custom-element

Declarative Custom Element (DCE) is a part of pure Declarative Web Application stack. A proof of concept as a part of WCCG in Declarative custom elements and Declarative Web Application discussion. NO-JS The functionality of DCE and its data access does not require programming using JavaScript.

It allows to define custom HTML tag with template filled from slots, attributes and data slice as of now from
local-storage, http-request, location. UI is re-rendered on each data slice change triggered by initialization or DOM event.

GitHub | Live demo: custom-element | Try in Sandbox | tests project | Chrome devtools pugin

What is DCE?

DCE provides the next level of abstraction in HTML - native composition. With native implementation which is streaming parser, streaming transformation, multithreading. native assumes the C/Rust compiled code. There is no place for JavaScript except of polyfill and ability to extend DCE, which otherwise has to be native.

The composition assumes the fully functional template and ability to call the template with parameters( custom tag + attributes) .

As the next to HTML abstraction layer - composition, it provides:

• ability to use dependencies as from withing the page as from external file/lib via src attribute and # in URL
• ability to treat external content via content-type like html, SVG, images, video with own template rendering
• provide styles and embedded DCE declarations in own and named(lib) scope, sharing the scoped registry.

After composition the layer of functional component provides

• data layer with access to attributes/payload(+slots), dataset, data bound slice
• means in template to use the data selector for condition/enumeration/text injection into attributes and DOM
• Set of native primitives to support browser APIs declaratively: location,storage, http request which bonded to slice and as result to reactive UI.
• support the data change trigger over events

While DCE is no-JS concept, DCE provides the basic declarative constructs to build most of simple apps. Assuming the extending via custom elements and JS. The evolution goal is to adopt most demanded APIs/construct natively into DCE stack over time.

DCE is compatible with closed/open/named root. Enabling as site-scoped styling and registry as encapsulated anonymous scopes in shadow root.

This project is a POC( Proof of Concept ) targeting to become a base for native DCE implementation polyfill.

use

Use the bootstrap project with all pre-configured or

install

use via CDN

<script type="module" src="https://unpkg.com/@epa-wg/custom-element@0.0/custom-element.js"></script>

NPM, yarn

npm i -P @epa-wg/custom-element
yarn add @epa-wg/custom-element

Enable IDE support

IDE.md

Live demo 🔗

Interactivity via data slice triggered by events

<custom-element>
      <input slice="typed"> //slice/typed : {//slice/typed}
</custom-element>

<custom-element>
    <template>
        <button slice="clickcount"
                slice-event="click"
                slice-value="//clickcount + 1" > + </button>
        <input slice="clickcount" type="number" value="{//clickcount ?? 0}">
        Click count: { //clickcount }
    </template>
</custom-element>

More on slice concept in slice and events demo page

Templating power

comes from XSLT and XPath. Which is natively implemented in all current browsers, globally tested and well documented.

<custom-element tag="pokemon-tile" hidden>
    <h3>{title}</h3> <!-- title is an attribute in instance
                                                 mapped into /*/attributes/title -->
    <if test="//smile">                 <!-- data-smile DCE instance attribute,
                                                 mapped into /*/dataset/smile
                                                 used in condition -->
                                            <!-- data-smile DCE instance attribute, used as HTML -->
        <div>Smile as: {//smile} </div>
    </if>
    <!-- image would not be visible in sandbox, see live demo -->
    <img src="https://unpkg.com/pokeapi-sprites@2.0.2/sprites/pokemon/other/dream-world/{pokemon-id}.svg"
         alt="{title} image"/>
                                            <!-- image-src and title are DCE instance attributes,
                                                 mapped into /*/attributes/
                                                 used within output attribute via curly brackets -->

                                            <!-- `slot name=xxx` replaced with elements with `slot=xxx` attribute -->
    <p><slot name="description"><i>description is not available</i></slot></p>
</custom-element>

<pokemon-tile title="bulbasaur" data-smile="👼" pokemon-id="1" >
    <p slot="description">Bulbasaur is a cute Pokémon born with a large seed firmly affixed to its back;
        the seed grows in size as the Pokémon  does.</p>
</pokemon-tile>

<pokemon-tile title="ninetales" pokemon-id="38" ></pokemon-tile>        

generates HTML

<pokemon-tile title="bulbasaur" data-smile="👼" 
              image-src="https://unpkg.com/pokeapi-sprites@2.0.2/sprites/pokemon/other/dream-world/1.svg"
    >
    <h3>bulbasaur</h3>
    <div>Smile as: 👼</div>
    <img src="https://unpkg.com/pokeapi-sprites@2.0.2/sprites/pokemon/other/dream-world/1.svg" alt="bulbasaur">
    <p>Bulbasaur is a cute Pokémon born with a large seed firmly affixed to its back;
                the seed grows in size as the Pokémon  does.</p>
</pokemon-tile>
<pokemon-tile title="ninetales" 
              image-src="https://unpkg.com/pokeapi-sprites@2.0.2/sprites/pokemon/other/dream-world/38.svg"
    >
    <h3>ninetales</h3>
    <img src="https://unpkg.com/pokeapi-sprites@2.0.2/sprites/pokemon/other/dream-world/38.svg" alt="ninetales">
    <p></p>
</pokemon-tile>

responsive hex-grid demo , look into sources for samples of CSS encapsulation and external template use.

Implementation notes

Life cycle

custom-element declaration

• constructor injects payload into XSL template
• creates a class for custom element extending HTMLElement
• registers element by tag attribute

NOTE: attempt to register custom element with already registered tag name would fail due to w3c standard limitations. The scoped custom element registry is still a proposal.

omitting tag leads to template instantiation

Whether template is inline or given by src attribute, the custom-element would be instantiated inline if no tag attribute is given.

custom element instance

constructor creates XML with

• root matching the tag
• payload
  • dom nodes with slot attribute stay inside
• attributes
• ?dataset

DOM content is replaced with results of instance XML transformation by declaration XSLT.

tag attribute

allows to define the Custom Element tag registered by window.customElements.define().

If omitted, the tag is auto-generated and the content is rendered inline.

<custom-element> my tag is {tag} </custom-element>

See demo for tag attribute use.

src attribute

allows to refer either external template or template within external library by #id hash in URL.

See demo with various samples.

types of template

• HTML with DCE syntax ( slots, data slices, xslt operators, etc. )
• SVG image, MathML, etc.
• XSLT template. The datadom is the XML payload for transformation. In order to be embedded into external document, this document has to have XML syntax like XHTML. Attempt of including XSLT within HTML file would break the template integrity by parser.

#id Local reference

allows to refer the template withing same document

url

allows to use the external document as template

url#id

allows to refer the template withing external document

template syntax

Scoped CSS live demo

styles encapsulation

DCE can have the own styles which would be scoped to the instances. In order to prevent the style leaking, it has to be defined withing template tag:

<custom-element>
  <template>
    <style>
        color: green;
        button{ color: blue; }
    </style>
    <label> green <button>blue</button> </label>
  </template>
</custom-element>

green blue

override style for instance

In same way as in DCE itself:

        <custom-element tag="dce-2">
            <template><!-- template needed to avoid styles leaking into global HTML -->
                <style>
                    button{ border: 0.2rem dashed blue; }
                </style>
                <button><slot>Blue borders</slot></button>
            </template>
        </custom-element>
        <dce-2>dashed blue</dce-2>
        <dce-2>
            <template> <!-- template needed to avoid styles leaking into global HTML -->
                <style>button{border-color:red;}</style>
                Red border
            </template>
        </dce-2>

Attributes

To be served by IDE and to track the attributes changes, they have to be declared via attribute:

    <custom-element tag="dce-with-attrs" hidden>
        <attribute name="p1" >default_P1                </attribute>
        <attribute name="p2" select="'always_p2'"       ></attribute>
        <attribute name="p3" select="//p3 ?? 'def_P3' " ></attribute>
        p1: {$p1} <br/> p2: {$p2} <br/> p3: {$p3}
    </custom-element>
    <dce-with-attrs p1="123" p3="qwe"></dce-with-attrs>

The curly braces {} in attributes implemented as attribute value template

The names in curly braces are matching the instance attributes. I.e. in XML node /my-component/attributes/.

To access payload XPath could start with /*/payload/. I.e. {/*/payload//label} refers to all label tags in payload.

Slots

<slot name="xxx"> is replaced by payload top elements with slot attribute matching the name, i.e. slot xxx is matching <i slot="xxx">...</i> in payload.

<custom-element tag="with-description" >
    <slot name="description">description is not available</slot>
    <!-- same as 
        <value-of select='/*/payload/*[@slot="description"]'/>
    -->
</custom-element>
<with-description>
    <p slot="description">Bulbasaur is a cute Pokémon ...</p>
</with-description>

loops, variables

Loop implemented via for-each

Variables in XSLT

XPath

is available in {} in attributes, in for-each, if, value-of, and other XSL tags.

XPath is a selector language to navigate over custom element instance data, attributes, and payload.

XSLT 1.0

The in-browser native implementation as of now supports XSLT 1.0. File the change request for support of another XSLT version or template engine.

troubleshooting

HTML parser is not compatible with templates

On many tags like table, or link a the attempt to use XSLT operations could lead to DOM order mismatch to given in template. In such cases the xhtml: prefix in front of troubled tag would solve the parsing.

<custom-element tag="dce-2" hidden>
  <local-storage key="basket" slice="basket" live type="json"></local-storage>
  <xhtml:table xmlns:xhtml="http://www.w3.org/1999/xhtml"  >
    <xhtml:tbody>
      <for-each select="//basket/@*">
        <xhtml:tr>
          <xhtml:th> {name()} </xhtml:th>
          <xhtml:td> {.}      </xhtml:td>
        </xhtml:tr>
      </for-each>
    </xhtml:tbody>
    <xhtml:tfoot>
      <xhtml:tr>
        <xhtml:td><slot>🤔</slot></xhtml:td>
        <xhtml:th> {sum(//slice/basket/@*)} </xhtml:th>
      </xhtml:tr>
    </xhtml:tfoot>
  </xhtml:table>
</custom-element>

See demo source for detailed sample.

Chrome devtools plugin

@epa-wg/custom-element plugin gives the view into

• current selected in DOM inspector node

• Parent customElement

• Declarative Custom Element dce for custom element ^^

• datadom for easier inspection

• xml as a string

• xslt as a string

template debugging

xml and xslt can be saved to file via for "copy string contents" into clipboard.

The XSLT debugger from your favorite IDE can set the breakpoints withing those files and run transformation under debugger.

{} does not give a value

• try to add as attribute you could observe and put the value of node name or text to identify the current location in data within template

<b title="{name(*)} : {text()}">xml tag name: <value-of select='name()'/></b>