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With fritz2, you can easily use WebComponents in any html-context. Some of the following code-snippets are not runnable on their own. Please find the complete example here

Use WebComponents

render {
    div("weather-card") {
        h2 { "Goslar" }
        custom("m3-stars") {
            attr("max", "5")
            attr("current", "3.5")
        // ...

Before you can use a custom element, you have to add the component to your site’s scripts. One way is adding a script link pointing to the component which is hosted somewhere you can access it:

<script type="module" src=""></script>

If the component you want to use is published on npm, you can add it as a dependency in your Gradle-build:

dependencies {
    // ...

… and import it in your Kotlin-Code:

abstract external class Stars : HTMLElement

Please see the official documentation for more details on this.

Depending on how the component is internally built, you might have to register it with the browser:

fun main() {
    window.customElements.define("m3-stars", Stars::class.js.unsafeCast<() -> dynamic>())
    // ...

For obvious reasons we cannot provide typesafe attributes for custom elements, but you can implement a Tag and provide an extension function for RenderContext:

class M3Stars(job: Job) : Tag<HTMLElement>("m3-stars", job = job), WithText<HTMLElement> {
    fun max(value: Flow<Int>) = attr("max", value.asString())
    fun current(value: Flow<Float>) = attr("current", value.asString())

fun RenderContext.m3Stars(content: M3Stars.() -> Unit): M3Stars = register(M3Stars(job), content)

Build a WebComponent

To build a WebComponent with fritz2, two steps are neccessary. First, implement your WebComponent-class:

object MyComponent : WebComponent<HTMLParagraphElement>() {
    override fun TagContext.init(element: HTMLElement, shadowRoot: ShadowRoot): Tag<HTMLParagraphElement> {
        return p {
            +"I am a WebComponent"

Next, register your component:

fun main() {
    registerWebComponent("my-component", MyComponent)

To observe one or more arguments, just add them to the registration:

registerWebComponent("my-component", MyComponent, "first-attr", "second-attr")

You can then use the values of these observed attributes in your init-method as a Flow:

val first = MyComponent.attributeChanges("first-attr")

render {
    first.render { firstAttr ->
        p { +firstAttr }

To react to the lifecyle of your component, you can override the according methods from the specification.

Packaging (i.e. as an npm-package) and publishing is out of scope of this documentation.

Again, to see it in action, please have a look at our webcomponents example.

Distributed by a MIT license.