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Flutter 3.0 - Better web-app lifecycles

It finally happened, version 3.0 of Flutter is here!!

One of the most exiting things for me is, that we can now better control web-app lifecycles. This post will give an overview of this new feature. If you want to see a more general changelog, check out this post, or this other one by @frezyx

We can also customize how a Flutter app is initialized on the web using the _flutter.loader JavaScript API. This API can be used to display a loading indicator, prevent the app from loading based on a condition, or wait until the user presses a button before showing the app.

This was a big missing piece for building web apps with Flutter. At least for me.

We can now:

  • Add a splash screen
  • Add a loading indicator, while loading resources.
  • Add a plain HTML interactive landing page displayed before the Flutter app.

The initialization process now looks like this:

  • Loading the entrypoint script
    • Loads the main.dart.js script and initializes the service worker.
  • Initializing the Flutter engine
    • Starts Flutter’s web engine by downloading required resources.
  • Running the app
    • Prepares the DOM for your Flutter app and runs it.

Let's see an example, which adds a loading indicator:

      <!-- ... -->
      <script src="flutter.js" defer></script>
      <div id="loading"></div>
         window.addEventListener('load', function(ev) {
           var loading = document.querySelector('#loading');
           loading.textContent = "Loading entrypoint...";
             serviceWorker: {
               serviceWorkerVersion: serviceWorkerVersion,
           }).then(function(engineInitializer) {
             loading.textContent = "Initializing engine...";
             return engineInitializer.initializeEngine();
           }).then(function(appRunner) {
             loading.textContent = "Running app...";
             return appRunner.runApp();
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This post is just an overview on the new lifecycle for web apps, for an in-depth guide, check out Customizing web app initialization

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