css-selectors dart polymer dart-polymer shadow-dom

How to query elements within shadow DOM from outside in Dart?

How can I select nodes within shadow DOM? Consider the following example:

structure of "unshadowed" DOM

<app-element>
  #shadow-root
    <h2></h2>
    <content>
      #outside shadow
      <h2></h2>
    </content>
    <ui-button>
      #shadow-root
        <h2></h2>
  </ui-button>
</app-element>

index.html

<body>
<app-element>
  <!-- OK: querySelect('app-element').querySelect('h2') -->
  <!-- OK: querySelect('app-element h2') -->
  <!-- There is no problem to select it -->
  <h2>app-element > content > h2</h2>
</app-element>
</body>

templates.html

<polymer-element name="ui-button" noscript>
  <template>
    <!-- FAIL: querySelect('app-element::shadow ui-button::shadow h2') -->
    <h2>app-element > ui-button > h2</h2>
  </template>
</polymer-element>

<polymer-element name="app-element" noscript>
  <template>
    <!-- FAIL: querySelect('app-element::shadow').querySelect('h2') -->
    <!-- FAIL: querySelect('app-element::shadow h2') -->
    <!-- FAIL: querySelect('app-element').shadowRoot.querySelect('h2') -->
    <h2>app-element > h2</h2>
    <content></content>
    <ui-button></ui-button>
  </template>
</polymer-element>

In comments like "OK: querySelect()" I show selectors I've tried to run from outside any shadowed DOM.

I've already read the following article: http://www.html5rocks.com/en/tutorials/webcomponents/shadowdom-201/?redirect_from_locale=ru and based on the fact that it was said in the article, query like: document.querySelector('app-element::shadow h2'); in JS should work as expected. However in Dart it doesn't work.

What do I wrong?

Solution

Outdated

ShadowDom was changed significantly since I posted this answer, this is why this answer doesn't apply anymore.

Original Answer

If you use a custom main, ensure that Polymer is properly initialized before you try to interact with your Polymer elements (see how to implement a main function in polymer apps for more details).

I usually suggest to avoid a custom main and create an app-element (or whatever name you prefer) and put your initialization code into attached (ensure to call super.attached();) or in ready() (doesn't need the super call).

It seems in this case it's not in the shadow DOM but a child.

This should work:

querySelector('h2');

It's only in the shadow DOM when it is within your elements <template>...</template> not when you wrap it in the tag of your custom element.

<polymer-element name="some-element">
  <template>
    <!-- this becomes the shadow DOM -->
    <content>
     <!-- 
       what gets captureD by the content element becomes a child or some-element
       -->
     </content>
  </template>
</polymer-element>

<body>
  <some-element>
    <!-- these elements here are captured by the 
         content tag and become children of some-element -->
    <div>some text</div>
  </some-element>
</body>

If you want to search

inside the shadow DOM of the current element

shadowRoot.querySelect('h2');

inside the shadow DOM of an element inside the shadow DOM

shadowRoot.querySelector('* /deep/ h2');
shadowRoot.querySelector('ui-button::shadow h2');

from outside the current element

import 'dart:html' as dom;
...
dom.querySelector('* /deep/ h2');
// or (only in the shadow DOM of <app-element>)
dom.querySelector('app-element::shadow h2');
dom.querySelector('app-element::shadow ui-button::shadow h2');
// or (arbitrary depth)
dom.querySelector('app-element /deep/ h2');