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Component Model Strawman: Element Registration
Overview
With the Element Registration mechanism. page authors can tell new element names and its behavior to agents.
The element element and HTMLRegistrationElement
The HTMLRegistrationElement represents an author-registered element definition.
Here is the simplest example which registeres an element named "x-comment" whose instance implements HTMLElement
// Imperative API
var element = document.createElement("element");
element.name = "x-comment";
document.head.appendChild(element);
// Shorter form.
HTMLRegistrationElement.register("x-comment");
<!-- Markup API -->
<head>
<element name="x-comment"></element>
</head>
Scripting
The script element
The element element allows script elements in its children. These script blocks are invoked as a function, whose receiver(this) is bound to enclosing element element. These script blocks can be used for providing lifecycle callback definitions for the registering element.
<head>
<element name="x-comment">
<script>
var shouldBeXComment = this.name;
<script>
</element>
</head>
The create callback
The HTMLRegistrationElement provide a pair of lifecycle callbacks for the element instance creation including the tree construction.
The first callback, create is invoked when an agent need a new element instance of the registered element. create callback should return new element instance which matches the registered element specification.
// Imperative API
var element = document.createElement("element");
element.name = "x-comment";
element.create = function() { return new HTMLElement("x-comment"); };
document.head.appendChild(element);
// Shorter form.
HTMLRegistrationElement.register("x-comment", function() { ... });
<!-- Markup API -->
<head>
<element name="x-comment">
<script>
this.create = function() { return new HTMLElement("x-comment"); };
</script>
</element>
</head>
If the callback function has HTMLElement in its prototype chain, it is invoked as a constructor. Thus following two markup and one imperative examples have same meaning.
<!-- Markup 1 -->
<element name="x-comment">
<script>
class Comment : HTMLElement {
....
};
this.create = Comment;
</script>
</element>
<!-- Markup 2 -->
<element name="x-comment">
<script>
class Comment : HTMLElement {
....
};
this.create = function() { return new Comment(); };
</script>
</element>
// Imperative
class Comment : HTMLElement {
...
};
HTMLRegistrationElement("x-comment", Comment);
If no create callback is given, its default behavior is something like this:
<element name="x-comment">
<script>
var element = this;
this.create = function() { return new HTMLElement(element.name); };
</script>
</element>
The setup callback
The second lifecycle callback, setup is called after an instance creation. If the instance is created by the agent's tree construction, it will be called as a part of the "close" phase. On setup, the attributes and child elements for the element are already set by the agent. So this callback is useful for building its visual like the shadow tree.
// Imperative API
var element = document.createElement("element");
...
element.setup = function() { this.shadow = new ShadowRoot(this); ...; };
...
<!-- Markup API -->
<head>
<element name="x-comment">
<script>
...
this.setup = function() {
// "this" points the newly created element instance.
this.shadow = new ShadowRoot(this);
...;
};
</script>
</element>
</head>
If no setup callback is given by the author, agent invokes the default behavior. It is something like this:
function defaultSetup() {
if (this.setup instanceof Function)
this.setup();
}
Styling
The element element also allows the style element as its children. The style given by the element is a part of document stylesheet. Authors can use the style element to provide the style for the registering element.
<head>
<element name="x-comment">
<style>
x-comment {
color: gray;
}
</style>
</element>
</head>
Shadow Tree construction
The element element helps to construct a shadow tree for newly created elements. If element element has a template element as its child, The agent creates shadow tree from the template element before invoking the setup callback, then invokes it with the shadow root.
<!-- Markup API -->
<head>
<element name="x-comment">
<template>
<div>...</div>
</template>
<script>
...
this.setup = function(shadow) {
// setup using the shadow instance.
};
</script>
</element>
</head>
HTMLRegistrationElement interface
[Callback=AcceptConstructor]
interface HTMLRegistrationCreateCallback {
HTMLElement create();
}:
[Callback]
interface HTMLRegistrationSetupCallback {
void setup(ShadowRoot shadow = null);
}:
[Constructor]
interface HTMLRegistrationElement {
attribute String name;
attribute HTMLRegistrationCreateCallback create;
attribute HTMLRegistrationSetupCallback setup;
};