portfolio/packages/frptools/src/computed.js

import { id, registerFire, registerSubscriptions, call } from './util.js';

export function computed(fn, dependencies = [], hash = id) {
	let subscribers = [];
	let isDirty = true;
	let val;
	let id;

	// Compute new value, call subscribers if changed.
	const accessor = function _computed() {
		if (isDirty) {
			const newVal = fn.apply(null, dependencies.map(runParam));
			isDirty = false;
			const newId = hash(newVal);
			if (id !== newId) {
				id = newId;
				val = newVal;
				accessor.fire(val);
			}
		}
		return val;
	};

	// Add child nodes to the logic graph (value-based)
	accessor.subscribe = registerSubscriptions(subscribers);
	accessor.fire = registerFire(subscribers);

	// Receive dirty flag from parent logic node (dependency).  Pass it down.
	accessor.setDirty = function setDirty() {
		if (!isDirty) {
			isDirty = true;
			subscribers.forEach(s => s.setDirty && s.setDirty());
		}
		return subscribers.length && accessor;
	};

	// Remove this node from the logic graph completely
	accessor.detach = () => {
		subscribers = [];
		dependentSubscriptions.forEach(call);
	};

	// Remove child nodes from the logic graph
	accessor.unsubscribeAll = () => {
		subscribers = [];
	};

	const dependentSubscriptions = dependencies.map(d => d.subscribe(accessor.setDirty));

	return accessor;
}

const runParam = a => (typeof a === 'function' ? a() : a);