学习[前端][Vue3.0] isReactive,isShadow,isProxy函数

isReactive,isShadow,isProxy这些函数通过预先埋在对象中的ReactiveFlag枚举，来判断一个响应式对象的状态,这么做的理由估计是为了提高变量的校验速度吧，只需要通过kv的方式来直接获取需要判断的变量。

export const enum ReactiveFlags {
  SKIP = '__v_skip',
  IS_REACTIVE = '__v_isReactive',
  IS_READONLY = '__v_isReadonly',
  IS_SHALLOW = '__v_isShallow',
  RAW = '__v_raw'
}

/**
 * Checks if an object is a proxy created by {@link reactive()} or
 * {@link shallowReactive()} (or {@link ref()} in some cases).
 *
 * @example
 * ```js
 * isReactive(reactive({}))            // => true
 * isReactive(readonly(reactive({})))  // => true
 * isReactive(ref({}).value)           // => true
 * isReactive(readonly(ref({})).value) // => true
 * isReactive(ref(true))               // => false
 * isReactive(shallowRef({}).value)    // => false
 * isReactive(shallowReactive({}))     // => true
 * ```
 *
 * @param value - The value to check.
 * @see {@link https://vuejs.org/api/reactivity-utilities.html#isreactive}
 */
export function isReactive(value: unknown): boolean {
  if (isReadonly(value)) {
    return isReactive((value as Target)[ReactiveFlags.RAW])
  }
  return !!(value && (value as Target)[ReactiveFlags.IS_REACTIVE])
}

/**
 * Checks whether the passed value is a readonly object. The properties of a
 * readonly object can change, but they can't be assigned directly via the
 * passed object.
 *
 * The proxies created by {@link readonly()} and {@link shallowReadonly()} are
 * both considered readonly, as is a computed ref without a set function.
 *
 * @param value - The value to check.
 * @see {@link https://vuejs.org/api/reactivity-utilities.html#isreadonly}
 */
export function isReadonly(value: unknown): boolean {
  return !!(value && (value as Target)[ReactiveFlags.IS_READONLY])
}

export function isShallow(value: unknown): boolean {
  return !!(value && (value as Target)[ReactiveFlags.IS_SHALLOW])
}

/**
 * Checks if an object is a proxy created by {@link reactive},
 * {@link readonly}, {@link shallowReactive} or {@link shallowReadonly()}.
 *
 * @param value - The value to check.
 * @see {@link https://vuejs.org/api/reactivity-utilities.html#isproxy}
 */
export function isProxy(value: unknown): boolean {
  return isReactive(value) || isReadonly(value)
}