[webcomponents] Proposal for Cross Origin Use Case and Declarative Syntax

Hi all,

We have been discussing cross-orign use case and declarative syntax of web components internally at Apple, and here are our straw man proposal to amend the existing Web Components specifications to support it.

1. Modify HTML Imports to run scripts in the imported document itself
This allows the importee and the importer to not share the same script context, etc…

2. Add “importcomponents" content attribute on link element
It defines the list of custom element tag names to be imported from the imported HTML document.
e.g. <link rel="import" href="~" importcomponents="tag-1 tag-2"> will export custom elements of tag names "tag-1" and "tag-2" from ~.  Any name that didn't have a definition in the import document is ignored (i.e. if "tag-2" was not defined in ~, it would be skipped but "tag-1" will be still imported).

This mechanism prevents the imported document from defining arbitrary components in the host document.

3. Support "static" (write-once) binding of a HTML template
e.g.
<template id=cardTemplate>Name: {{name}}<br>Email:{{email}}</template>
<script>
document.body.appendChild(cardTemplate.instantiate({name: "Ryosuke Niwa", email:"rniwa@webkit.org"}));
</script>

4. Add “interface" content attribute to template element
This content attribute specifies the name of the JavaScript constructor function to be created in the global scope. The UA creates one and will be used to instantiate a given custom element.  The author can then setup the prototype chain as needed:

<template defines="name-card" interface="NameCardElement">
Name: {{name}}<br>Email:{{email}}
</template>
<script>
NameCardElement.prototype.name = function () {...}
NameCardElement.prototype.email = function () {...}
</script>

This is similar to doing:
var NameCardElement = document.register(’name-card');

5. Add "defines" content attribute on HTML template element to define a custom element
This new attribute defines a custom element of the given name for the template content.
e.g. <template defines="nestedDiv"><div><div></div></div></template> will let you use <nestedDiv></nestedDiv>

We didn’t think having a separate custom element was useful because we couldn’t think of a use case where you wanted to define a custom element declaratively and not use template by default, and having to associate the first template element with the custom element seemed unnecessary complexity.

5.1. When a custom element is instantiated, automatically instantiate template inside a shadow root after statically binding the template with dataset
This allows statically declaring arguments to a component.
e.g.
<template defines="name-card">Name: {{name}}<br>Email:{{email}}</template>
<name-card data-name="Ryosuke Niwa" data-email="rniwa@webkit.org”>

5.2. When a new custom element object is constructed, "created" callback is called with a shadow root
Unfortunately, we can't let the author define a constructor because the element hadn't been properly initialized with the right JS wrapper at the time of its construction.  So just like we can't do "new HTMLTitleElement", we're not going to let the author do an interesting things inside a custom element's constructor.  Instead, we're going to call "created" function on its prototype chain:

<template defines="name-card" interface="NameCardElement">
Name: {{name}}<br>Email:{{email}}
</template>
<script>
NameCardElement.prototype.name = function () {...}
NameCardElement.prototype.email = function () {...}
NameCardElement.prototype.created = function (shadowRoot) {
    ... // Initialize the shadowRoot here.
}
</script>

This is similar to the way document.register works in that document.register creates a constructor automatically.

6. The cross-origin component does not have access to the shadow host element, and the host document doesn’t have access to the element object.
When member functions of the element is called, “this” object will be undefined.  This is necessary because exposing the object to a cross-origin content will result in tricky security issues, forcing us to have proxy objects, etc…

Inside the document that imported a component, the element doesn’t use the prototype defined by the component as that exposes JS objects cross-origin.  e.g. even if LikeButtonElement was defined in facebook.com/~/like-button.html, the document that uses this component wouldn’t see the prototype or the constructor.  It’ll be HTMLUnknownElement.  (We could create a new custom element type such as HTMLCrossOriginCustomElement if think that’s necessary).

7. Expose shadow host’s dataset on shadow root
This allows the component to communicate with the host document in a limited fashion without exposing the element directly.

This design allows us to have an iframe-like boundary between the shadow host (custom element itself) and the shadow root (implementation details), and address our cross-origin use case elegantly as follows:

rniwa.com/webkit.html
---------------------------------
<!DOCTYPE html>
<html>
<head>
<link rel=import href="https://webkit.org/components.html" defines="share-button like-button">
</head>
<body>
<like-button data-url="https://build.webkit.org/">Like build.webkit.org</like-button>
</body>
</html>

webkit.org/components.html
---------------------------------
<template defines="like-button" interface="LikeButtonElement">
<!-- implicitly does shadowRoot.appendChild(myTemplate.instantiate(shadowHost.dataset)); -->
<form ...>
<input type=hidden value="{{url}}">
<button type=submit>Like!</button>
</form>
<script>
LikeButtonElement.prototype.created = function (shadowRoot) {
    shadowRoot.query('form').onsubmit = function () {
        // ...
    }
}
</script>
</template>

- R. Niwa