watcher ​

class Watcher{
    constructor(vm,key,cb) {
        this.vm = vm
        this.key = key
        this.cb = cb

        Dep.target = this
        this.oldValue = vm[key]

        Dep.target = null
    }

    update(){
        const newVal = this.vm[this.key]
        if(newVal === this.oldValue) return

        this.cb(newVal)
    }
}

class Watcher{
    constructor(vm,key,cb) {
        this.vm = vm
        this.key = key
        this.cb = cb

        Dep.target = this
        this.oldValue = vm[key]

        Dep.target = null
    }

    update(){
        const newVal = this.vm[this.key]
        if(newVal === this.oldValue) return

        this.cb(newVal)
    }
}