Polymerize - Polymer 2.0 Dart-DDC

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This package is a command line tool to build Polymer 2 application with Dart-DDC and Bazel.

The benefits of this approach compared to the dart2js standard polymer-1.x are :

  • support for polymer 2.0-preview (web components 1.0)
  • using DDC to generate ES6 output code
  • using bazel as build system (see also rules
  • 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)
  • 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


Actually this tool will now work only for Linux. To be used on macosx should be only a matter of changing some builtin path and this will be fixed very soon.

Bazel doesn't work on windows system for now because dart dev_compiler is not ready for windows, so... sorry guys.


This tool is actually used internally by bazel rules so you don't really need to install it (bazel will do that for you).

In theory all you have to do is start using bazel rules and enjoy your building.

But unfortunately maintaining bazel build file is not an easy task, expecially when dealing with transitive dependencies.

polymerize can be used to automatically generate and update all the bazel stuff for you from your pubspec.yaml (and resolved lock file).

With that all you have to do is to write and maitain your usual pubspec.yaml and then run polymerize init -y <path-to-root-pubspec.yaml (after installing the polymerize tool):

pub global activate polymerize  # <-- to install the tool

polymerize init -y pubspec.yaml # <-- to auto generate or update bazel build files

If you update any pubspec.yaml just launch polymerize init -y pubspec.yaml again to have your WORKSPACE and BUILDs file updated accordingly.

Develping with polymerize

Sample project

A sample project demostrating 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' show OtherComponent;

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 uses attribute of the @PolymerRegister annotation is still there only for cosmetic reasons (this avoids the annoying unused import analyzer warning when a component is imported but referenced only in the html template and not in the dart code, and also the IDE will help you adding the right import).

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 for it and use the @BowerImport annotation along with @PolymerRegister with native=true, for instance:

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


Then in the main BUILD file trigger the automatic download and installation of the corresponding JS dependencies by adding a bower rule:

  name = "main",
  resolutions = {
    "polymer": "2.0-preview",
  deps = [

This rule 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:

dart ../bin/polymerize.dart 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

The component-refs file is a yaml describing the components to analyze to create the stub (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, ready to be used


  • more polymer APIs
  • better convert to/from JS
  • annotations for properties (computed props, etc.)
  • support for mixins
  • ~~support for external element wrappers~~
  • ~~support for auto gen HTML imports~~