import { readActivePrefsSlot, type ActivePrefs } from './active-prefs.svelte';
import type { MotionEffective, MotionSource } from '$libs/motion';

/**
 * The `motion` slot of an open `ActivePrefs`, as the `MotionSource` port the
 * animation runtimes consume.
 *
 * This is the ONE place the effective reduced-motion value is decided for
 * everything downstream: the motion engine, the scene engine, sema's haptic
 * channel and every eidos component read it from here, so an explicit
 * `motion: 'allow'` reaches them all and a `reduce` degrades them all. Asking
 * the OS again (`ActiveDom.prefersReducedMotion`) is asking a question prefs
 * has already answered — the media query is an ENVIRONMENT hint, and
 * `resolveMotion` folded it with the user's intent long before this call.
 *
 * The slot is read DEFENSIVELY (the shape `dom-projection.ts` uses): an app is
 * free to compose a schema without a `motion` dimension, and the answer then is
 * `undefined` — no source, so each consumer keeps its own documented default
 * rather than inventing a policy nobody declared.
 *
 * Reads go through `slot.get()` on every call: the slot is `$state`-backed, so
 * a caller inside a `$derived` / `$effect` tracks it, and a caller outside one
 * still sees the current value.
 */
export function createMotionSourceFromPrefs(prefs: ActivePrefs): MotionSource | undefined {
	const slot = readActivePrefsSlot<MotionEffective>(prefs, 'motion');
	if (!slot) return undefined;
	return {
		get: () => slot.get(),
		onChange: (handler) => slot.onChange(handler)
	};
}