Function createWatch

packages/core/primitives/signals/src/watch.ts:65–137

(
  fn: (onCleanup: WatchCleanupRegisterFn) => void,
  schedule: (watch: Watch) => void,
  allowSignalWrites: boolean,
)

}

export function createWatch(
  fn: (onCleanup: WatchCleanupRegisterFn) => void,
  schedule: (watch: Watch) => void,
  allowSignalWrites: boolean,
): Watch {
  const node: WatchNode = Object.create(WATCH_NODE);
  if (allowSignalWrites) {
    node.consumerAllowSignalWrites = true;
  }

  node.fn = fn;
  node.schedule = schedule;

  const registerOnCleanup = (cleanupFn: WatchCleanupFn) => {
    node.cleanupFn = cleanupFn;
  };

  function isWatchNodeDestroyed(node: WatchNode) {
    return node.fn === null && node.schedule === null;
  }

  function destroyWatchNode(node: WatchNode) {
    if (!isWatchNodeDestroyed(node)) {
      consumerDestroy(node); // disconnect watcher from the reactive graph
      node.cleanupFn();

      // nullify references to the integration functions to mark node as destroyed
      node.fn = null;
      node.schedule = null;
      node.cleanupFn = NOOP_CLEANUP_FN;
    }
  }

  const run = () => {
    if (node.fn === null) {
      // trying to run a destroyed watch is noop
      return;
    }

    if (isInNotificationPhase()) {
      throw new Error(
        typeof ngDevMode !== 'undefined' && ngDevMode
          ? 'Schedulers cannot synchronously execute watches while scheduling.'
          : '',
      );
    }

    node.dirty = false;
    if (node.version > 0 && !consumerPollProducersForChange(node)) {
      return;
    }
    node.version++;

    const prevConsumer = consumerBeforeComputation(node);
    try {
      node.cleanupFn();
      node.cleanupFn = NOOP_CLEANUP_FN;
      node.fn(registerOnCleanup);