEnabled: true in the initializeEngine method as shown:

flutter_bootstrap.js
js
{{flutter_js}}
{{flutter_build_config}}

_flutter.loader.load({
  onEntrypointLoaded: async function onEntrypointLoaded(engineInitializer) {
    let engine = await engineInitializer.initializeEngine({
      multiViewEnabled: true, // Enables embedded mode.
    });
    let app = await engine.runApp();
    // Make this `app` object available to your JS app.
  }
});

Manage Flutter views from JS

#

To add or remove views, use the app object returned by the runApp method:

js
// Adding a view...
let viewId = app.addView({
  hostElement: document.querySelector('#some-element'),
});

// Removing viewId...
let viewConfig = app.removeView(viewId);

Handling view changes from Dart

#

View additions and removals are surfaced to Flutter through the didChangeMetrics method of the WidgetsBinding class.

The complete list of views attached to your Flutter app is available through the WidgetsBinding.instance.platformDispatcher.views iterable. These views are of type FlutterView.

To render content into each FlutterView, your Flutter app needs to create a View widget. View widgets can be grouped together under a ViewCollection widget.

The following example, from the Multi View Playground, encapsulates the above in a MultiViewApp widget that can be used as the root widget for your app. A WidgetBuilder function runs for each FlutterView:

multi_view_app.dart
dart
import 'dart:ui' show FlutterView;
import 'package:flutter/widgets.dart';

/// Calls [viewBuilder] for every view added to the app to obtain the widget to
/// render into that view. The current view can be looked up with [View.of].
class MultiViewApp extends StatefulWidget {
  const MultiViewApp({super.key, required this.viewBuilder});

  final WidgetBuilder viewBuilder;

  @override
  State<MultiViewApp> createState() => _MultiViewAppState();
}

class _MultiViewAppState extends State<MultiViewApp> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
    _updateViews();
  }

  @override
  void didUpdateWidget(MultiViewApp oldWidget) {
    super.didUpdateWidget(oldWidget);
    // Need to re-evaluate the viewBuilder callback for all views.
    _views.clear();
    _updateViews();
  }

  @override
  void didChangeMetrics() {
    _updateViews();
  }

  Map<Object, Widget> _views = <Object, Widget>{};

  void _updateViews() {
    final Map<Object, Widget> newViews = <Object, Widget>{};
    for (final FlutterView view in WidgetsBinding.instance.platformDispatcher.views) {
      final Widget viewWidget = _views[view.viewId] ?? _createViewWidget(view);
      newViews[view.viewId] = viewWidget;
    }
    setState(() {
      _views = newViews;
    });
  }

  Widget _createViewWidget(FlutterView view) {
    return View(
      view: view,
      child: Builder(
        builder: widget.viewBuilder,
      ),
    );
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return ViewCollection(views: _views.values.toList(growable: false));
  }
}

For more information, check out WidgetsBinding mixin in the API docs, or the Multi View Playground repo that was used during development.

Replace runApp by runWidget in Dart

#

Flutter's runApp function assumes that there's at least one view available to render into (the implicitView), however in Flutter web's multi-view mode, the implicitView doesn't exist anymore, so runApp will start failing with Unexpected null value errors.

In multi-view mode, your main.dart must call the runWidget function instead. It doesn't require an implicitView, and will only render into the views that have been explicitly added into your app.

The following example uses the MultiViewApp described above to render copies of the MyApp() widget on every FlutterView available:

main.dart
dart
void main() {
  runWidget(
    MultiViewApp(
      viewBuilder: (BuildContext context) => const MyApp(),
    ),
  );
}

Identifying views

#

Each FlutterView has an identifier assigned by Flutter when attached. This viewId can be used to uniquely identify each view, retrieve its initial configuration, or decide what to render in it.

The viewId of the rendered FlutterView can be retrieved from its BuildContext like this:

dart
class SomeWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // Retrieve the `viewId` where this Widget is being built:
    final int viewId = View.of(context).viewId;
    // ...

Similarly, from the viewBuilder method of the MultiViewApp, the viewId can be retrieved like this:

dart
MultiViewApp(
  viewBuilder: (BuildContext context) {
    // Retrieve the `viewId` where this Widget is being built:
    final int viewId = View.of(context).viewId;
    // Decide what to render based on `viewId`...
  },
)

Read more about the View.of constructor.

Initial view configuration

#

Flutter views can receive any initialization data from JS when starting up. The values are passed through the initialData property of the addView method, as shown:

js
// Adding a view with initial data...
let viewId = app.addView({
  hostElement: someElement,
  initialData: {
    greeting: 'Hello, world!',
    randomValue: Math.floor(Math.random() * 100),
  }
});

In Dart, the initialData is available as a JSAny object, accessible through the top-level views property in the dart:ui_web library. The data is accessed through the viewId of the current view, as shown:

dart
final initialData = ui_web.views.getInitialData(viewId) as YourJsInteropType;

To learn how to define the YourJsInteropType class to map the initialData object passed from JS so it's type-safe in your Dart program, check out: JS Interoperability on dart.dev.

View constraints

#

By default, an embedded Flutter web view considers the size of its hostElement as an immutable property, and tightly constrains its layout to the available space.

On the web, it's common for the intrinsic size of an element to affect the layout of the page (like img or p tags that can reflow content around them).

When adding a view to Flutter web, you might configure it with constraints that inform Flutter of how the view needs to be laid out:

js
// Adding a view with initial data...
let viewId = app.addView({
  hostElement: someElement,
  viewConstraints: {
    maxWidth: 320,
    minHeight: 0,
    maxHeight: Infinity,
  }
});

The view constraints passed from JS need to be compatible with the CSS styling of the hostElement where Flutter is being embedded. For example, Flutter won’t try to "fix" contradictory constants like passing max-height: 100px in CSS, but maxHeight: Infinity to Flutter.

To learn more, check out the ViewConstraints class, and Understanding constraints.

Custom element (hostElement)

#

Between Flutter 3.10 and 3.24
You can embed a single-view Flutter web app into any HTML element of your web page.

To tell Flutter web which element to render into, pass an object with a config field to the _flutter.loader.load function that specifies a HTMLElement as the hostElement.

js
_flutter.loader.load({
  config: {
    hostElement: document.getElementById('flutter_host'),
  }
});

To learn more about other configuration options, check out Customizing web app initialization.

Unless stated otherwise, the documentation on this site reflects the latest stable version of Flutter. Page last updated on 2024-11-12. View source or report an issue.