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<topic_start> |
Choosing which option to use |
Choose the auto option (default) if you are optimizing for download size on |
mobile browsers and optimizing for performance on desktop browsers.Choose the html option if you are optimizing download size over performance on |
both desktop and mobile browsers.Choose the canvaskit option if you are prioritizing performance and |
pixel-perfect consistency on both desktop and mobile browsers.<topic_end> |
<topic_start> |
Examples |
Run in Chrome using the default renderer option (auto):Build your app in release mode, using the default (auto) option:Build your app in release mode, using just the CanvasKit renderer:Run your app in profile mode using the HTML renderer: |
<topic_end> |
<topic_start>Customizing web app initialization |
You can customize how a Flutter app is initialized on the web |
using the _flutter.loader JavaScript API provided by flutter.js. |
This API can be used to display a loading indicator in CSS, |
prevent the app from loading based on a condition, |
or wait until the user presses a button before showing the app.The initialization process is split into the following stages:This page shows how to customize the behavior |
at each stage of the initialization process.<topic_end> |
<topic_start> |
Getting started |
By default, the index.html file |
generated by the flutter create command |
contains a script tag |
that calls loadEntrypoint from the flutter.js file:info Note |
In Flutter 2.10 or earlier, |
this script doesn’t support customization. |
To upgrade your index.html file to the latest version, |
see Upgrading an older project.The loadEntrypoint function calls the onEntrypointLoaded callback |
once the Service Worker is initialized, and the main.dart.js entrypoint |
has been downloaded and run by the browser. Flutter also calls |
onEntrypointLoaded on every hot restart during development.The onEntrypointLoaded callback receives an engine initializer object as |
its only parameter. Use the engine initializer to set the run-time |
configuration, and start the Flutter Web engine.The initializeEngine() function returns a Promise |
that resolves with an app runner object. The app runner has a |
single method, runApp(), that runs the Flutter app.<topic_end> |
<topic_start> |
Customizing web app initialization |
In this section, |
learn how to customize each stage of your app’s initialization.<topic_end> |
<topic_start> |
Loading the entrypoint |
The loadEntrypoint method accepts these parameters:The serviceWorker JavaScript object accepts the following properties:<topic_end> |
<topic_start> |
Initializing the engine |
As of Flutter 3.7.0, you can use the initializeEngine method to |
configure several run-time options of the Flutter web engine through a |
plain JavaScript object.You can add any of the following optional parameters:info Note |
Some of the parameters described above might have been overridden |
in previous releases by using properties in the window object. |
That approach is still supported, but displays a deprecation |
notice in the JS console, as of Flutter 3.7.0.<topic_end> |
<topic_start>Engine configuration example |
The initializeEngine method lets you pass any of the configuration |
parameters described above to your Flutter app.Consider the following example.Your Flutter app should target an HTML element with id="flutter_app" and |
use the canvaskit renderer. The resulting JavaScript code would resemble |
the following:For a more detailed explanation of each parameter, take a look at the |
“Runtime parameters” documentation section of the configuration.dart |
file of the web engine.<topic_end> |
<topic_start>Skipping this step |
Instead of calling initializeEngine() on the engine initializer (and then |
runApp() on the application runner), you can call autoStart() to |
initialize the engine with its default configuration, and then start the app |
immediately after the initialization is complete:<topic_end> |
<topic_start> |
Example: Display a progress indicator |
To give the user of your application feedback |
during the initialization process, |
use the hooks provided for each stage to update the DOM:For a more practical example using CSS animations, |
see the initialization code for the Flutter Gallery.<topic_end> |
<topic_start> |
Upgrading an older project |
If your project was created in Flutter 2.10 or earlier, |
you can create a new index.html file |
with the latest initialization template by running |
flutter create as follows.First, remove the files from your /web directory.Then, from your project directory, run the following: |
<topic_end> |
<topic_start>Display 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. This page explains these limitations |
and offers ways to work around them.<topic_end> |
<topic_start> |
Background |
This section summarizes the technologies available |
across Flutter and the web, |
on which the solutions below are based on.<topic_end> |
<topic_start> |
Images in Flutter |
Flutter offers the Image widget as well as the low-level |
dart:ui/Image class for rendering images. |
The Image widget has enough functionality for most use-cases. |
The dart:ui/Image class can be used in |
advanced situations where fine-grained control |
of the image is needed.<topic_end> |
<topic_start> |
Images on the web |
The web offers several methods for displaying images. |
Below are some of the common ones:Each option has its own benefits and drawbacks. |
For example, the built-in elements fit nicely among |
other HTML elements, and they automatically take |
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