前提

element-plus以按需加载方式引入
https://element-plus.gitee.io/zh-CN/guide/quickstart.html#%E6%8C%89%E9%9C%80%E5%AF%BC%E5%85%A5

问题现象

在使用elementPlus按需引入时,使用options api，全局$meeage,notify,$loading使用时会报未定义

问题原因

如果是全量引入element-plus 则不存在该问题。
按需引入时element-plus未给全局vue实例app.config.globalProperties绑定$meeage,notify,$loading等全局方法，导致报错，需要手工绑定。

问题解决

采用插件方式安装全局方法

components/elementPlus目录新建插件文件

elementPlusMessage.js,elementPlusLoading.js,elementPlusNotify.js

elementPlusMessage.js

import 'element-plus/es/components/message/style/css'
import {ElMessage} from "element-plus"


/**
 * 安装 按需引入情况下支持 $message方法
 */
export const ElementPlusMessage = {
    install: (Vue) => {
        let elMessage = (op) => {
            return ElMessage(op)
        }
        elMessage.success = (op) => {
            return ElMessage.success(op)
        }
        elMessage.warning = (op) => {
            return ElMessage.warning(op)
        }
        elMessage.error = (op) => {
            return ElMessage.error(op)
        }
        elMessage.info = (op) => {
            return ElMessage.info(op)
        }
        Vue.config.globalProperties.$message = elMessage
    }
}

export default ElementPlusMessage

elementPlusLoading.js

import 'element-plus/es/components/loading/style/css'
import {ElLoading} from "element-plus"

/**
 * 安装 按需引入情况下支持 $loading方法
 */
export const ElementPlusLoading = {
    install: (Vue) => {
        Vue.config.globalProperties.$loading = (op) => {
            return ElLoading.service(op)
        }
    }
}

export default ElementPlusLoading

elementPlusNotify.js

import 'element-plus/es/components/notification/style/css'
import {ElNotification} from "element-plus"


/**
 * 安装 按需引入情况下支持 $notify 方法
 */
export const ElementPlusNotify = {
    install: (Vue) => {
        let notification = (op) => {
            return ElNotification(op);
        }
        notification.success = (op) => {
            return ElNotification.success(op)
        }
        notification.warning = (op) => {
            return ElNotification.warning(op)
        }
        notification.error = (op) => {
            return ElNotification.error(op)
        }
        notification.info = (op) => {
            return ElNotification.info(op)
        }
        Vue.config.globalProperties.$notify = notification
    }
}

export default ElementPlusNotify

main.js修改安装插件

import ElementPlusLoading from "@components/elementPlus/elementPlusLoading";
import ElementPlusMessage from "@components/elementPlus/elementPlusMessage";
import ElementPlusNotify from "@components/elementPlus/elementPlusNotify";
const app = createApp(App)
    .use(ElementPlusLoading)
    .use(ElementPlusMessage)
    .use(ElementPlusNotify)
    .use(router).mount('#app')

至此，可以正常使用$meeage,notify,$loading等全局方法