@efflore/ui-element/src/core/context.ts

UIElement - minimal reactive framework based on Web Components

import type { UIElement } from "../ui-element"
import { isFunction } from "./is-type"

/** @see https://github.com/webcomponents-cg/community-protocols/blob/main/proposals/context.md */

/* === Type definitions === */

/**
 * A context key.
 *
 * A context key can be any type of object, including strings and symbols. The
 *  Context type brands the key type with the `__context__` property that
 * carries the type of the value the context references.
 */
type Context<K, V> = K & {__context__: V}

/**
 * An unknown context type
 */
type UnknownContext = Context<unknown, unknown>

/**
 * A helper type which can extract a Context value type from a Context type
 */
/* eslint-disable @typescript-eslint/no-unused-vars */
type ContextType<T extends UnknownContext> = T extends Context<infer _, infer V> ? V : never

/**
 * A callback which is provided by a context requester and is called with the value satisfying the request.
 * This callback can be called multiple times by context providers as the requested value is changed.
 */
type ContextCallback<V> = (value: V, unsubscribe?: () => void) => void

declare global {
	interface HTMLElementEventMap {
		/**
		 * A 'context-request' event can be emitted by any element which desires
		 * a context value to be injected by an external provider.
		 */
		'context-request': ContextRequestEvent<Context<unknown, unknown>>
	}
}

/* === Constants === */

const CONTEXT_REQUEST = 'context-request'

/* === Exported class === */

/**
 * Class for context-request events
 * 
 * An event fired by a context requester to signal it desires a named context.
 *
 * A provider should inspect the `context` property of the event to determine if it has a value that can
 * satisfy the request, calling the `callback` with the requested value if so.
 *
 * If the requested context event contains a truthy `subscribe` value, then a provider can call the callback
 * multiple times if the value is changed, if this is the case the provider should pass an `unsubscribe`
 * function to the callback which requesters can invoke to indicate they no longer wish to receive these updates.
 * 
 * @class ContextRequestEvent
 * @extends {Event}
 * 
 * @property {T} context - context key
 * @property {ContextCallback<ContextType<T>>} callback - callback function for value getter and unsubscribe function
 * @property {boolean} [subscribe=false] - whether to subscribe to context changes
 */
class ContextRequestEvent<T extends UnknownContext> extends Event {
	public constructor(
		public readonly context: T,
		public readonly callback: ContextCallback<ContextType<T>>,
		public readonly subscribe: boolean = false
	) {
		super(CONTEXT_REQUEST, {
			bubbles: true,
			composed: true
		})
	}
}

/**
 * Initialize context provider / consumer for a UIElement instance
 * 
 * @param {UIelement} host - UIElement instance to initialize context for
 */
const initContext = (host: UIElement) => {
	const proto = host.constructor as typeof UIElement

	// context consumers
	const consumed = proto.consumedContexts || []
	for (const context of consumed)
		host.set(String(context), undefined, false)
	setTimeout(() => { // wait for all custom elements to be defined
		for (const context of consumed)
			host.dispatchEvent(new ContextRequestEvent(
				context,
				(value: unknown) => host.set(String(context), value)
			))
	})

	// context providers
	const provided = proto.providedContexts || []
	if (!provided.length) return
	host.addEventListener(CONTEXT_REQUEST, (e: ContextRequestEvent<UnknownContext>) => {
		const { context, callback } = e
		if (!provided.includes(context) || !isFunction(callback)) return
		e.stopPropagation()
		callback(host.signal(String(context)))
	})
}

export {
	type Context, type UnknownContext,
	CONTEXT_REQUEST, ContextRequestEvent, initContext
}