Contents

Build and release a web app

Contents

During a typical development cycle, you test an app using flutter run -d chrome (for example) at the command line. This builds a debug version of your app.

This page helps you prepare a release version of your app and covers the following topics:

Handling images on the web

The web supports the standard Image widget to display images. However, because web browsers are built to run untrusted code safely, there are certain limitations in what you can do with images compared to mobile and desktop platforms.

For more information, see Displaying images on the web.

Choosing a web renderer

By default, the flutter build and flutter run commands use the auto choice for the web renderer. This means that your app runs with the HTML renderer on mobile browsers and CanvasKit on desktop browsers. This is our recommended combination to optimize for the characteristics of each platform.

For more information, see Web renderers.

Minification

Minification is handled for you when you create a release build.

A debug build of a web app is not minified and tree shaking has not been performed.

A profile build is not minified and tree shaking has been performed.

A release build is both minified and tree shaking has been performed.

Building the app for release

Build the app for deployment using the flutter build web command. You can also choose which renderer to use by using the --web-renderer option (See Web renderers). This generates the app, including the assets, and places the files into the /build/web directory of the project.

The release build of a simple app has the following structure:

/build/web
  assets
    AssetManifest.json
    FontManifest.json
    NOTICES
    fonts
      MaterialIcons-Regular.ttf
      <other font files>
    <image files>
    packages
      cupertino_icons
        assets
          CupertinoIcons.ttf
    shaders
      ink_sparkle.frag
  canvaskit
    canvaskit.js
    canvaskit.wasm
    profiling
      canvaskit.js
      canvaskit.wasm
  favicon.png
  flutter.js
  flutter_service_worker.js
  index.html
  main.dart.js
  manifest.json
  version.json

Launch a web server (for example, python -m http.server 8000, or by using the dhttpd package), and open the /build/web directory. Navigate to localhost:8000 in your browser (given the python SimpleHTTPServer example) to view the release version of your app.

Embedding a Flutter app into an HTML page

hostElement

Added in Flutter 3.10
You can embed a Flutter web app into any HTML element of your web page, with flutter.js and the hostElement engine initialization parameter.

To tell Flutter web in which element to render, use the hostElement parameter of the initializeEngine function:

<html>
  <head>
    <!-- ... -->
    <script src="flutter.js" defer></script>
  </head>
  <body>

    <!-- Ensure your flutter target is present on the page... -->
    <div id="flutter_host">Loading...</div>

    <script>
      window.addEventListener("load", function (ev) {
        _flutter.loader.loadEntrypoint({
          onEntrypointLoaded: async function(engineInitializer) {
            let appRunner = await engineInitializer.initializeEngine({
              // Pass a reference to "div#flutter_host" into the Flutter engine.
              hostElement: document.querySelector("#flutter_host")
            });
            await appRunner.runApp();
          }
        });
      });
    </script>
  </body>
</html>

To learn more, check out Customizing web app initialization.

Iframe

You can embed a Flutter web app, as you would embed other content, in an iframe tag of an HTML file. In the following example, replace “URL” with the location of your HTML page:

<iframe src="URL"></iframe>

Deploying to the web

When you are ready to deploy your app, upload the release bundle to Firebase, the cloud, or a similar service. Here are a few possibilities, but there are many others:

PWA Support

As of release 1.20, the Flutter template for web apps includes support for the core features needed for an installable, offline-capable PWA app. Flutter-based PWAs can be installed in the same way as any other web-based PWA; the settings signaling that your Flutter app is a PWA are provided by manifest.json, which is produced by flutter create in the web directory.

PWA support remains a work in progress, so please give us feedback if you see something that doesn’t look right.