js/page/element.html

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<h1 class="js module">element.js</h1>

<code-block>import element from 'element.js'</code-block>

<section>
	<h2>Description</h2>
	<p>
		The <code>element</code> helper automates many utility features that
		often have to be added to every new custom element, like dispatching
		to different handler methods depending on the name of a changed attribute
		or adding getters and setters for these attributes.
	</p>
</section>
<section>
	<h2>Basic Usage</h2>

	<code-block>
		element(class MyElement extends HTMLElement {
			constructor() {
				super()
				console.log("An element walks into a bar...")
			}
		})
	</code-block>

	<h2>Features</h2>

	<section>
		<h3>Attributes</h3>
		<p>
			The static <code>attributes</code> property, when present,
			is used to automatically add the static <code>observedAttributes</code>
			property for the custom element API, as well as define getters and setters
			for the listed attributes.
		</p><p>
			A pair of filters can be given to modify the value after getting or before
			setting it. This can be used to convert between the string values that
			attributes restrict us to and a more sensible representation as well as
			some validation (attributes can be changed externally, after all).
		</p><p>
			Object keys in camelCase will be converted to kebab-case before being
			used as attribute names in the auto-generated <code>observedAttributes</code>.
		</p><p>
			The rules are:
			<ul>
				<li>Any truthy value registers the attribute with getter and setter
				<li>If the value has a <code>get</code> property, it will be incorporated as a filter into the getter.
				<li>If the value has a <code>set</code> property, it will be incorporated as a filter into the setter.
				<li><b>Unless</b> this value is <code>false</code> no setter will be added (read only attribute).
			</ul>
		</p>
		<code-block>
			element(class extends HTMLElement {
				static attributes = {
					plain = true,
					filtered = {
						get(string) { return Number(string) }
						set(value) { return Math.floor(value*100+0.5)/100 }
					},
					plainReadOnly = { set: false },
				}
			})
		</code-block>
	</section>

	<section>
		<h3>Change Methods</h3>
		<p>
			The <code>attributeChangedCallback</code> method is added automatically to the class.
			This auto-generated callback will first look up a <code>${attributeName}Changed</code>
			method and call it, if found, with the old and new values as arguments.
			Then it will look for a <code>changed</code> method and call it with the same arguments
			as <code>attributeChangedCallback</code> normally receives.
		</p><p>
			Attribute names will be converted from camelCase to kebab-case when needed before
			being used to form a method name to look up.
			Changing an attribute <code>foo-bar</code> will look for a <code>fooBarChanged</code> method.
		</p>
		<code-block>
			element(class extends HTMLElement {
				static attributes = { fooBar = true }
				fooBarChanged(oldValue, newValue) {
					console.log(`foo-bar changed from ${oldValue} to ${newValue}`)
				}
			})
		</code-block>
	</section>

	<section>
		<h3>Batched Methods</h3>
		<p>
			Certain methods, like re-rendering the content of a component
			should often happen in response to events that can happen repeatedly,
			or in response to many different events that can all happen at once.
		</p><p>
			To avoid having to repeatedly add complicated checks to handle these
			event bursts, <code>element</code> introduces the concept of <em>dollar-methods</em>.
		</p>
		<code-block>
			element(class extends HTMLElement {
				$render() {
					console.warn("Full Re-render...")
				}
			})
		</code-block>
		<p>
			Any method with a name starting with <code>$</code> will automatically
			have a sibling-method defined, with the dollar removed. Each time this
			auto-generated method is called, its argument list will be pushed to an array
			and a call of the original dollar-method will be scheduled as a micro-task.
		</p><p>
			The original method will then be called with the argument-lists of the individual
			calls to its dollar-less counterpart all at once.
		</p><p>
			It is of course still possible to manually call the dollar-method when
			immediate execution is wanted.
		</p>

		<p>
			Note how in the following example, all three change methods would be called
			in rapid succession if the browser applies the custom element class to an element
			it found on the page with all three attributes <code>foo</code>, <code>bar</code>
			and <code>baz</code> defined.
		</p>

		<code-block>
			element(class extends HTMLElement {
				$render() { console.warn("Full Re-render...") }

				fooChanged() { this.render() }
				barChanged() { this.render() }
				bazChanged() { this.render() }
			})
		</code-block>

		<p>
			Here, <code>this.render()</code> will not instantly re-render the whole component
			but instead schedule the re-render in a micro-task, potentially avoiding lots of
			repeated work.
		</p>
	</section>
</section>