effect

effect runs a function now and again whenever one of its listed dependencies changes.

Import

import { effect } from '@bedrock-core/observable';

Signature

effect(run: () => void, deps: readonly ReadonlyObservable<unknown>[], options?: { label?: string }): Unsubscribe

Parameters

Parameter	Type	Default	Description
run *	() => void	—	The side effect; runs once now, then on every dependency change
deps *	readonly ReadonlyObservable<unknown>[]	—	What it reacts to — anything with get and subscribe
options.label	string	'effect'	Named in the log when run throws

Returns

An Unsubscribe that stops the effect. It does not run run again.

Usage

const phase = observable<'lobby' | 'fight' | 'end'>('lobby');

const stop = effect(() => {
  bossBar.setTitle(phase.get());   // runs now with 'lobby', then on every change
}, [phase]);

stop();

Examples

Keeping engine state in step with a value

effect(() => {
  world.gameRules.pvp = config.server.pvp.get();
}, [config.server.pvp]);

Several dependencies

effect(() => {
  hud.render(shared.event.get(), aliveCount.get());
}, [shared.event, aliveCount]);

Notes

• Dependencies are listed, not tracked. A read inside run of an observable not in deps does not re-run the effect.
• A run that throws is reported under label; the effect stays subscribed.
• Inside a batch, an effect whose dependencies both changed runs exactly once, at the end.
• effect is computed without a value: the same scheduling, no result.