SOURCE

const router = new VueRouter({
  routes : [{
      path: '/user/:username',
      component: {
        template: '<div>{{$route.params.username}}</div>',
        watch: {
          '$route' (to, from) {
            console.log(from);
          }
        }
      }
  }]
});

var vm = new Vue({
  el: '#app',
  router
});

<script src="https://unpkg.com/vue/dist/vue.js"></script>
<script src="https://unpkg.com/vue-router/dist/vue-router.js"></script>

<div id="app">
  <div>
    <router-link to="/user/user1">进入用户1</router-link>
  </div>
  <div>
    <router-link to="/user/user2">进入用户2</router-link>  
  </div>
  <router-view></router-view>
</div>