Vue3响应式系统原理

1. proxy的优点

• Proxy对象实现属性监听
• 多层嵌套，在访问属性过程中处理下一级属性
• 默认监听动态添加的属性
• 默认监听属性的删除操作
• 默认监听数组的索引和length属性
• 可以作为单独模块使用

2. Vue3响应式API的模拟实现

const isObject = (val) => val !== null && typeof val === 'object'
const convert = (target) => (isObject(target) ? reactive(target) : target)
const hasOwnProperty = Object.prototype.hasOwnProperty
const hasOwn = (target, key) => hasOwnProperty.call(target, key)

/*
  1.接收一个参数，判断这参数是否是对象
  2.创建拦截器对象handler，设置get/set/deleteProperty
  3.返回Proxy对象
*/
function reactive(target) {
  if (!isObject(target)) return target

  const handler = {
    get(target, key, receiver) {
      // 收集依赖
      track(target, key)
      const result = Reflect.get(target, key, receiver)
      return convert(result)
    },
    set(target, key, value, receiver) {
      const oldValue = Reflect.get(target, key, receiver)
      let result = true
      if (oldValue !== value) {
        result = Reflect.set(target, key, value, receiver)
        // 触发更新
        trigger(target, key)
      }
      return result
    },
    deleteProperty(target, key) {
      const hadKey = hasOwn(target, key)
      const result = Reflect.deleteProperty(target, key)
      if (hadKey && result) {
        // 触发更新
        trigger(target, key)
      }
      return result
    },
  }

  return new Proxy(target, handler)
}

let activeEffect = null
function effect(callback) {
  activeEffect = callback
  callback() // 访问响应式对象属性，去收集依赖
  activeEffect = null
}

//收集依赖
let targetMap = new WeakMap()
function track(target, key) {
  if (!activeEffect) return
  let depsMap = targetMap.get(target)
  if (!depsMap) {
    targetMap.set(target, (depsMap = new Map()))
  }
  let dep = depsMap.get(key)
  if (!dep) {
    depsMap.set(key, (dep = new Set()))
  }
  dep.add(activeEffect)
}

//触发更新
function trigger(target, key) {
  const depsMap = targetMap.get(target)
  if (!depsMap) return
  const dep = depsMap.get(key)
  if (dep) {
    dep.forEach((effect) => {
      effect()
    })
  }
}

function ref(raw) {
  // 判断 raw 是否是ref 创建的对象，如果是的话直接返回
  if (isObject(raw) && raw.__v_isRef) {
    return
  }
  let value = convert(raw)
  const r = {
    __v_isRef: true,
    get value() {
      track(r, 'value')
      return value
    },
    set value(newValue) {
      if (newValue !== value) {
        raw = newValue
        value = convert(raw)
        trigger(r, 'value')
      }
    },
  }
  return r
}

function toRefs(proxy) {
  const ret = proxy instanceof Array ? new Array(proxy.length) : {}
  for (const key in proxy) {
    ret[key] = toProxyRef(proxy, key)
  }
  return ret
}

function toProxyRef(proxy, key) {
  const r = {
    __v_isRef: true,
    get value() {
      return proxy[key]
    },
    set value(newValue) {
      proxy[key] = newValue
    },
  }
  return r
}

//计算属性实现
function computed(getter) {
  const result = ref()
  effect(() => (result.value = getter()))
  return result
}