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Web Component Implementations

The user interface components used in Anklang are implemented as custom HTML elements and are generally composed of a single file that provides:

a) A brief documentation block; b) CSS style information, that is extracted at build time via JsExtract; c) An HTML layout specified with lit-html expressions; d) An assorted JavaScript class that defines a new custom HTML element, possibly via Lit.

Simple components that have no or at most one child element and do not require complex HTML layouts with lit-html can be implemented directly via customElements.define().

Components with complex layouts that need lit-html or that act as containers with several HTMLSlotElements (for multiple types of children) which require a ShadowRoot, should be implemented as LitElements by extending LitComponent (our convenience wrapper around LitElement.

Note that a Lit component is an HTML element, it extends ReactiveElement which always extends HTMLElement and none of the other HTML element interfaces.

Guidelines for Web Components

Guidelines capturing past experiences:

  • Create a component to encapsulate possible state and specific UI/UX behavior or API functionality.

  • Avoid adding components purely for layout or style reasons. Horizontal or vertical flex boxes, grid layout, dialog and menu styling are much better addressed with CSS classes and are more often subject to change than encapsulated behavior.

  • Use CSS utility classes instead of custom or adhoc styles that have to be given a dedicated name. TailwindCSS gives good guidance here.

  • Use simple custom element components if that suffices, components that do not expect external children can get away without a shadow DOM.

  • Components with external children need <slot/> elements, these require shadow DOM handling as provided by LitElement, use it via our LitComponent class.

  • Focussable elements MUST NOT be buried inside the shadow DOM tree. It is perfectly fine to add focussable elements as (external) children to a ready-made web component that has a shadow DOM with <slot/> elements, but proper focus handling, focus related styling, keyboard and focus chain handling is not reliably possible when focussable elements are inside the shadow DOM, instead of staying part of the light DOM by adding them into <slot/> elements.

Legacy Vue Components

Some components are still implemented via Vue and are slowly phased out. We often use <canvas> elements for Anklang specific displays, and Vue canvas handling comes with certain caveats:

1) Use of the Util.vue_mixins.dom_updates mixin (now default) allows to trigger the dom_update() component method for $forceUpdate() invocations and related events. 2) A methods: { dom_update() {}, } component entry should be provided that triggers the actual canvas rendering logic. 3) Using a document.fonts.ready promise, Anklang re-renders all Vue components via $forceUpdate() once all webfonts have been loaded, <canvas> elements containing text usually need to re-render themselves in this case.

Envue components:

Envue components are created to simplify some of the oddities of dealing with Vue-3 components. The function Envue.Component.vue_export creates a Vue component definition, so that the Vue component instance ($vm) is tied to an Envue.Component instance ($object). Notes: - The Vue lifetime component can be accessed as $object.$vm. - The Envue component can be accessed as $vm.$object. - Accesses to $vm.* fields e.g. from within a <template/> definition are forwarded to access $object.* fields. - Vue3 components are Proxy objects, but assignments to these Proxy objects is not reactive. - To construct reactive instance data with async functions, use observable_from_getters().

Vue uses a template compiler to construct a render() function from HTML <template/> strings. The Javascript expressions in templates are sandboxed and limited in scope, but may refer to Vue component properties that are exposed through hasOwnProperty(). In order to support Envue instance methods and fields in template expressions, all members present after Envue construction are forwarded into the Vue component.