/**
 * @Description 由于涉及到跨組件之間通信，因此我們只有自己實現(xiàn)發(fā)布訂閱的模式，來實現(xiàn)組件之間通信,靈感主要來源于element-ui組件庫源碼中跨層級父子組件通信方案，本質(zhì)上也是發(fā)布訂閱和$emit和$on
 * @param { String } componentName 組件名
 * @param { String } eventName 事件名
 * @param { argument } params 參數(shù)
 **/
// 廣播通知事件
function _broadcast(componentName, eventName, params) {
  // 遍歷當前組件的子組件
  this.$children.forEach(function (child) {
    // 取出componentName,組件options上面可以自己配置
    var name = child.$options.componentName;
    // 如果找到了需要通知的組件名，觸發(fā)組件上面的$eimit方法，觸發(fā)自定義事件
    if (name === componentName) {
      child.$emit.apply(child, [eventName].concat(params));
    } else {
      // 沒找到，遞歸往下找
      _broadcast.apply(child, [componentName, eventName].concat([params]));
    }
  });
}
const emiiter = {
  methods: {
    // 派發(fā)事件（通知父組件）
    dispatch(componentName, eventName, params) {
      var parent = this.$parent || this.$root;
      var name = parent.$options.componentName;
      // 循環(huán)往上層父組件，知道知道組件名和需要觸發(fā)的組件名相同即可，然后觸發(fā)對應組件的事件
      while (parent && (!name || name !== componentName)) {
        parent = parent.$parent;
        if (parent) {
          name = parent.$options.componentName;
        }
      }
      if (parent) {
        parent.$emit.apply(parent, [eventName].concat(params));
      }
    },
    // 廣播事件（通知子組件）
    broadcast(componentName, eventName, params) {
      _broadcast.call(this, componentName, eventName, params);
    },
  },
};
export default emiiter;

<template>
  <div
    class="my-dropdown"
    @click.stop="trigger == 'click' ? (showMenu = !showMenu) : ''"
    @mouseenter="trigger == 'hover' ? (showMenu = true) : ''"
    @mouseleave="trigger == 'hover' ? (showMenu = false) : ''"
    ref="myDropDdown"
  >
    <div class="tip-text" ref="tipText">
      <slot></slot>
    </div>
    <slot name="list"></slot>
  </div>
</template>
<script>
import emitter from "./emitter";
export default {
  name: "MyDropdown",
  componentName: "MyDropdown",
  mixins: [emitter],
  props: {
    // 觸發(fā)顯示方式
    trigger: {
      type: String,
      default: "click",
    },
    // 下來菜單的出現(xiàn)位置（上方，下方）
    placement: {
      type: String,
      default: "bottom",
      validator: function (value) {
        // 這個值必須匹配下列字符串中的一個
        return ["bottom", "top"].includes(value);
      },
    },
  },
  data() {
    return {
    //控制菜單是否顯示
      showMenu: false,
    };
  },
  mounted() {
      //初始化自定義事件
    this.initEvent();
  },
  methods: {
    // 初始化
    initEvent() {
    //訂閱當item點擊的時候，發(fā)布on-click事件，告知外部
      this.$on("item-click", (params) => {
        this.$emit("on-click", params);
        this.showMenu = false;
      });
      //空白點擊要隱藏菜單，需要執(zhí)行的函數(shù)需要綁定this指向
      this.handleEmptyDomElementClickBindThis =
        this.handleEmptyDomElementClick.bind(this);
      window.addEventListener("click", this.handleEmptyDomElementClickBindThis);
    },
    // 處理空白區(qū)域點擊，隱藏菜單列表
    handleEmptyDomElementClick(e) {
      if (!Array.from(this.$refs.myDropDdown.childNodes).includes(e.target)) {
        this.showMenu = false;
      }
    },
  },
  beforeDestroy() {
    // 移除window上面的事件
    window.removeEventListener(this.handleEmptyDomElementClickBindThis);
  },
  watch: {
  //變化的時候，通知子組件隱藏菜單列表
    showMenu() {
      this.broadcast("MyDropdownMenu", "set-menu-status", this.showMenu);
    },
  },
};
</script>
<style lang="less">
.my-dropdown {
  position: relative;
}
</style>

<template>
<!-- 涉及到高度，位移，過渡使用js鉤子函數(shù)的方式比較好處理些 -->
  <transition
    @before-enter="beforeEnter"
    @enter="enter"
    @leave="leave"
    v-bind:css="false"
  >
    <div class="my-dropdown-menu" v-if="showMeune" ref="myDroupdownMenu">
      <slot></slot>
    </div>
  </transition>
</template>
<script>
import emitter from "./emitter";
export default {
  name: "MyDropdownMenu",
  componentName: "MyDropdownMenu",
  mixins: [emitter],
  data() {
    return {
      showMeune: false,
      timer: null,
    };
  },
  mounted() {
    this.$on("set-menu-status", (status) => {
      this.showMeune = status;
    });
  },
  methods: {
  //進入前，初始化需要過渡的屬性
    beforeEnter: function (el) {
      // 初始化
      el.style.opacity = 0;
      el.style.transform = "scaleY(0)";
      el.style.transformOrigin = "top center";
    },
  //dom進入
    enter: function (el, done) {
    //獲取文檔可視區(qū)高度
      const htmlClientHeight = document.documentElement.clientHeight;
      //菜單列表相對于父元素的top偏移量
      const offsetTop = el.offsetTop;
      const scrollHeight = el.scrollHeight;
      //獲取當前元素和可視區(qū)的一些長度（top,left,bottom等）
      const { bottom } = el.getBoundingClientRect();
      // 說明底部高度不夠顯示菜單了,這時候我們需要調(diào)整菜單朝上面顯示
      if (htmlClientHeight - bottom < scrollHeight) {
        el.style.transformOrigin = "bottom center";
        el.style.top = -(scrollHeight + 20) + "px";
      } else {
      //查看是否placement屬性，是的話我們主動處理
        if (this.$parent.placement == "top") {
          el.style.transformOrigin = "bottom center";
          el.style.top = -(scrollHeight + 20) + "px";
        } else {
          el.style.top = offsetTop + "px";
        }
      }
      el.style.transform = "scaleY(1)";
      el.style.opacity = 1;
    //根據(jù)官網(wǎng)事例，必須在enter和leave里面調(diào)用done函數(shù)，不然過渡動畫不生效（切記）
      done();
    },
    //dom元素離開 
    leave: function (el, done) {
      el.style.transform = "scaleY(0)";
      el.style.opacity = 0;
      clearTimeout(this.timer);
      this.timer = setTimeout(() => {
      //根據(jù)官網(wǎng)事例，必須在enter和leave里面調(diào)用done函數(shù)，不然過渡動畫不生效（切記）
        done();
      }, 250);
    },
  },
};
</script>
<style lang="less">
.my-dropdown-menu {
  min-width: 100px;
  max-height: 200px;
  overflow: auto;
  margin: 5px 0;
  padding: 5px 0;
  background-color: #fff;
  box-sizing: border-box;
  border-radius: 4px;
  box-shadow: 0 1px 6px rgb(0 0 0 / 20%);
  z-index: 900;
  transform-origin: top center;
  position: absolute;
  transition: transform 0.25s ease, opacity 0.25s ease;
}
</style>

<template>
  <div
    :class="[
      'my-dropdownItem',
      divided ? 'my-dropdownItem-divided' : '',
      disabled ? 'my-dropdownItem-disabled' : '',
    ]"
    @click.stop="handleItemClick"
  >
    <slot></slot>
  </div>
</template>
<script>
import emitter from "./emitter";
export default {
  name: "MyDropdownItem",
  componentName: "MyDropdownItem",
  mixins: [emitter],
  props: {
    divided: {
      type: Boolean,
      default: false,
    },
    disabled: {
      type: Boolean,
      default: false,
    },
    name: {
      type: String,
      default: "",
    },
  },
  data() {
    return {};
  },
  methods: {
    handleItemClick() {
      if (this.disabled) return;
      // item項點擊通知dropdown組件派發(fā)到外部的自定義事件
      this.dispatch("MyDropdown", "item-click", this.name);
    },
  },
};
</script>
<style lang="less">
.my-dropdownItem {
  margin: 0;
  line-height: normal;
  padding: 7px 16px;
  clear: both;
  color: #515a6e;
  font-size: 14px !important;
  white-space: nowrap;
  list-style: none;
  cursor: pointer;
  transition: background 0.2s ease-in-out;
  &:hover {
    background: #f3f3f3;
  }
}
.my-dropdownItem-divided {
  border-bottom: 1px solid #eee;
}
.my-dropdownItem-disabled {
  color: #cacdd2;
  cursor: not-allowed;
  &:hover {
    background: #fff;
  }
}
</style>