index.js

import attributesObserver from '@webreflection/custom-elements-attributes';
import {expando} from '@webreflection/custom-elements-upgrade';
import qsaObserver from 'qsa-observer';

const {
  customElements, document,
  Element, MutationObserver, Object, Promise,
  Map, Set, WeakMap, Reflect
} = self;

const {createElement} = document;
const {define, get, upgrade} = customElements;
const {construct} = Reflect || {construct(HTMLElement) {
  return HTMLElement.call(this);
}};

const {defineProperty, getOwnPropertyNames, setPrototypeOf} = Object;

const shadowRoots = new WeakMap;
const shadows = new Set;

const classes = new Map;
const defined = new Map;
const prototypes = new Map;
const registry = new Map;

const shadowed = [];
const query = [];

const getCE = is => registry.get(is) || get.call(customElements, is);

const handle = (element, connected, selector) => {
  const proto = prototypes.get(selector);
  if (connected && !proto.isPrototypeOf(element)) {
    const redefine = expando(element);
    override = setPrototypeOf(element, proto);
    try { new proto.constructor; }
    finally {
      override = null;
      redefine();
    }
  }
  const method = `${connected ? '' : 'dis'}connectedCallback`;
  if (method in proto)
    element[method]();
};

const {parse} = qsaObserver({query, handle});

const {parse: parseShadowed} = qsaObserver({
  query: shadowed,
  handle(element, connected) {
    if (shadowRoots.has(element)) {
      if (connected)
        shadows.add(element);
      else
        shadows.delete(element);
      if (query.length)
        parseShadow.call(query, element);
    }
  }
});

// qsaObserver also patches attachShadow
// be sure this runs *after* that
const {attachShadow} = Element.prototype;
if (attachShadow)
  Element.prototype.attachShadow = function (init) {
    const root = attachShadow.call(this, init);
    shadowRoots.set(this, root);
    return root;
  };

const whenDefined = name => {
  if (!defined.has(name)) {
    let _, $ = new Promise($ => { _ = $; });
    defined.set(name, {$, _});
  }
  return defined.get(name).$;
};

const augment = attributesObserver(whenDefined, MutationObserver);

let override = null;

getOwnPropertyNames(self)
  .filter(k => /^HTML.*Element$/.test(k))
  .forEach(k => {
    const HTMLElement = self[k];
    function HTMLBuiltIn() {
      const {constructor} = this;
      if (!classes.has(constructor))
        throw new TypeError('Illegal constructor');
      const {is, tag} = classes.get(constructor);
      if (is) {
        if (override)
          return augment(override, is);
        const element = createElement.call(document, tag);
        element.setAttribute('is', is);
        return augment(setPrototypeOf(element, constructor.prototype), is);
      }
      else
        return construct.call(this, HTMLElement, [], constructor);
    }
    setPrototypeOf(HTMLBuiltIn, HTMLElement);
    defineProperty(
      HTMLBuiltIn.prototype = HTMLElement.prototype,
      'constructor',
      {value: HTMLBuiltIn}
    );
    defineProperty(self, k, {value: HTMLBuiltIn});
  });

document.createElement = function (name, options) {
  const is = options && options.is;
  if (is) {
    const Class = registry.get(is);
    if (Class && classes.get(Class).tag === name)
      return new Class;
  }
  const element = createElement.call(document, name);
  if (is)
    element.setAttribute('is', is);
  return element;
};

customElements.get = getCE;
customElements.whenDefined = whenDefined;
customElements.upgrade = function (element) {
  const is = element.getAttribute('is');
  if (is) {
    const constructor = registry.get(is);
    if (constructor) {
      augment(setPrototypeOf(element, constructor.prototype), is);
      // apparently unnecessary because this is handled by qsa observer
      // if (element.isConnected && element.connectedCallback)
      //   element.connectedCallback();
      return;
    }
  }
  upgrade.call(customElements, element);
};
customElements.define = function (is, Class, options) {
  if (getCE(is))
    throw new Error(`'${is}' has already been defined as a custom element`);
  let selector;
  const tag = options && options.extends;
  classes.set(Class, tag ? {is, tag} : {is: '', tag: is});
  if (tag) {
    selector = `${tag}[is="${is}"]`;
    prototypes.set(selector, Class.prototype);
    registry.set(is, Class);
    query.push(selector);
  }
  else {
    define.apply(customElements, arguments);
    shadowed.push(selector = is);
  }
  whenDefined(is).then(() => {
    if (tag) {
      parse(document.querySelectorAll(selector));
      shadows.forEach(parseShadow, [selector]);
    }
    else
      parseShadowed(document.querySelectorAll(selector));
  });
  defined.get(is)._(Class);
};

function parseShadow(element) {
  const root = shadowRoots.get(element);
  parse(root.querySelectorAll(this), element.isConnected);
}