Vue vuexでfirebaseのログイン保持 ref: https://qiita.com/ErgoFriend/items/bd1bb445e185bf45e272

About.vue

<template>
  <div class="about">
    <div class="box">
      <div class="media">
        <figure class="image is-64x64 media-left">
          <img class="is-rounded" :src="user.photoURL">
        </figure>
        <div class="media-content">
          <div class="content">
            <p>
              <strong>{{ user.displayName }} </strong> <small>{{ user.email }} </small>
              <br>
              {{ user.uid }}
            </p>
          </div>
          <nav class="level is-mobile">
            <div class="level-left">
              <a class="level-item">
                <span class="icon is-small"><i class="fas fa-reply"></i></span>
              </a>
              <a class="level-item">
                <span class="icon is-small"><i class="fas fa-retweet"></i></span>
              </a>
              <a class="level-item">
                <span class="icon is-small"><i class="fas fa-heart"></i></span>
              </a>
            </div>
          </nav>
        </div>
      </div>
    </div>
    <footer class="footer">
      <div class="content has-text-centered">
        <p>
      <strong>Makina</strong> by <a href="https://twitter.com/ergofriend">ergofriend</a>. The source code is licensed
      <a href="http://opensource.org/licenses/mit-license.php">MIT</a>. The website content
      is licensed <a href="http://creativecommons.org/licenses/by-nc-sa/4.0/">CC BY NC SA 4.0</a>.
        </p>
      </div>
    </footer>
  </div>
</template>
<script>
export default {
  name: "about",
  computed: {
    user() {
      return this.$store.getters.user;
    }
  },
  components: {}
};
</script>
<style scoped>
.box {
  margin: 5% 10%;
}
</style>

App.vue

<template>
  <div id="app">
    <Navigation />
    <router-view/>
  </div>
</template>
<script>
import Navigation from "@/components/Navigation.vue";
export default {
  name: "home",
  components: {
    Navigation
  }
};
</script>
<style>
#app {
  font-family: "Avenir", Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-align: center;
  color: #2c3e50;
}
#nav {
  padding: 30px;
}
#nav a {
  font-weight: bold;
  color: #2c3e50;
}
#nav a.router-link-exact-active {
  color: #42b983;
}
</style>

Authentication.vue

<template>
    <div class="authentication">
      <!-- ログイン時にはフォームとログアウトボタンを表示 -->
      <div v-if="userStatus" key="login" class="navbar-item">
        <p class="navbar-item">{{ user.displayName }}</p>
        <button type="button" class="button is-small is-info is-outlined" @click="doLogout">
          Sign out
        </button>
      </div>
      <!-- 未ログイン時にはログインボタンを表示 -->
      <div v-else key="logout">
        <button type="button" class="google-button" @click="doLogin">
          <span class="google-button__icon">
            <svg viewBox="0 0 366 372" xmlns="http://www.w3.org/2000/svg"><path d="M125.9 10.2c40.2-13.9 85.3-13.6 125.3 1.1 22.2 8.2 42.5 21 59.9 37.1-5.8 6.3-12.1 12.2-18.1 18.3l-34.2 34.2c-11.3-10.8-25.1-19-40.1-23.6-17.6-5.3-36.6-6.1-54.6-2.2-21 4.5-40.5 15.5-55.6 30.9-12.2 12.3-21.4 27.5-27 43.9-20.3-15.8-40.6-31.5-61-47.3 21.5-43 60.1-76.9 105.4-92.4z" id="Shape" fill="#EA4335"/><path d="M20.6 102.4c20.3 15.8 40.6 31.5 61 47.3-8 23.3-8 49.2 0 72.4-20.3 15.8-40.6 31.6-60.9 47.3C1.9 232.7-3.8 189.6 4.4 149.2c3.3-16.2 8.7-32 16.2-46.8z" id="Shape" fill="#FBBC05"/><path d="M361.7 151.1c5.8 32.7 4.5 66.8-4.7 98.8-8.5 29.3-24.6 56.5-47.1 77.2l-59.1-45.9c19.5-13.1 33.3-34.3 37.2-57.5H186.6c.1-24.2.1-48.4.1-72.6h175z" id="Shape" fill="#4285F4"/><path d="M81.4 222.2c7.8 22.9 22.8 43.2 42.6 57.1 12.4 8.7 26.6 14.9 41.4 17.9 14.6 3 29.7 2.6 44.4.1 14.6-2.6 28.7-7.9 41-16.2l59.1 45.9c-21.3 19.7-48 33.1-76.2 39.6-31.2 7.1-64.2 7.3-95.2-1-24.6-6.5-47.7-18.2-67.6-34.1-20.9-16.6-38.3-38-50.4-62 20.3-15.7 40.6-31.5 60.9-47.3z" fill="#34A853"/></svg>
          </span>
          <span class="google-button__text">Sign in with Google</span>
        </button>
      </div>
    </div>
</template>
<script>
import Firebase from './../firebase';
export default {
  name: 'authentication',
  created: function() {
    Firebase.onAuth();
  },
  computed: {
    user() {
      return this.$store.getters.user;
    },
    userStatus() {
    // ログインするとtrue
      return this.$store.getters.isSignedIn;
    }
  },
  methods: {
    // ログイン処理
    doLogin() {
      Firebase.login();
    },
    // ログアウト処理
    doLogout() {
      Firebase.logout();
    }
  }
};
</script>
<style scoped>
.authentication {
  display: inline-block;
}
.google-button {
  height: 40px;
  border-width: 0;
  background: white;
  color: #737373;
  border-radius: 5px;
  white-space: nowrap;
  box-shadow: 1px 1px 0px 1px rgba(0, 0, 0, 0.05);
  transition-property: background-color, box-shadow;
  transition-duration: 150ms;
  transition-timing-function: ease-in-out;
  padding: 0;
}
.google-button:focus,
.google-button:hover {
  box-shadow: 1px 4px 5px 1px rgba(0, 0, 0, 0.1);
}
.google-button:active {
  background-color: #e5e5e5;
  box-shadow: none;
  transition-duration: 10ms;
}
.google-button__icon {
  display: inline-block;
  vertical-align: middle;
  margin: 8px 0 8px 8px;
  width: 18px;
  height: 18px;
  box-sizing: border-box;
}
.google-button__icon--plus {
  width: 27px;
}
.google-button__text {
  display: inline-block;
  vertical-align: middle;
  padding: 0 24px;
  font-size: 14px;
  font-weight: bold;
  font-family: "Roboto", arial, sans-serif;
}
html,
body {
  height: 100%;
}
body {
  background-color: #f0f0f0;
  display: flex;
  align-items: center;
  justify-content: center;
}
</style>

file0.txt

node 8.11.4
npm 5.6.0
vue 3.0.1

npm install -g @vue/cli
npm install -g @vue/cli-service-global

file1.txt

git clone https://github.com/ErgoFriend/makina.git
cd makina
npm install
vue ui

file4.txt

export default {
  init() {},
  login() {},
  logout() {},
  onAuth() {}
};

firebase.js

import firebase from "@firebase/app";
import "@firebase/auth";
import store from "./store";

const config = {
  apiKey: "ほげほげ",
  authDomain: "ほげほg..."
  data....   //省略します！
};

export default {
  init() {
    firebase.initializeApp(config);
    firebase.auth().setPersistence(firebase.auth.Auth.Persistence.SESSION);
  },
  login() {
    const provider = new firebase.auth.GoogleAuthProvider();
    firebase.auth().signInWithPopup(provider)
  },
  logout() {
    firebase.auth().signOut()
  },
  onAuth() {
    firebase.auth().onAuthStateChanged(user => {
      user = user ? user : {};
      store.commit('onAuthStateChanged', user);
      store.commit('onUserStatusChanged', user.uid ? true : false);
    });
  }
};

Home.vue

<template>
  <div class="home">
    <h1>hello</h1>
  </div>
</template>
<script>
export default {
  name: "home",
  components: {}
};
</script>
<style scoped>
.home {
  height: 90vh;
  background-repeat: no-repeat;
  background-position: center;
  background-size: contain;
  background-image: url("https://pbs.twimg.com/media/DgOcLUnU8AEgN56.jpg");
}
</style>

main.js

import Vue from "vue";
import App from "./App.vue";
import router from "./router";
import store from "./store";

import Buefy from "buefy";
import "buefy/dist/buefy.min.css";
Vue.use(Buefy);

import Firebase from "./firebase";
Firebase.init();

Vue.config.productionTip = false;
new Vue({
  router,
  store,
  render: h => h(App)
}).$mount("#app");

Navigation.vue

<template>
    <div id="nav">
      <nav class="navbar is-transparent" v-bind:class="{'is-fixed-bottom':menuPosition}">
        <div class="navbar-brand">
          <router-link class="navbar-item" to="/">
            <img src="https://bulma.io/images/bulma-logo.png" alt="Bulma: a modern CSS framework based on Flexbox" width="112" height="28">
          </router-link>
          <div class="navbar-burger burger " v-bind:class="{ 'is-active': menuActive }" v-on:click="menuToggle()" data-target="navbarExampleTransparentExample">
            <span></span>
            <span></span>
            <span></span>
          </div>
        </div>
        <div id="navbarExampleTransparentExample" class="navbar-menu" v-bind:class="{ 'is-active': menuActive }">
          <div class="navbar-start">
            <router-link class="navbar-item" to="/">Home</router-link>
            <router-link class="navbar-item" to="/about">About</router-link>
          </div>
          <div class="navbar-end">
            <Authentication />
          </div>
        </div>
      </nav>
    </div>
</template>
<script>
import Authentication from "@/components/Authentication.vue";
export default {
  name: "navigation",
  components: {
    Authentication
  },
  created() {
    if (window.innerWidth < 920) {
      this.menuPosition = !this.menuPosition;
    }
  },
  data() {
    return {
      menuActive: false,
      menuPosition: false
    };
  },
  methods: {
    menuToggle() {
      this.menuActive = !this.menuActive;
    }
  }
};
</script>
<style scoped>
#nav {
  padding: 30px;
}
#nav a {
  font-weight: bold;
  color: #555;
}
#nav a.router-link-exact-active {
  color: #3273dc;
}
</style>

src

│  App.vue
│  main.js
│  router.js
│  store.js
|  firebase.js  //追加
├─assets
│      logo.png
├─components
│      // HelloWorld.vue を削除します
|      Authentication.vue  //追加
|      Navigation.vue  //追加
└─views
        About.vue
        Home.vue

store.js

import Vue from 'vue';
import Vuex from 'vuex';

Vue.use(Vuex);

export default new Vuex.Store({
  state: {
    user: {},
    status: false
  },
  mutations: {
    onAuthStateChanged(state, user) {
      state.user = user; //firebaseが返したユーザー情報
    },
    onUserStatusChanged(state, status) {
      state.status = status; //ログインしてるかどうか true or false
    }
  },
  getters: {
    user(state) {
      return state.user;
    },
    isSignedIn(state) {
      return state.status;
    }
  }
});