ven DOM element.
   *
   * @param {Element} element The DOM element to generate the identifier for.
   * @return {ElementIdentifier} The identifier for the DOM element that can be passed between
   * processes as a message.
   */
  get(element) {
    if (!element) {
      throw new Error(
        "Can't create a ContentDOMReference identifier for " +
          "non-existant nodes."
      );
    }

    let browsingContext = BrowsingContext.getFromWindow(element.ownerGlobal);
    let mappings = gRegistry.get(browsingContext);
    if (!mappings) {
      mappings = {
        IDToElement: new Map(),
        elementToID: new WeakMap(),
      };
      gRegistry.set(browsingContext, mappings);
    }

    let id = mappings.elementToID.get(element);
    if (id) {
      // We already had this element registered, so return the pre-existing ID.
      return { browsingContextId: browsingContext.id, id };
    }

    // We must be registering a new element at this point.
    id = Math.random();
    mappings.elementToID.set(element, id);
    mappings.IDToElement.set(id, Cu.getWeakReference(element));

    let identifier = { browsingContextId: browsingContext.id, id };

    finalizerRoots.set(
      element,
      lazy.finalizationService.make(
        FINALIZATION_TOPIC,
        JSON.stringify(identifier)
      )
    );

    return identifier;
  },

  /**
   * Resolves an identifier back into the DOM Element that it was generated from.
   *
   * @param {ElementIdentifier} The identifier generated via ContentDOMReference.get for a
   * DOM element.
   * @return {Element} The DOM element that the identifier was generated for, or
   * null if the element does not still exist.
   */
  resolve(identifier) {
    let browsingContext = BrowsingContext.get(identifier.browsingContextId);
    let { id } = identifier;
    return this._resolveIDToElement(browsingContext, id);
  },

  /**
   * Removes an identifier from the registry so that subsequent attempts
   * to resolve it will result in null. This is done automatically when the
   * target node is GCed.
   *
   * @param {ElementIdentifier} The identifier to revoke, issued by ContentDOMReference.get for
   * a DOM element.
   */
  _revoke(identifier) {
    let browsingContext = BrowsingContext.get(identifier.browsingContextId);
    let { id } = identifier;

    let mappings = gRegistry.get(browsingContext);
    if (!mappings) {
      return;
    }

    mappings.IDToElement.delete(id);
  },

  /**
   * Private helper function that resolves a BrowsingContext and ID (the
   * pair that makes up an identifier) to a DOM element.
   *
   * @param {BrowsingContext} browsingContext The BrowsingContext that was hosting
   * the DOM element at the time that the identifier was generated.
   * @param {ElementID} id The ID generated for the DOM element.
   *
   * @return {Element} The DOM element that the identifier was generated for, or
   * null if the element does not still exist.
   */
  _resolveIDToElement(browsingContext, id) {
    let mappings = gRegistry.get(browsingContext);
    if (!mappings) {
      return null;
    }

    let weakReference = mappings.IDToElement.get(id);
    if (!weakReference) {
      return null;
    }

    return weakReference.get();
  },
};

ContentDOMReference._init();
PK