Vue3 为什么要用 Proxy 代替 Object.defineProperty 实现响应式

var obj = {
  a: 1,
  o: {
    b: 2,
    o1: {}
  }
}

function defineReactive(target, key, value) {
    observer(value) // 对 value 深层监听
    
    Object.defineProperties(target, key, {
        get() {
            // dep.addSubs(watcher) // 添加到监听队列
            return value
        },
        set(newValue) {
            if (newValue !== value) {
                observer(newValue) // 再次劫持新 value
                
                value = newValue
                // dep.notify() // 通知依赖触发监听队列的更新
            }
        }
    })
}

function observer(target) {
  if (typeof target !== ‘object‘ || !target) {
    return target
  }
  
  if (Array.isArray(target)) {
    target.__proto__ = newArrProto
  }
  //遍历递归
  for (let key of target) {
    defineReactive(target, key, target[key])
  }
}
//创建原生数组原型
const oldArrProto = Array.prototype

const newArrProto = Object.create(oldArrProto)
//重写数组方法
[‘push‘, ‘pop‘, ‘shift‘, ‘unshift‘, ‘splice‘, ‘sort‘, ‘reverse‘].forEach(methodName => {
    newArrProto[methodName] = function(...args) {
        // dep.notify()
        oldArrProto[methodName].apply(this, args)
    }
})

function observer(target) {
  ...
  for (let key of target) {
    defineReactive(target, key, target[key])
  }
}

function defineReactive(target, key, value) {
  observer(target) // 首次监听时就对 value 的属性进行递归
  ...
}
监听原生数组的部分方法需要重写覆盖 Array.prototype
[‘push‘, ‘pop‘, ‘shift‘, ‘unshift‘, ‘splice‘, ‘sort‘, ‘reverse‘] 会改变原数组的原生方法不会被 Object.defineProperty 劫持，需要重新写数组的原生方法添加更新触发

// 创建响应式
function reactive(target = {}) {
    if (typeof target !== ‘object‘ || target == null) {
        return target
    }
    
    const proxyConfig = {
        get(target, key, receiver) {
        // 只处理本身（非原型的）属性
            // target是对象，ownKeys是属性的数组
            // target是数组，ownKeys是索引数组，最后一个是length
            const ownKeys = Reflect.ownKeys(target)
            if (ownKeys.includes(key)) {
              // dep.subs(watcher) // 添加监听
            }
            const result = Reflect.get(target, key, receiver)
        　　// 深度监听
            // 性能如何提升的？
            return reactive(result) // 只在 getter 时才再次劫持
        },
        set(target, key, val, receiver) {
        // 重复的数据，不处理
          if (val === target[key]) {
            return
          }
            
            const ownKeys = Reflect.ownKeys(target)
        // 判断是否是新增属性
            if (ownKeys.includes(key)) {
              // 已有值
            } else {
              // 新增值
            }
            
            const result = Reflect.set(target, key, val, receiver)
            // dep.noitfy() // 通知监听队列进行更新
        // 是否设置成功
            return result
        },
        deleteProperty(target, key) {
            const result = Reflect.deleteProperty(target, key)
            // 是否删除成功
            return result
        }
    }
    
    const observed = new Proxy(target, proxyConfig)
    return observed
}                            

    get(target, key, receiver) {
            const ownKeys = Reflect.ownKeys(target)
            if (ownKeys.includes(key)) {
              // dep.subs(watcher) // 添加监听
            }
            const result = Reflect.get(target, key, receiver)
            return reactive(result) // 只在 getter 时才再次劫持
        },