Function effect

packages/core/src/render3/reactivity/effect.ts:137–197

(
  effectFn: (onCleanup: EffectCleanupRegisterFn) => void,
  options?: CreateEffectOptions,
)

 * @publicApi 20.0
 */
export function effect(
  effectFn: (onCleanup: EffectCleanupRegisterFn) => void,
  options?: CreateEffectOptions,
): EffectRef {
  ngDevMode &&
    assertNotInReactiveContext(
      effect,
      'Call `effect` outside of a reactive context. For example, schedule the ' +
        'effect inside the component constructor.',
    );

  if (ngDevMode && !options?.injector) {
    assertInInjectionContext(effect);
  }

  if (ngDevMode && options?.allowSignalWrites !== undefined) {
    console.warn(
      `The 'allowSignalWrites' flag is deprecated and no longer impacts effect() (writes are always allowed)`,
    );
  }

  const injector = options?.injector ?? inject(Injector);
  let destroyRef = options?.manualCleanup !== true ? injector.get(DestroyRef) : null;

  let node: EffectNode;

  const viewContext = injector.get(ViewContext, null, {optional: true});
  const notifier = injector.get(ChangeDetectionScheduler);
  if (viewContext !== null) {
    // This effect was created in the context of a view, and will be associated with the view.
    node = createViewEffect(viewContext.view, notifier, effectFn);
    if (destroyRef instanceof NodeInjectorDestroyRef && destroyRef._lView === viewContext.view) {
      // The effect is being created in the same view as the `DestroyRef` references, so it will be
      // automatically destroyed without the need for an explicit `DestroyRef` registration.
      destroyRef = null;
    }
  } else {
    // This effect was created outside the context of a view, and will be scheduled independently.
    node = createRootEffect(effectFn, injector.get(EffectScheduler), notifier);
  }
  node.injector = injector;

  if (destroyRef !== null) {
    // If we need to register for cleanup, do that here.
    node.onDestroyFns = [destroyRef.onDestroy(() => node.destroy())];
  }

  const effectRef = new EffectRefImpl(node);

  if (ngDevMode) {
    node.debugName = options?.debugName ?? '';
    const prevInjectorProfilerContext = setInjectorProfilerContext({injector, token: null});
    try {
      emitEffectCreatedEvent(effectRef);
    } finally {
      setInjectorProfilerContext(prevInjectorProfilerContext);
    }
  }