Writing a decorator for <iron-form>: a timing question

[shipagency]

I am writing a Polymer 2 element that decorated <iron-form>
My element needs to work with <iron-form> and the form itself that is decorated by <iron-form> -- which is stored as _form in the <iron-form> element.

At the moment I have this:

ready () {
  super.ready()
  var self = this

  this.ironForm = this.allChildrenNamed('IRON-FORM', true)
  if (!this.ironForm) throw new Error('hot-form must contain an iron-form element which will be decorated')

  setTimeout(function () {
    self.form = self.ironForm._form
  }, 0)
}

Is this a reliable way to go about this?
WIth that setTimeout, <iron-form> has enough time to assign _form. Will this be the case longer-term?
@TimvdLippe commeted here: Polymer/polymer#5149

There might be a case where iron-form is not attached yet, as the assignment to _form is in that callback (

iron-form/iron-form.html

Lines 172 to 204 in dc9c963

	attached: function() {
	// We might have been detached then re-attached.
	// Avoid searching again for the <form> if we already found it.
	if (this._form) {
	return;
	}
	// Search for the `<form>`, if we don't find it, observe for
	// mutations.
	this._form = Polymer.dom(this).querySelector('form');
	if (this._form) {
	this._init();
	// Since some elements might not be upgraded yet at this time,
	// we won't be able to look into their shadowRoots for submittables.
	// We wait a tick and check again for any missing submittable default
	// values.
	this.async(this._saveInitialValues.bind(this));
	} else {
	this._nodeObserver = Polymer.dom(this).observeNodes(
	function(mutations) {
	for (var i = 0; i < mutations.addedNodes.length; i++) {
	if (mutations.addedNodes[i].tagName === 'FORM') {
	this._form = mutations.addedNodes[i];
	// At this point in time, all custom elements are expected
	// to be upgraded, hence we'll be able to traverse their
	// shadowRoots.
	this._init();
	Polymer.dom(this).unobserveNodes(this._nodeObserver);
	this._nodeObserver = null;
	}
	}
	}.bind(this));
	}
	},

) I do not think setTimeout is 100% reliable, but it will probably work in most cases.

Since this is mostly an iron-form specific question, I would advise to open an issue there and potentially request a new event to be notified when _form is set, in the attached callback.

Would you accept a PR with the addition of the newly emitted event?
Or, what would you advise?