1、建一个components文件夹，里面建一个Toast.vue文件

<template>
  <div class="toast">{{message}}</div>
</template>

<script>
import { reactive } from 'vue'
export default {
  props: ['message']
}
export const useToastEffect = () => {
  const toastData = reactive({
    showToast: false,
    toastMessage: ''
  })
  const showToast = (message) => {
    toastData.showToast = true
    toastData.toastMessage = message
    setTimeout(() => {
      toastData.showToast = false
      toastData.toastMessage = ''
    }, 2000)
  }
  return { toastData, showToast }
}
</script>

<style lang="scss" scoped>
.toast {
  position: fixed;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  padding: .1rem;
  background: rgba(0, 0, 0, .35);
  border-radius: .05rem;
  color: #FFF;
}
</style>

调用Toast组件

<template>
  <div class="wrapper">
    <img class="wrapper__img" src="http://www.dell-lee.com/imgs/vue3/user.png"/>
    <div class="wrapper__input">
      <input
        class="wrapper__input__content"
        placeholder="用户名"
        v-model="data.username"
      />
    </div>
    <div class="wrapper__input">
      <input
        type="password"
        class="wrapper__input__content"
        placeholder="请输入密码"
        v-model="data.password"
      />
    </div>
    <div class="wrapper__login-button" @click="handleLogin">登陆</div>
    <div class="wrapper__login-link" @click="handleRegisterClick">立即注册</div>
    <Toast v-if="toastData.showToast" :message="toastData.toastMessage" /> <!-- 2、页面中调用 -->
  </div>
</template>

<script>
import { reactive } from 'vue'
import { useRouter } from 'vue-router'
import { post } from '../../utils/request'
import Toast, { useToastEffect } from '../../components/Toast' // 1、在需要的页面引用

export default {
  name: 'Login',
  components: { Toast },
  setup () {
    const router = useRouter()
    const data = reactive({ username: '', password: '' })
    const { toastData, showToast } = useToastEffect() // 3、
    const handleLogin = async () => {
      try {
        const result = await post('/api/user/login', {
          username: data.username,
          password: data.password
        })
        if (result?.errno === 0) {
          localStorage.isLogin = true
          router.push({ name: 'Home' })
        } else {
          showToast('登录失败') // 4、调用showToast组件
        }
      } catch (e) {
        showToast('请求失败') // // 5、调用showToast组件
      }
    }
    const handleRegisterClick = () => {
      router.push({ name: 'Register' })
    }
    return { handleLogin, handleRegisterClick, data, toastData } // 6、返回toastData
  }
}
</script>

<style lang="scss" scoped>
@import '../../style/viriables.scss';
.wrapper {
  position: absolute;
  top: 50%;
  left: 0;
  right: 0;
  transform: translateY(-50%);
  &__img {
    display: block;
    margin: 0 auto .4rem auto;
    width: .66rem;
    height: .66rem;
  }
  &__input {
    height: .48rem;
    margin: 0 .4rem .16rem .4rem;
    padding: 0 .16rem;
    background: #F9F9F9;
    border: 1px solid rgba(0,0,0,0.10);
    border-radius: 6px;
    border-radius: 6px;
    &__content {
      line-height: .48rem;
      border: none;
      outline: none;
      width: 100%;
      background: none;
      font-size: .16rem;
      color: $content-notice-fontcolor;
      &::placeholder {
        color: $content-notice-fontcolor;
      }
    }
  }
  &__login-button {
    margin: .32rem .4rem .16rem .4rem;
    line-height: .48rem;
    background: #0091FF;
    box-shadow: 0 .04rem .08rem 0 rgba(0,145,255,0.32);
    border-radius: .04rem;
    border-radius: .04rem;
    color: #fff;
    font-size: .16rem;
    text-align: center;
  }
  &__login-link {
    text-align: center;
    font-size: .14rem;
    color: $content-notice-fontcolor;
  }
}
</style>