SOURCE

 class Dep{
            constructor(){
                this.subs=[];
            }
            addSub(sub){
                this.subs.push(sub)
            }
            removeSub(sub){
                remove(this.subs,sub)
            }
            depend(){
                if(window.target){
                    this.addSub(window.target);
                }
            }
            notify(){
                const subs=this.subs.slice();
                console.log("subs",subs)
                for(let i=0,l=subs.length;i<l;i++){
                    subs[i].update();
                }
            }

        }
        function remove(arr,item){
            if(arr.length){
                const index=arr.indexOf(item);
                if(index>-1){
                    return arr.slice(index,1);
                }
            }
        }
        //let dep=[];
        //
        function defineReactive(data,key,val){
            let dep=new Dep();//修改
            Object.defineProperty(data,key,{
                enumerable:true,
                configurable:true,
                get:function(t){
                     console.log("get",window.target)
                    // dep.push(window.target)
                    dep.depend();
                    return val;
                },
                set:function(newVal){
                    console.log("set",newVal)
                    if(val===newVal){
                        return;
                    }
                    for(let i=0;i<dep.length;i++){
                        dep[i](newVal,val);
                    }
                    val=newVal;
                    dep.notify();
                }
            });
        };
        const bailRE=/[^\w.$]/;
        function parsePath(path){
            if(bailRE.test(path)){
                return;
            }
            const segments=path.split(".");
            //这里返回的是一个函数，obj，就是最外层的obj对象
            return function(obj){
                for(let i=0;i<segments.length;i++){
                    if(!obj){
                        return;
                    }
                    obj=obj[segments[i]];
                };
                return obj;
            }
        };
        class Watcher{
            constructor(vm,expOrFn,cb){
                this.vm=vm;
                this.getter=parsePath(expOrFn);
                console.log("this.getter",this.getter,typeof this.getter)
                this.cb=cb;
                this.value=this.get();
            }
            get(){
                window.target=this;
                console.log("window.target",window.target,this.getter)
                let value=this.getter.call(this.vm,this.vm);
                window.target=undefined;
                return value;
            }
            update() {
                const oldValue=this.value;
                this.value=this.get();
                this.cb.call(this.vm,this.value,oldValue); 
            }
        }
        let obj={};
        let vm= defineReactive(obj,"z",100);
        new Watcher(obj,"z",function(oldv,newV){
            console.log("==watch==",oldv,newV)
        })
       
    //    console.log("vm",vm)
    //     obj.z=200;
         console.log(obj,obj.z)
        

      //  obj.z=300;

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