signIn(){
    this.formData = $(".form").serialize();
    var that = this;
    this.$http.get("/api/user", this.formData)
     .then(response => {
      that.userName = response.data.data.user.userName;
      that.userHead = response.data.data.userHead;
      that.$emit('userSignIn', that.userName);
     })
     .catch(error => {
      console.log(error);
     });
   }

 that.$emit('userSignIn', that.userName);

<keep-alive>
   <router-view @userSignIn="userSignIn"></router-view>
</keep-alive>

export default {

 data(){
  return{
   userName: sessionStorage.userName
  }
 },

 methods:{
  //子组件(register)将用户名传过来
  userSignIn(userName){
   sessionStorage.userName = userName;
   this.userName = sessionStorage.userName;
  }
 }
}