export function onSnapshot()

in packages/firestore/src/api/reference_impl.ts [589:699]

• 82 lines of code
• 7 McCabe index (conditional complexity)

export function onSnapshot<T>(
  query: Query<T>,
  onNext: (snapshot: QuerySnapshot<T>) => void,
  onError?: (error: FirestoreError) => void,
  onCompletion?: () => void
): Unsubscribe;
/**
 * Attaches a listener for `QuerySnapshot` events. You may either pass
 * individual `onNext` and `onError` callbacks or pass a single observer
 * object with `next` and `error` callbacks. The listener can be cancelled by
 * calling the function that is returned when `onSnapshot` is called.
 *
 * NOTE: Although an `onCompletion` callback can be provided, it will
 * never be called because the snapshot stream is never-ending.
 *
 * @param query - The query to listen to.
 * @param options - Options controlling the listen behavior.
 * @param onNext - A callback to be called every time a new `QuerySnapshot`
 * is available.
 * @param onCompletion - Can be provided, but will not be called since streams are
 * never ending.
 * @param onError - A callback to be called if the listen fails or is
 * cancelled. No further callbacks will occur.
 * @returns An unsubscribe function that can be called to cancel
 * the snapshot listener.
 */
export function onSnapshot<T>(
  query: Query<T>,
  options: SnapshotListenOptions,
  onNext: (snapshot: QuerySnapshot<T>) => void,
  onError?: (error: FirestoreError) => void,
  onCompletion?: () => void
): Unsubscribe;
export function onSnapshot<T>(
  reference: Query<T> | DocumentReference<T>,
  ...args: unknown[]
): Unsubscribe {
  reference = getModularInstance(reference);

  let options: SnapshotListenOptions = {
    includeMetadataChanges: false
  };
  let currArg = 0;
  if (typeof args[currArg] === 'object' && !isPartialObserver(args[currArg])) {
    options = args[currArg] as SnapshotListenOptions;
    currArg++;
  }

  const internalOptions = {
    includeMetadataChanges: options.includeMetadataChanges
  };

  if (isPartialObserver(args[currArg])) {
    const userObserver = args[currArg] as PartialObserver<QuerySnapshot<T>>;
    args[currArg] = userObserver.next?.bind(userObserver);
    args[currArg + 1] = userObserver.error?.bind(userObserver);
    args[currArg + 2] = userObserver.complete?.bind(userObserver);
  }

  let observer: PartialObserver<ViewSnapshot>;
  let firestore: Firestore;
  let internalQuery: InternalQuery;

  if (reference instanceof DocumentReference) {
    firestore = cast(reference.firestore, Firestore);
    internalQuery = newQueryForPath(reference._key.path);

    observer = {
      next: snapshot => {
        if (args[currArg]) {
          (args[currArg] as NextFn<DocumentSnapshot<T>>)(
            convertToDocSnapshot(
              firestore,
              reference as DocumentReference<T>,
              snapshot
            )
          );
        }
      },
      error: args[currArg + 1] as ErrorFn,
      complete: args[currArg + 2] as CompleteFn
    };
  } else {
    const query = cast<Query<T>>(reference, Query);
    firestore = cast(query.firestore, Firestore);
    internalQuery = query._query;
    const userDataWriter = new ExpUserDataWriter(firestore);

    observer = {
      next: snapshot => {
        if (args[currArg]) {
          (args[currArg] as NextFn<QuerySnapshot<T>>)(
            new QuerySnapshot(firestore, userDataWriter, query, snapshot)
          );
        }
      },
      error: args[currArg + 1] as ErrorFn,
      complete: args[currArg + 2] as CompleteFn
    };

    validateHasExplicitOrderByForLimitToLast(reference._query);
  }

  const client = ensureFirestoreConfigured(firestore);
  return firestoreClientListen(
    client,
    internalQuery,
    internalOptions,
    observer
  );
}