Tools for Web UI

Written by Sigmund Cherem
October 2012 (Updated December 2012)

The Web UI package (Web UI for short) provides web components and templates to help you write web applications at scale. You can learn about this package in our explainer article. This article describes tools that will help you create and deploy projects that use Web UI.

The overall approach: a compiler in disguise

We use a compiler to generate efficient code for applications that use the Web UI package. You’ll sometimes invoke this compiler from the command line, but often you might not even know that it’s there.

Special support is available for both Dartium and the Dart Editor to provide you with a smooth edit/refresh cycle. Below you’ll find quick instructions to setup the Dart Editor to do background compilation, and to set up Dartium to compile your code on demand. Under the hood, the Dart Editor and Dartium will be invoking our compiler. When it’s time to deploy, you’ll then use the command-line API and then use dart2js to compile all the way to JavaScript.

Set up

Web UI is available under the web_ui pub package. Simply add a dependency to your pubspec.yaml file:

  web_ui: any

Then run pub get and you will have everything you need to get started. You can use package:web_ui/... imports in your code, and you will be able to access our compiler under packages/web_ui/dwc.dart.

Command-line API

You can invoke the compiler from the command line by invoking the dwc script on the entry point of your application. For instance, the following example shows how to compile an application under the directory web, whose entry point is app.html.

> dart --package-root=packages/ packages/web_ui/dwc.dart --out web/out/ web/app.html

The --out web/out/ option indicates that we want the generated files to be written under web/out/. If the option is not specified, the compiler will generate files in the same folder of your entry point by default. In that case, all generated files will be mangled to avoid name collisions. For instance, file names will start with a leading underscore and have double extensions (for example, _app.html.dart). When you specify an output dir, the compiler avoids mangling file names.

In our example above, the application’s main HTML file is web/out/app.html. Navigate Dartium to that file and see it run. If you don’t specify an output directory, the main file will be web/_app.html.html.

You can use dart2js to generate JavaScript code that will run in other browsers. The entry point Dart script you need to compile is app.html_boostrap.dart.

> cd web/out/
> dart2js app.html_bootstrap.dart -oapp.html_bootstrap.dart.js

Note that we made the output file name exactly the input file name followed by an extra .js extension. This makes it possible to reuse the same HTML file under web/out/app.html on any modern browsers.

Recent versions of Chrome have experimental support for shadow DOM, and our generated code can use it if it is available. This feature is not enabled by default. To turn it on, you need to:

  • Import package:web_ui/web_ui.dart and set useShadowDom = true in your app’s main script.
  • Provide the special flag --enable-experimental-webkit-features to Chrome or Dartium.

When developing with the Shadow DOM, you’ll find these two additional flags very useful: --allow-file-access-from-files and --enable-devtools-experiments. The first lets your files make HttpRequests for resources from your file system. The second will display #shadowroot in the web inspector for elements that have a ShadowRoot.

Editor background compilation

We can make the Dart Editor compile your code every time a project file is modified, so when you are ready to test your app, the compiled code will be ready to go.

The Dart Editor provides a hook that lets you execute a script whenever changes are detected on a project’s files. We provide a library that makes it really easy to create a script that automatically calls our compiler. Create a file named build.dart in the same directory as your pubspec.yaml file with the following contents:

import 'package:web_ui/component_build.dart';
import 'dart:io';

void main(List<String> args) {
  build(args, ['web/app.html']);

This script will invoke dwc on web/app.html every time the Dart Editor detects that you changed a file in your project. When you want to launch your app, you can directly launch the input HTML file web/app.html and the editor will automatically redirect to run the generated file. You can also find the generated file under web/out/app.html, and request the editor to launch it directly.

For more information about hooking into the Dart Editor build, see Build.dart and the Dart Editor Build System.