track.js

const rAF = window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || (fn => setTimeout(fn, 16));

function autorun(fn) {
	const derivation = new Derivation(fn);
	derivation.register();
	return derivation;
}

function schedule(derivation) {
	if (!!schedule.queue) return schedule.queue.add(derivation);
	schedule.queue = new Set([ derivation ]);
	rAF(() => {
		schedule.queue.forEach(derivation => derivation.evaluate());
		schedule.queue = null;
	});
}

class Derivation {
	constructor(fn) {
		this.dependency = new Set();
		this.evaluate = fn;
	}

	start() { this.dependency.forEach(track => track.derivations.add(this)); }

	stop() { this.dependency.forEach(track => track.derivations.delete(this)); }

	register() {
		Derivation.current = this;
		this.evaluate();
		Derivation.current = null;
	}
}

class Track {
	constructor() {
		this.derivations = new Set();
	}

	depend() {
		const cur = Derivation.current
		if (cur) {
			cur.dependency.add(this);
			this.derivations.add(cur);
		}
	}

	update(changed = true) {
		if (!changed) return;
		this.derivations.forEach(schedule);
	}
}

function property(obj, key, value) {
	const track = new Track();
	Object.defineProperty(obj, key, {
		enumarable: true,
		get() { return track.depend(), value; },
		set(v) { track.update(v !== value, value = v); }
	});
}