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Polymerize - Polymer 2.0 Dart-DDC

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This package is a community effort to bring support for Polymer 2 and latest HTML standards to Dart (DDC).

It features :

  • support for polymer 2.0.0 (web components 1.0)
  • using DDC to generate ES6 output code
  • leverages html5, a new html lib for Dart based on js interoperability only,
  • using bazel as build system (see also rules )
  • NOW working with dart 1.24 + pub (ddc)
  • dynamic load of polymer components definitions through imd (require js implementation using html imports)
  • interoperability with other JS frameworks
  • Incremental build (dependencies are built only once, thanks to DDC modularity system and bazel build)
  • possibility to distribute ONLY the build result to thirdy party users and devs
  • simplified API
    • automatic getter and setter (no explicit notify for first level properties)
    • NO Annotations required to expose properties
    • NO Annotations required to expose methods
  • seamless integration with widely used js tools like bower


Polymerize works on every platforms where DDC runs that's MacOS and Linux for now.

The upcoming version of pub will support DDC build and polymerize will support it too thus extending the platform where you can use it.

Browser compatibility

Polymerize uses DDC and Polymer-2, this means that it will only work on modern browsers. So far only chrome and firefox have been tested but Safari should work too along with latest IE11 builds.

Eventually some "transpiling" support can be added along with some optimizing post processing (like vulcanize or similar) could be added to the build chain to broaden the compatibility range.

** UPDATE ** : after moving to ** PUB+DDC ** there's something broken with firefox , will fix soon. But I wanted to publish this release quickly after 1.24 was officially released.


Prepare a project

In order to build a project with polymerize you have to add the following transformer and dev dependencies to your pubspec:

  - polymerize: ^0.9.0
  - polymerize:
     entry-point: web/index.dart

The entry point is the main dart file. Your html file should look as this:

     <script src="bower_components/webcomponentsjs/webcomponents-loader.js"></script>
     <script type="application/dart" src="index.dart"></script>
     <script src="polymerize_require/start.js"></script>

See the demo todo project for more details.

Build a project

  1. pub build --web-compiler dartdevc (or add the necessary entries in the pubspec as well)

** DISCLAIMER ** : When using dartdevc for production build you will loose al the optimizations that dart2js normally makes to your code. This means that you will have bigger fat code even if modular and loaded only on demand.

Developing with polymerize

Sample project

A sample project illustrating how to build polymer-2 components using polymerize can be found here :

See the README for more information.

Component definition

This is a sample component definition:

import 'package:polymer_element/polymer_element.dart';
import 'package:my_component/other_component.dart';

abstract class MyTag extends PolymerElement {

  int count = 0;  // <- no need to annotate this !!!

  onClickIt(Event ev,details) {  // <- NO need to annotate this!!!!
    count = count + 1;    // <- no need to call `set` API , magical setter in action here

  void countChanged(val) {
    print("Count has changed : ${count}");

  MyTag() { // <- Use a simple constructor for created callback !!!
    print("HELLO THERE !")

  factory MyTag.tag() => Element.tag('my-tag'); // <- If you want to create it programmatically use this

  connectedCallback() {
    super.connectedCallback(); // <- super MUST BE CALLED if you override this callback (needed by webcomponents v1) !!!!

The Html template is just the usual dom-module template without any JS code. The import dependencies will generate the appropriate html imports so there is no need to add them to the template.

The index.html should preload imd, webcomponents polyfill and polymer.html (see the demo).

Importing a Bower component

To import a bower component and use it in your project simply create a stub (that can created automatically, see below) for it and use the @BowerImport annotation along with @PolymerRegister with native=true, for instance:

abstract class PaperButton extends PolymerElement implements imp0.PaperButtonBehavior {
   * If true, the button should be styled with a shadow.
  external bool get raised;
  external set raised(bool value);


During the build phase polymerize will check any @BowerImport annotation on classes of dependencies, generate a bower.json file (using resolutions if you need to override something) and then runs bower install.

You can also automatically generate a stub from the HTML polymer component using polymerize generate_wrapper, for instance:

pub run polymerize:polymerize generate-wrapper --component-refs comps.yaml --dest-path out -p polymer_elements --bower-needs-map Polymer.IronFormElementBehavior=package:polymer_elements/iron_form_element_behavior.dart

(You have to add polymerize as a dev dependency of your project).

The generator uses a yaml file describing the components to analyze passed through the component-refs options (see gen/comps.yam in this repo for an example).

The project polymerize_elements is an example of wrappers generated using this tool for the polymer-elements components.


After complilation everything will be found in the bazel output folder (bazel-bin), ready to be used.


  • more polymer APIs
  • support for mixins
  • annotations for properties (computed props, etc.)
  • support for external element wrappers
  • support for auto gen HTML imports


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