class Watcher{
  constructor(data, key, cb){
  }
}
//转换成可监听对象
function observe(data){
  new Observer(data)
}
//修改数据的getter和setter
function defineReactive(obj, key){
  let value = obj[key];
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get(){
      return value;
    },
    set(newVal){
      value = newVal
    }
  })
}

class Observer {
  constructor(data){
    this.walk(data);
  }

  walk(data){
    for(var key in data) {
      // 这里不考虑嵌套的问题，否则的话需要递归调用walk
      defineReactive(data, key)
    }
  }
}

class Watcher{
  constructor(data, key, cb){
    this.cb = cb;
    Dep.target = this; //每次新建watcher的时候讲给target赋值，对target的管理这里简化了vue的实现
    data[key];//调用getter，执行addSub, 将target传入对应的dep; vue的实现本质就是如此
  }
}
class Dep {
  constructor(){
    this.subs = [];
  }
  addSub(sub){
    this.subs.push(sub);
  }
  notify(){
    this.subs.forEach(sub => sub.cb())
  }
}
function defineReactive(obj, key){
  let value = obj[key];
  let dep = new Dep(); //每一个属性都有一个对应的dep，作为闭包保存
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get(){
      dep.addSub(Dep.target)
      Dep.target = null;
      return value;
    },
    set(newVal){
      value = newVal
      dep.notify();
    }
  })
}

class Watcher {
  constructor(proxy, key, cb) {
    this.cb = cb;
    Dep.target = this;
    this.value = proxy[key];
  }
}
class Dep {
  constructor(){
    this.subs = []
  }
  addSub(sub){
    this.subs.push(sub);
  }
  notify(newVal){
    this.subs.forEach(sub => {
      sub.cb(newVal, sub.value);
      sub.value = newVal;
    })
  }
}
const observe = (obj) => {
  const deps = {};
  return new Proxy(obj, {
    get: function (target, key, receiver) {
      const dep = (deps[key] = deps[key] || new Dep);
      Dep.target && dep.addSub(Dep.target)
      Dep.target = null;
      return Reflect.get(target, key, receiver);
    },
    set: function (target, key, value, receiver) {
      const dep = (deps[key] = deps[key] || new Dep);
      Promise.resolve().then(() => {
        dep.notify(value);
      })
      return Reflect.set(target, key, value, receiver);
    }
  });
}
var state = observe({x:0})
new Watcher(state, 'x', function(n, o){
  console.log(n, o)
});
new Watcher(state, 'y', function(n, o){
  console.log(n, o)
});
state.x = 3;
state.y = 3;