portfolio/packages/frptools/src/computed.js

import { eq } from './util.js';

export function computed(fn, dependencies = [], comparator = eq) {
	const subscribers = new Set();
	const dependents = new Set();
	let isDirty = true;
	let val;

	// Receive dirty flag from parent logic node (dependency).  Pass it down.
	function _computedDirtyReporter(_, skipPropagation) {
		if (!isDirty) {
			isDirty = true;
			dependents.forEach(d => d(_, skipPropagation));
		}

		if (subscribers.size && !skipPropagation) {
			accessor();
		}
	}

	const dependentSubscriptions = Array.from(dependencies).map(d => d._d(_computedDirtyReporter));

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

	// Add child nodes to the logic graph (value-based)
	accessor.subscribe = fn => {
		subscribers.add(fn);
		return () => {
			subscribers.delete(fn);
			return subscribers.size;
		};
	};

	// Add child nodes to the logic graph (dirty-based)
	accessor._d = fn => {
		dependents.add(fn);
		return () => dependents.delete(fn);
	};

	// Remove this node from the logic graph completely
	accessor.detach = () => {
		subscribers.clear();
		dependents.clear();
		dependentSubscriptions.forEach(runParam);
	};

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

	return accessor;
}

const runParam = a => a();