怎么在watch监听开始之后立即被调用？

Answer 1

有个参数immediate
https://cn.vuejs.org/v2/api/#vm-watch

Answer 2

  let vm=new Vue({
        el:"#first",
        data:{msg:'liuneng'},
        watch:{

            msg:{
                handler (newMsg,oldMsg){
                    console.log(newMsg);
                },
                immediate:true
            }
        }
    })

设置immediate属性为true