Vue.use做了什么，以element-ui为例简要说明

Vue.use做了什么

我们在使用vue+element-ui做前端开发时经常会在工程的main.js里面写上如下几句代码（注：多余的代码已忽略）

import Vue from 'vue'
import Element from 'element-ui'
...
Vue.use(Element, {
  size: Cookies.get('size') || 'medium' // set element-ui default size
  // locale: enLang // 如果使用中文，无需设置，请删除
})

通过以上代码设定之后我们便可以在各个vue页面中任意使用element-ui的组件了，这中间到底发生了什么事情呢
Vue.use方法内部代码大致如下（多余部分已经省略）

// 这里的plugin参数就是，就是我们通过Vue.use(Element)引入的
Vue.use = function (plugin: Function | Object) {
    
	// ......
	
	const args = toArray(arguments, 1)
	// 这一句很重要，这里的this，就是Vue，把他添加到args数组的第一个元素
	args.unshift(this)
	// 判断我们传递进来的"Element"，也即这里的"plugin"内部是否有一个叫"install"的方法
	// 如果有，就执行我们的"Element"，也即"plugin.install"方法
    if (typeof plugin.install === 'function') {
      plugin.install.apply(plugin, args)
    } else if (typeof plugin === 'function') {
      plugin.apply(null, args)
    }
	
	// ......
}

也就是说Vue.use内部会执行第一个参数的install方法，那么我们看下 element-ui内部怎么写的

import Pagination from '../packages/pagination/index.js';
import Dialog from '../packages/dialog/index.js';
import Autocomplete from '../packages/autocomplete/index.js';
import Dropdown from '../packages/dropdown/index.js';
import DropdownMenu from '../packages/dropdown-menu/index.js';
import DropdownItem from '../packages/dropdown-item/index.js';
import Menu from '../packages/menu/index.js';
import Submenu from '../packages/submenu/index.js';
import MenuItem from '../packages/menu-item/index.js';
import MenuItemGroup from '../packages/menu-item-group/index.js';
import Input from '../packages/input/index.js';
import InputNumber from '../packages/input-number/index.js';
import Radio from '../packages/radio/index.js';
import RadioGroup from '../packages/radio-group/index.js';
import RadioButton from '../packages/radio-button/index.js';
import Checkbox from '../packages/checkbox/index.js';
import CheckboxButton from '../packages/checkbox-button/index.js';
import CheckboxGroup from '../packages/checkbox-group/index.js';
import Switch from '../packages/switch/index.js';
import Select from '../packages/select/index.js';
import Option from '../packages/option/index.js';
import OptionGroup from '../packages/option-group/index.js';
import Button from '../packages/button/index.js';
import ButtonGroup from '../packages/button-group/index.js';
import Table from '../packages/table/index.js';
import TableColumn from '../packages/table-column/index.js';
import DatePicker from '../packages/date-picker/index.js';
import TimeSelect from '../packages/time-select/index.js';
import TimePicker from '../packages/time-picker/index.js';
import Popover from '../packages/popover/index.js';
import Tooltip from '../packages/tooltip/index.js';
import MessageBox from '../packages/message-box/index.js';
import Breadcrumb from '../packages/breadcrumb/index.js';
import BreadcrumbItem from '../packages/breadcrumb-item/index.js';
import Form from '../packages/form/index.js';
import FormItem from '../packages/form-item/index.js';
import Tabs from '../packages/tabs/index.js';
import TabPane from '../packages/tab-pane/index.js';
import Tag from '../packages/tag/index.js';
import Tree from '../packages/tree/index.js';
import Alert from '../packages/alert/index.js';
import Notification from '../packages/notification/index.js';
import Slider from '../packages/slider/index.js';
import Loading from '../packages/loading/index.js';
import Icon from '../packages/icon/index.js';
import Row from '../packages/row/index.js';
import Col from '../packages/col/index.js';
import Upload from '../packages/upload/index.js';
import Progress from '../packages/progress/index.js';
import Spinner from '../packages/spinner/index.js';
import Message from '../packages/message/index.js';
import Badge from '../packages/badge/index.js';
import Card from '../packages/card/index.js';
import Rate from '../packages/rate/index.js';
import Steps from '../packages/steps/index.js';
import Step from '../packages/step/index.js';
import Carousel from '../packages/carousel/index.js';
import Scrollbar from '../packages/scrollbar/index.js';
import CarouselItem from '../packages/carousel-item/index.js';
import Collapse from '../packages/collapse/index.js';
import CollapseItem from '../packages/collapse-item/index.js';
import Cascader from '../packages/cascader/index.js';
import ColorPicker from '../packages/color-picker/index.js';
import Transfer from '../packages/transfer/index.js';
import Container from '../packages/container/index.js';
import Header from '../packages/header/index.js';
import Aside from '../packages/aside/index.js';
import Main from '../packages/main/index.js';
import Footer from '../packages/footer/index.js';
import Timeline from '../packages/timeline/index.js';
import TimelineItem from '../packages/timeline-item/index.js';
import Link from '../packages/link/index.js';
import Divider from '../packages/divider/index.js';
import Image from '../packages/image/index.js';
import Calendar from '../packages/calendar/index.js';
import Backtop from '../packages/backtop/index.js';
import InfiniteScroll from '../packages/infinite-scroll/index.js';
import PageHeader from '../packages/page-header/index.js';
import CascaderPanel from '../packages/cascader-panel/index.js';
import Avatar from '../packages/avatar/index.js';
import Drawer from '../packages/drawer/index.js';
import Popconfirm from '../packages/popconfirm/index.js';
import locale from 'element-ui/src/locale';
import CollapseTransition from 'element-ui/src/transitions/collapse-transition';

//定义一个组件数组，存放element-ui开放出来的所有组件
const components = [
  Pagination,
  Dialog,
  Autocomplete,
  Dropdown,
  DropdownMenu,
  DropdownItem,
  Menu,
  Submenu,
  MenuItem,
  MenuItemGroup,
  Input,
  InputNumber,
  Radio,
  RadioGroup,
  RadioButton,
  Checkbox,
  CheckboxButton,
  CheckboxGroup,
  Switch,
  Select,
  Option,
  OptionGroup,
  Button,
  ButtonGroup,
  Table,
  TableColumn,
  DatePicker,
  TimeSelect,
  TimePicker,
  Popover,
  Tooltip,
  Breadcrumb,
  BreadcrumbItem,
  Form,
  FormItem,
  Tabs,
  TabPane,
  Tag,
  Tree,
  Alert,
  Slider,
  Icon,
  Row,
  Col,
  Upload,
  Progress,
  Spinner,
  Badge,
  Card,
  Rate,
  Steps,
  Step,
  Carousel,
  Scrollbar,
  CarouselItem,
  Collapse,
  CollapseItem,
  Cascader,
  ColorPicker,
  Transfer,
  Container,
  Header,
  Aside,
  Main,
  Footer,
  Timeline,
  TimelineItem,
  Link,
  Divider,
  Image,
  Calendar,
  Backtop,
  PageHeader,
  CascaderPanel,
  Avatar,
  Drawer,
  Popconfirm,
  CollapseTransition
];

//定义install方法，其中第一个参数就是Vue对象 也就是Vue.use内部调用install时传入的args数组的第一个参数Vue
const install = function(Vue, opts = {}) {
  locale.use(opts.locale);
  locale.i18n(opts.i18n);
  //这句很重要，循环element-ui的组件数组并调用Vue.component方法进行全局组件注册，全局注册之后所有页面都可以使用了
  components.forEach(component => {
    //全局注册
    Vue.component(component.name, component);
  });

  Vue.use(InfiniteScroll);
  Vue.use(Loading.directive);

  Vue.prototype.$ELEMENT = {
    size: opts.size || '',
    zIndex: opts.zIndex || 2000
  };

  Vue.prototype.$loading = Loading.service;
  Vue.prototype.$msgbox = MessageBox;
  Vue.prototype.$alert = MessageBox.alert;
  Vue.prototype.$confirm = MessageBox.confirm;
  Vue.prototype.$prompt = MessageBox.prompt;
  Vue.prototype.$notify = Notification;
  Vue.prototype.$message = Message;

};

经过以上代码可以看出来Vue.use相当于一个反向调用的钩子，通过在内部反向调用各个组件的install方法来实现全局注册，最后达到一次注册全局使用的目的。