手写实现vue2下拉菜单dropdown组件实例

// 广播通知事件
function _broadcast(componentName, eventName, params) {
  // 遍历当前组件的子组件
  this.$children.forEach(function (child) {
    // 取出componentName,组件options上面可以自己配置
    var name = child.$options.componentName;
    // 如果找到了需要通知的组件名，触发组件上面的$eimit方法，触发自定义事件
    if (name === componentName) {
      child.$emit.apply(child, [eventName].concat(params));
    } else {
      // 没找到，递归往下找
      _broadcast.apply(child, [componentName, eventName].concat([params]));
    }
  });
}
const emiiter = {
  methods: {
    // 派发事件（通知父组件）
    dispatch(componentName, eventName, params) {
      var parent = this.$parent || this.$root;
      var name = parent.$options.componentName;
      // 循环往上层父组件，知道知道组件名和需要触发的组件名相同即可，然后触发对应组件的事件
      while (parent && (!name || name !== componentName)) {
        parent = parent.$parent;
        if (parent) {
          name = parent.$options.componentName;
        }
      }
      if (parent) {
        parent.$emit.apply(parent, [eventName].concat(params));
      }
    },
    // 广播事件（通知子组件）
    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: {
    // 触发显示方式
    trigger: {
      type: String,
      default: "click",
    },
    // 下来菜单的出现位置（上方，下方）
    placement: {
      type: String,
      default: "bottom",
      validator: function (value) {
        // 这个值必须匹配下列字符串中的一个
        return ["bottom", "top"].includes(value);
      },
    },
  },
  data() {
    return {
    //控制菜单是否显示
      showMenu: false,
    };
  },
  mounted() {
      //初始化自定义事件
    this.initEvent();
  },
  methods: {
    // 初始化
    initEvent() {
    //订阅当item点击的时候，发布on-click事件，告知外部
      this.$on("item-click", (params) => {
        this.$emit("on-click", params);
        this.showMenu = false;
      });
      //空白点击要隐藏菜单，需要执行的函数需要绑定this指向
      this.handleEmptyDomElementClickBindThis =
        this.handleEmptyDomElementClick.bind(this);
      window.addEventListener("click", this.handleEmptyDomElementClickBindThis);
    },
    // 处理空白区域点击，隐藏菜单列表
    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: {
  //变化的时候，通知子组件隐藏菜单列表
    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，过渡使用js钩子函数的方式比较好处理些 -->
  <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.$on("set-menu-status", (status) => {
      this.showMeune = status;
    });
  },
  methods: {
  //进入前，初始化需要过渡的属性
    beforeEnter: function (el) {
      // 初始化
      el.style.opacity = 0;
      el.style.transfORM = "scaleY(0)";
      el.style.transformOrigin = "top center";
    },
  //dom进入
    enter: function (el, done) {
    //获取文档可视区高度
      const htmlClientHeight = document.documentElement.clientHeight;
      //菜单列表相对于父元素的top偏移量
      const offsetTop = el.offsetTop;
      const scrollHeight = el.scrollHeight;
      //获取当前元素和可视区的一些长度（top,left,bottom等）
      const { bottom } = el.getBoundinGClientRect();
      // 说明底部高度不够显示菜单了,这时候我们需要调整菜单朝上面显示
      if (htmlClientHeight - bottom < scrollHeight) {
        el.style.transformOrigin = "bottom center";
        el.style.top = -(scrollHeight + 20) + "px";
      } else {
      //查看是否placement属性，是的话我们主动处理
        if (this.$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;
    //根据官网事例，必须在enter和leave里面调用done函数，不然过渡动画不生效（切记）
      done();
    },
    //dom元素离开 
    leave: function (el, done) {
      el.style.transform = "scaleY(0)";
      el.style.opacity = 0;
      clearTimeout(this.timer);
      this.timer = setTimeout(() => {
      //根据官网事例，必须在enter和leave里面调用done函数，不然过渡动画不生效（切记）
        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项点击通知dropdown组件派发到外部的自定义事件
      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>