Function watch

packages/runtime-core/src/apiWatch.ts:131–144

(
  source: T | WatchSource<T>,
  cb: any,
  options?: WatchOptions<Immediate>,
)

// implementation
export function watch<T = any, Immediate extends Readonly<boolean> = false>(
  source: T | WatchSource<T>,
  cb: any,
  options?: WatchOptions<Immediate>,
): WatchHandle {
  if (__DEV__ && !isFunction(cb)) {
    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.`,
    )
  }
  return doWatch(source as any, cb, options)
}

function doWatch(
  source: WatchSource | WatchSource[] | WatchEffect | object,