目录

Web 渲染器

目录

Flutter web offers two build modes, and two renderers. A build mode is chosen when building the app and it determines which of the two renderers are available in the app, and how a renderer is chosen. A renderer is chosen at runtime when the app is being launched, and it determines the set of web technologies used to render the UI.

The two build modes are: default mode, and WebAssembly mode.

The two renderers are: canvaskit and skwasm.

Renderers

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Rendering UI in Flutter begins in the framework with widgets and render objects. Once processed, render objects generate a Scene object made of layers. The scene is then passed to the Flutter engine that turns it into pixels. All of the framework, including all widgets and custom app code, and much of the engine are written in Dart. However, a big part of the engine is written in C++, which includes Skia, as well as custom Flutter engine code. Multiple options are available on the web for how to compile Dart and C++, what graphics system to use to convert UI into pixels, how to split the workload across threads, etc.

Flutter web does not offer all possible combinations of options. Instead, it provides just two bundles of carefully chosen combinations.

canvaskit

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When using the canvaskit renderer, the Dart code is compiled to JavaScript, and the UI is rendered on the main thread into WebGL. It is compatible with all modern browsers. It includes a copy of Skia compiled to WebAssembly, which adds about 1.5MB in download size.

skwasm

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When using skwasm the Dart code is compiled to WebAssembly. Additionally, if the hosting server meets the SharedArrayBuffer security requirements, Flutter will use a dedicated web worker to offload part of the rendering workload to a separate thread, taking advantage of multiple CPU cores. This renderer includes a more compact version of Skia compiled to WebAssembly, adding about 1.1MB in download size.

Build modes

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Default build mode

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The default mode is used when flutter run and flutter build web commands are used without passing --wasm or by passing --no-wasm. This build mode only uses the canvaskit renderer.

WebAssembly build mode

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This mode is enabled by passing --wasm to flutter run and flutter build web commands.

This mode makes both skwasm and canvaskit available. skwasm requires wasm garbage collection, which is not yet supported by all modern browsers. Therefore, at runtime Flutter will pick skwasm if garbage collection is supported, and fallback to canvaskit if not. This allows apps compiled in the WebAssembly mode to still run in all modern browsers.

The --wasm flag is not supported by non-web platforms.

Choosing a renderer at runtime

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By default, when building in WebAssembly mode, Flutter will decide when to use skwasm, and when to fallback to canvaskit. This can be overridden by passing a configuration object to the loader, as follows:

  1. Build the app with the --wasm flag to make both skwasm and canvaskit renderers available to the app.
  2. Set up custom web app initialization as described in Write a custom flutter_bootstrap.js.
  3. Prepare a configuration object with the renderer property set to "canvaskit" or "skwasm".
  4. Pass your prepared config object as the config property of a new object to the _flutter.loader.load method that is provided by the earlier injected code.

Example:

html
<body>
  <script>
    {{flutter_js}}
    {{flutter_build_config}}

    // TODO: Replace this with your own code to determine which renderer to use.
    const useCanvasKit = true;

    const config = {
      renderer: useCanvasKit ? "canvaskit" : "skwasm",
    };
    _flutter.loader.load({
      config: config,
    });
  </script>
</body>

The web renderer can't be changed after calling the load method. Therefore, any decisions about which renderer to use, must be made prior to calling _flutter.loader.load.

Choosing which build mode to use

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Compiling Dart to WebAssembly comes with a few new requirements that must be met by all app code, and all plugins and packages used by the app:

  • The code must only use the new JS interop library dart:js_interop. Old-style dart:js, dart:js_util, and package:js are no longer supported.
  • Code using Web APIs must use the new package:web instead of dart:html.
  • WebAssembly implements Dart's numeric types int and double exactly the same as the Dart VM. In JavaScript these types are emulated using the JS Number type. It is possible that your code accidentally or purposefully relies on the JS behavior for numbers. If so, such code needs to be updated to behave correctly with the Dart VM behavior.

General recommendations can be summarized as follows:

  • Choose the default mode if your app relies on plugins and packages that do not yet support WebAssembly.
  • Choose the WebAssembly mode if your app's code and packages are compatible with WebAssembly and app performance is important. skwasm has noticeably better app start-up time and frame performance compared to canvaskit. skwasm is particularly effective in multi-threaded mode, so consider configuring the server such that it meets the SharedArrayBuffer security requirements.

Examples

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Run in Chrome using the default build mode:

flutter run -d chrome

Build your app for release using the default build mode:

flutter build web

Build your app for release using the WebAssembly mode:

flutter build web --wasm

Run your app for profiling using the default build mode:

flutter run -d chrome --profile

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