Function watch

src/apis/watch.ts:468–494

(
  source: WatchSource<T> | WatchSource<T>[],
  cb: WatchCallback<T>,
  options?: WatchOptions
)

// implementation
export function watch<T = any>(
  source: WatchSource<T> | WatchSource<T>[],
  cb: WatchCallback<T>,
  options?: WatchOptions
): WatchStopHandle {
  let callback: WatchCallback<unknown> | null = null
  if (isFunction(cb)) {
    // source watch
    callback = cb as WatchCallback<unknown>
  } else {
    // effect watch
    if (__DEV__) {
      warn(
        `\`watch(fn, options?)\` signature has been moved to a separate API. ` +
          `Use \`watchEffect(fn, options?)\` instead. \`watch\` now only ` +
          `supports \`watch(source, cb, options?) signature.`
      )
    }
    options = cb as Partial<WatchOptions>
    callback = null
  }

  const opts = getWatcherOption(options)
  const vm = getWatcherVM()

  return createWatcher(vm, source, callback, opts)
}

function traverse(value: unknown, seen: Set<unknown> = new Set()) {
  if (!isObject(value) || seen.has(value) || rawSet.has(value)) {