Flutter WebAssembly Build Fails: App_links & Dart:html

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Flutter WebAssembly Build Failure: app_links and dart:html Issues

Hey guys! Ever run into a snag when building your Flutter web app? I recently hit a pretty significant roadblock with my project, and I wanted to share my experience and how I (kinda) fixed it. The issue centers around WebAssembly (Wasm), a key technology in modern web development, and its interaction with the app_links package in Flutter. Let's dive in!

The Problem: WebAssembly Dry Run and dart:html

So, the core problem? When you're trying to build a Flutter web app using the flutter build web --release command, a crucial step involves a WebAssembly dry run. This run checks your code for compatibility with Wasm, ensuring everything will play nice in the browser. Unfortunately, I encountered an error during this dry run. The culprit? The app_links plugin. Specifically, its app_links_web.dart file, which was importing dart:html.

Now, here's the kicker: the new Wasm pipeline in Flutter doesn't like dart:html. It's a no-go! Instead, plugins are expected to use the package:web library, which provides a Wasm-friendly API. This incompatibility caused the build process to halt, leaving me with a non-functional web app. It's like trying to fit a square peg in a round hole – it just doesn't work!

To be specific, the error message I received was: "Wasm dry run findings: Found incompatibilities with WebAssembly. package:app_links/src/app_links_web.dart 1:1 - dart:html unsupported (0)". This was a clear signal that something needed to be fixed. The build was failing, preventing me from deploying my web app.

Detailed Steps to Reproduce the Flutter WebAssembly Failure

If you want to experience this issue yourself, or if you're troubleshooting a similar problem, here's how to reproduce it:

  1. Set up your Flutter Project: Make sure you have a Flutter project ready to go, or create a new one. You'll want to be on a stable Flutter channel, which is generally the most reliable for production builds. I was using Flutter version 3.35.5 when I encountered this. It's a good practice to keep your Flutter SDK updated, but sometimes newer versions can introduce unforeseen issues like this.
  2. Add app_links: Include the app_links package in your pubspec.yaml file. The exact version I used was app_links: ^3.5.1. Make sure to run flutter pub get after adding this dependency to fetch the package and its dependencies. This is a crucial step; otherwise, the plugin won't be available in your project.
  3. Run the Build Command: Execute the build command flutter build web --release. This command compiles your Flutter code into a web-compatible format, including the Wasm dry run. The --release flag optimizes the build for production, which is what usually reveals these types of Wasm compatibility issues.

If everything goes as expected, you should see the same Wasm dry run failure I experienced. If you don't get the error, make sure you followed all the steps and double-check your Flutter and Dart versions.

The Environment Details

Here are the specifics of my development environment, which might help you understand the context of the issue:

  • Flutter: 3.35.5 (channel stable, revision ac4e799d23)
  • Dart: 3.9.2
  • app_links: 3.5.1
  • Platform: macOS 15.0 (Apple Silicon)

These details can be crucial when trying to pinpoint the source of a build error, as different versions and platforms can interact in unexpected ways. This environment helps to isolate the problem.

The Workaround: Vendoring and Replacing dart:html with package:web

So, how did I get things working again? Well, it involved a temporary fix to get me unblocked. My workaround was to essentially modify the app_links plugin locally. This meant copying the plugin's code into my project and making the necessary changes.

Here's what I did:

  1. Locally Vendor the Plugin: First, I copied the app_links plugin code into my project. This gives you direct control over the plugin's files. It's important to note that this is a temporary fix, as it means you'll have to maintain the changes yourself until the official package is updated.
  2. Replace dart:html: Next, I opened the app_links_web.dart file and replaced the import for dart:html with package:web. This change is vital because it makes the code compatible with Wasm.
  3. Use package:web: Within the code, I changed the usages of dart:html specific objects with package:web equivalents. For instance, the window.location.href was changed to web.window.location.href. This is because the package:web library provides a Wasm-friendly way to access web APIs.
  4. Add web Dependency: Finally, I added a dependency on web: ^1.0.0 in the plugin’s pubspec.yaml. This ensures that the package:web library is available when the plugin runs.

By making these changes, the Wasm dry run then succeeded, and the build proceeded without errors. It's important to test these changes thoroughly to make sure everything functions correctly.

Proposed Fix: Updating the app_links Plugin

The long-term solution lies in updating the app_links plugin itself. The official fix requires the developers of the plugin to update their web implementation to use package:web instead of dart:html. This change would ensure that the plugin is compatible with Flutter's Wasm builds out of the box, without requiring any manual workarounds from users like me. Specifically:

  • Replace dart:html imports: The primary change involves replacing all instances of dart:html imports with package:web.
  • Declare web dependency: The plugin's pubspec.yaml needs to declare a dependency on the package:web library.

By implementing this fix, the app_links plugin can be made Wasm-friendly, resolving the build failure and making it easier for developers to use the plugin in their Flutter web projects. This is the ideal fix because it avoids the need for developers to resort to manual workarounds. The plugin should be updated to address the Wasm compatibility issue.

Conclusion: WebAssembly and Flutter Web Development

Building web apps with Flutter is awesome, and WebAssembly is a crucial part of that. However, as this experience shows, you might hit some bumps along the road. This issue underscores the importance of staying up-to-date with Flutter's changes, understanding the implications of Wasm, and being prepared to make modifications (even temporarily) to third-party packages. My temporary workaround saved the day, and hopefully, the plugin will be updated to make things easier for everyone. Keep an eye on package updates, and you'll be able to navigate these challenges like a pro! I hope this helps you guys!