怎样动态加载路由？

<div id="first">    <router-link to="/a">老八一号</router-link>    <router-link to="/b">老八二号</router-link>    <router-link to="/c">老八三号</router-link>    <router-link to="/d">老八四号</router-link>    <router-view></router-view></div><script>    let a={        template:`<h1>奥里给 干了 兄弟们</h1>`    };    let b={        template:`<h2>虽然不是同一个时间，但是是同一个撤所儿</h2>`    };    let c={        template:`<h3>就吃老八秘制晓汉堡儿</h3>`    };    let d={        template:`<h2>老八秘制晓汉堡  既便宜它还管饱</h2>`    };    let routerObj=new VueRouter({        routes:[ {path:'/a',component:a,name:'一号'}, {path:'/b',component:b,name:'二号'}        ]    });    routerObj.addRoutes([        {path:'/c',component:c,name:'三号'},        {path:'/d',component:d,name:'四号'}    ]);    let vm=new Vue({        el:"#first",        data:{},        router:routerObj    })</script>

vue-router的addRoutes方法