import Vue from 'vue'
import ElementUI from 'element-ui'
import 'element-ui/lib/theme-chalk/index.css'

Vue.use(ElementUI)

Vue.use(ElementUI)实现

function initUse (Vue) {
  Vue.use = function (plugin) {
    var installedPlugins = (this._installedPlugins || (this._installedPlugins = []));
    if (installedPlugins.indexOf(plugin) > -1) {
      return this
    }

    // additional parameters
    var args = toArray(arguments, 1);
    args.unshift(this);
    
    if (typeof plugin.install === 'function') {
      plugin.install.apply(plugin, args);
    } else if (typeof plugin === 'function') {
      plugin.apply(null, args);
    }

    installedPlugins.push(plugin);
    return this
  };
}

step1:查看Vue.use 函数，其中我们看到

if (typeof plugin.install === 'function') {
      plugin.install.apply(plugin, args);
    } else if (typeof plugin === 'function') {
      plugin.apply(null, args);
    }

step2:查看plugin.install的函数,这个install 函数在element-ui的module中

const install = function(Vue, opts = {}) {
  debugger
  locale.use(opts.locale);
  locale.i18n(opts.i18n);
  components.forEach(component => {
    //这里是把建立Vue组件
    Vue.component(component.name, component);
  });

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

   //这里增加prototype的属性
  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;

};

下图我们就可以看到在install函数中写入Vue.prototype的属性和方法已经成功

image.png