Function instanceWatch

packages/runtime-core/src/apiWatch.ts:245–268

(
  this: ComponentInternalInstance,
  source: string | Function,
  value: WatchCallback | ObjectWatchOptionItem,
  options?: WatchOptions,
)

// this.$watch
export function instanceWatch(
  this: ComponentInternalInstance,
  source: string | Function,
  value: WatchCallback | ObjectWatchOptionItem,
  options?: WatchOptions,
): WatchHandle {
  const publicThis = this.proxy
  const getter = isString(source)
    ? source.includes('.')
      ? createPathGetter(publicThis!, source)
      : () => publicThis![source as keyof typeof publicThis]
    : source.bind(publicThis, publicThis)
  let cb
  if (isFunction(value)) {
    cb = value
  } else {
    cb = value.handler as Function
    options = value
  }
  const reset = setCurrentInstance(this)
  const res = doWatch(getter, cb.bind(publicThis), options)
  reset()
  return res
}

export function createPathGetter(
  ctx: ComponentPublicInstance,