js面向对象封装级联下拉菜单列表的实现步骤

本实例开发的级联下拉菜单是根据已有json数据创建的DOM元素。点击文本框后，显示一级菜单。如果菜单中包含子菜单，菜单右侧会有指示箭头。点击菜单之后，会再显示下一级菜单，以此类推。当菜单下无子菜单时，选择菜单后会在文本框中显示。

打开后的级联菜单效果如图所示：

使用实例中封装好的插件，只需要有一个input元素，即可通过插件自动生成级联下拉菜单，html代码如下所示：

<div style="margin-top:100px;text-align:center;">
  <input type="text" id="input">
 </div>

接下来看下具体封装的js代码怎么实现。

1. 声明级联菜单的构造函数

构造函数需要传入一个文本框元素和菜单关联数据两个参数。

//elem为文本框，data为菜单关联数据
 function CascadeMenu(elem,data){

 }

2. 在构造函数中创建级联菜单相关元素，并放到页面中，具体代码如下：

function CascadeMenu(elem,data){
  //获取文本框
  this.eInput = elem;
  //设置文本框为只读
  this.eInput.setAttribute('readonly',true);
  //设置文本框提示
  this.eInput.placeholder = '请选择';
  //获取文本框父元素
  var eInputParent = this.eInput.parentNode;
  //创建级联菜单容器
  this.eCascade = document.createElement('div');
  this.eCascade.className = 'cascade_container';
  //创建菜单下拉列表容器
  this.eCascadeInto = document.createElement('div');
  this.eCascadeInto.className = 'cascade_into';
  //下拉列表容器默认隐藏
  this.eCascadeInto.style.display = 'none';
  //将各个元素放到页面中
  this.eCascade.appendChild(this.eInput);
  this.eCascade.appendChild(this.eCascadeInto);
  eInputParent.appendChild(this.eCascade);
  //获取菜单数据
  this.aData = data;
  //记录已选择的菜单数据
  this.aSelected = [];
  //菜单打开状态，默认为false，表示隐藏
  this.bShow = false;
 }

3. 在文本框上绑定点击事件，生成级联下拉菜单

刚才已经把需要的元素都放到了页面中，现在可以通过点击文本框显示和隐藏级联菜单元素；
在显示级联菜单元素时，应该要通过数据生成级联下拉菜单。
因为每次点击都需要生成，所以可以在构造函数的原型上添加一个方法。如下所示： 

function CascadeMenu(elem,data){
  /*…*/

  this.eInput.addEventListener('click',()=>{
   //判断菜单打开状态
   if(this.bShow){ 
    //如果已打开，则隐藏菜单
    this.eCascadeInto.style.display = 'none';
    //修改菜单打开状态
    this.bShow = false;
   }else{
    //显示级联菜单元素
    this.eCascadeInto.style.display = 'none';
    //保存已选择的菜单数据
    this.aSelected = this.eInput.value.split('>');
    //生成级联菜单
    this.generateMenu();
   }
  });
 }
 //根据数据生成级联菜单
 CascadeMenu.prototype.generateMenu = function(){
  //在fnCreatHTML调用实例对象需要声明一个变量指向this
  var _self = this;
  //因为不确定子菜单有多少组，所以需要声明一个函数来递归调用
  //data:传入数据，step:当前级别
  function fnCreatHTML(data,step){
   //用于存储子菜单数据
   var aChildArr = null;
   //生成菜单DOM的字符串
   var sHTML = '<ul>';
   //循环数据
   for(let i=0;i<data.length;i++){
    //判断如果有子菜单，添加child的class，用于显示菜单右侧箭头
    if(data[i].child){ 
     //判断是否是当前选择，如果是，加上cur class，并且存储子菜单数据
     if(data[i].name==this.aSelected[step]){
      aChildArr = data[i].child;
      sHTML += '<li class="child cur" data-po="'+step+'">';
     }else{
      sHTML += '<li class="child" data-po="'+step+'">';
     }
    }else{ 
     //如果没有子菜单，直接加到菜单列表中
     sHTML += data[i].name == this.aSelected[step]?
                  '<li class="cur" data-po="'+step+'">':
                  '<li data-po="'+step+'">';
    }
    //添加菜单名称
    sHTML += data[i].name;
    //结束当前菜单
    sHTML += '</li>';
   }
   sHTML += '</ul>';
   //如果已选择多个菜单，递归调用函数，生成子菜单
   if(this.aSelected.length>step+1){
    sHTML += fnCreatHTML(aChildArr,step+1);
   }
   return sHTML;
  }
  this.eCascadeInto.innerHTML = fnCreatHTML(this.aData,0);
 }

4. 菜单上绑定事件，用于选择菜单

级联菜单有两种类型，一种是有下级菜单的，点击时显示下级菜单；
一种是没有下级菜单的，点击时直接选择菜单并在文本框中按级别显示所选择的菜单。代码如下所示：

function CascadeMenu(elem,data){
  /*…*/

  //利用事件委托选择菜单
  this.eCascadeInto.addEventListener('click',(event)=>{
   //获取菜单
   var eTarget = event.target;
   //获取选择的级别
   var po = +eTarget.dataset.po;
   //删除当前选择级后面的数据
   this.aSelected.splice(po+1,this.aSelected.length-(po+1));
   //修改当前选择数据
   this.aSelected[po] = eTarget.innerHTML;
   //判断是否有子菜单
   if(eTarget.className.indexOf('child')==-1){ //没有子菜单直接选择
    this.eInput.value = this.aSelected.join('>');
    this.eCascadeInto.style.display = 'none';
    this.bShow = false;
   }else{ //有子菜单显示下一级
    //重新生成DOM元素，数组中增加空字符串用于显示下一级
    this.aSelected.push('')
    //重新生成级联菜单
    this.generateMenu();
   }  
  });
 }

5. 在页面空白处点击时，隐藏菜单

现在只能在文本框上点击显示和隐藏菜单。一般来说任何打开的弹框，都希望在弹框以外的位置可以关闭掉。这样需要修改一下文本框上的点击事件函数：当打开菜单时，要在document元素上绑定点击事件，用于关闭菜单；当隐藏菜单时，需要取消document上绑定的点击事件。如下所示：

function CascadeMenu(elem,data){
  /*…*/

  this.eInput.addEventListener('click',()=>{
   //判断菜单打开状态
   if(this.bShow){ 
    //如果已打开，则隐藏菜单
    this.eCascadeInto.style.display = 'none';
    //修改菜单打开状态
    this.bShow = false;
    //取消document上的事件
    document.onclick = null;
   }else{
    //显示级联菜单元素
    this.eCascadeInto.style.display = 'none';
    //保存已选择的菜单数据
    this.aSelected = this.eInput.value.split('>');
    //生成级联菜单
    this.generateMenu();
    document.onclick = () => {
     //隐藏菜单
     this.eCascadeInto.style.display = 'none';
     //修改菜单打开状态
     this.bShow = false;
     //取消document上的事件
     document.onclick = null;
    }
   }
  });
  //阻止冒泡
  this.eCascade.addEventListener('click',(event)=>{
   event.stopPropagation();
  });

  /*…*/
 }

6. 最后，准备好数据，调用构造函数，生成级联下拉菜单，如下所示：

var json = [
  {
   "name":"北京市","id":"110000","child":[
    {"name":"市辖区","id":"110100","child":[
     {"name":"东城区","id":"110101","child":null},{"name":"西城区","id":"110102","child":null},{"name":"朝阳区","id":"110105","child":null},{"name":"丰台区","id":"110106","child":null},{"name":"石景山区","id":"110107","child":null},{"name":"海淀区","id":"110108","child":null},{"name":"门头沟区","id":"110109","child":null},{"name":"房山区","id":"110111","child":null},{"name":"通州区","id":"110112","child":null},{"name":"顺义区","id":"110113","child":null},{"name":"昌平区","id":"110114","child":null},{"name":"大兴区","id":"110115","child":null},{"name":"怀柔区","id":"110116","child":null},{"name":"平谷区","id":"110117","child":null},{"name":"密云区","id":"110118","child":null},{"name":"延庆区","id":"110119","child":null}]
    },
    {"name":"北京市","id":"110000","child":null}
   ]
  },
  {
   "name":"河北省","id":"130000","child":[
    {"name":"石家庄市","id":"130100","child":[
     {"name":"市辖区","id":"130101","child":null},{"name":"长安区","id":"130102","child":null},{"name":"桥西区","id":"130104","child":null},{"name":"新华区","id":"130105","child":null},{"name":"井陉矿区","id":"130107","child":null},{"name":"裕华区","id":"130108","child":null},{"name":"藁城区","id":"130109","child":null},{"name":"鹿泉区","id":"130110","child":null},{"name":"栾城区","id":"130111","child":null},{"name":"井陉县","id":"130121","child":null},{"name":"正定县","id":"130123","child":null},{"name":"行唐县","id":"130125","child":null},{"name":"灵寿县","id":"130126","child":null},{"name":"高邑县","id":"130127","child":null},{"name":"深泽县","id":"130128","child":null},{"name":"赞皇县","id":"130129","child":null},{"name":"无极县","id":"130130","child":null},{"name":"平山县","id":"130131","child":null},{"name":"元氏县","id":"130132","child":null},{"name":"赵县","id":"130133","child":null},{"name":"晋州市","id":"130183","child":null},{"name":"新乐市","id":"130184","child":null}]
    },
    {"name":"唐山市","id":"130200","child":[
     {"name":"市辖区","id":"130201","child":null},{"name":"路南区","id":"130202","child":null},{"name":"路北区","id":"130203","child":null},{"name":"古冶区","id":"130204","child":null},{"name":"开平区","id":"130205","child":null},{"name":"丰南区","id":"130207","child":null},{"name":"丰润区","id":"130208","child":null},{"name":"曹妃甸区","id":"130209","child":null},{"name":"滦县","id":"130223","child":null},{"name":"滦南县","id":"130224","child":null},{"name":"乐亭县","id":"130225","child":null},{"name":"迁西县","id":"130227","child":null},{"name":"玉田县","id":"130229","child":null},{"name":"遵化市","id":"130281","child":null},{"name":"迁安市","id":"130283","child":null}]
    },
    {"name":"秦皇岛市","id":"130300","child":[
     {"name":"市辖区","id":"130301","child":null},{"name":"海港区","id":"130302","child":null},{"name":"山海关区","id":"130303","child":null},{"name":"北戴河区","id":"130304","child":null},{"name":"抚宁区","id":"130306","child":null},{"name":"青龙满族自治县","id":"130321","child":null},{"name":"昌黎县","id":"130322","child":null},{"name":"卢龙县","id":"130324","child":null}]
    },
    {"name":"邯郸市","id":"130400","child":[
     {"name":"市辖区","id":"130401","child":null},{"name":"邯山区","id":"130402","child":null},{"name":"丛台区","id":"130403","child":null},{"name":"复兴区","id":"130404","child":null},{"name":"峰峰矿区","id":"130406","child":null},{"name":"邯郸县","id":"130421","child":null},{"name":"临漳县","id":"130423","child":null},{"name":"成安县","id":"130424","child":null},{"name":"大名县","id":"130425","child":null},{"name":"涉县","id":"130426","child":null},{"name":"磁县","id":"130427","child":null},{"name":"肥乡县","id":"130428","child":null},{"name":"永年县","id":"130429","child":null},{"name":"邱县","id":"130430","child":null},{"name":"鸡泽县","id":"130431","child":null},{"name":"广平县","id":"130432","child":null},{"name":"馆陶县","id":"130433","child":null},{"name":"魏县","id":"130434","child":null},{"name":"曲周县","id":"130435","child":null},{"name":"武安市","id":"130481","child":null}]
    }
   ]
  },
  {
   "name":"湖南省","id":"430000","child":[
    {"name":"长沙市","id":"430100","child":[
     {"name":"市辖区","id":"430101","child":null},{"name":"芙蓉区","id":"430102","child":null},{"name":"天心区","id":"430103","child":null},{"name":"岳麓区","id":"430104","child":null},{"name":"开福区","id":"430105","child":null},{"name":"雨花区","id":"430111","child":null},{"name":"望城区","id":"430112","child":null},{"name":"长沙县","id":"430121","child":null},{"name":"宁乡县","id":"430124","child":null},{"name":"浏阳市","id":"430181","child":null}]
    },
    {"name":"株洲市","id":"430200","child":[
     {"name":"市辖区","id":"430201","child":null},{"name":"荷塘区","id":"430202","child":null},{"name":"芦淞区","id":"430203","child":null},{"name":"石峰区","id":"430204","child":null},{"name":"天元区","id":"430211","child":null},{"name":"株洲县","id":"430221","child":null},{"name":"攸县","id":"430223","child":null},{"name":"茶陵县","id":"430224","child":null},{"name":"炎陵县","id":"430225","child":null},{"name":"醴陵市","id":"430281","child":null}]
    },
    {"name":"湘潭市","id":"430300","child":[
     {"name":"市辖区","id":"430301","child":null},{"name":"雨湖区","id":"430302","child":null},{"name":"岳塘区","id":"430304","child":null},{"name":"湘潭县","id":"430321","child":null},{"name":"湘乡市","id":"430381","child":null},{"name":"韶山市","id":"430382","child":null}]
    },
    {"name":"衡阳市","id":"430400","child":[
     {"name":"市辖区","id":"430401","child":null},{"name":"珠晖区","id":"430405","child":null},{"name":"雁峰区","id":"430406","child":null},{"name":"石鼓区","id":"430407","child":null},{"name":"蒸湘区","id":"430408","child":null},{"name":"南岳区","id":"430412","child":null},{"name":"衡阳县","id":"430421","child":null},{"name":"衡南县","id":"430422","child":[
      {"name":"三塘镇",id:"430422",child:null},{"name":"车江镇",id:"430422",child:null}
     ]},{"name":"衡山县","id":"430423","child":null},{"name":"衡东县","id":"430424","child":null},{"name":"祁东县","id":"430426","child":null},{"name":"耒阳市","id":"430481","child":null},{"name":"常宁市","id":"430482","child":null}]
    }
   ]
  },
  {
   "name":"广东省","id":"440000","child":[
    {"name":"广州市","id":"440100","child":[
     {"name":"市辖区","id":"440101","child":null},{"name":"荔湾区","id":"440103","child":null},{"name":"越秀区","id":"440104","child":null},{"name":"海珠区","id":"440105","child":null},{"name":"天河区","id":"440106","child":null},{"name":"白云区","id":"440111","child":null},{"name":"黄埔区","id":"440112","child":null},{"name":"番禺区","id":"440113","child":null},{"name":"花都区","id":"440114","child":null},{"name":"南沙区","id":"440115","child":null},{"name":"从化区","id":"440117","child":null},{"name":"增城区","id":"440118","child":null}]
    },
    {"name":"韶关市","id":"440200","child":[
     {"name":"市辖区","id":"440201","child":null},{"name":"武江区","id":"440203","child":null},{"name":"浈江区","id":"440204","child":null},{"name":"曲江区","id":"440205","child":null},{"name":"始兴县","id":"440222","child":null},{"name":"仁化县","id":"440224","child":null},{"name":"翁源县","id":"440229","child":null},{"name":"乳源瑶族自治县","id":"440232","child":null},{"name":"新丰县","id":"440233","child":null},{"name":"乐昌市","id":"440281","child":null},{"name":"南雄市","id":"440282","child":null}]
    },
    {"name":"深圳市","id":"440300","child":[
     {"name":"市辖区","id":"440301","child":null},{"name":"罗湖区","id":"440303","child":null},{"name":"福田区","id":"440304","child":null},{"name":"南山区","id":"440305","child":null},{"name":"宝安区","id":"440306","child":null},{"name":"龙岗区","id":"440307","child":null},{"name":"盐田区","id":"440308","child":null}]
    },
    {"name":"珠海市","id":"440400","child":[
     {"name":"市辖区","id":"440401","child":null},{"name":"香洲区","id":"440402","child":null},{"name":"斗门区","id":"440403","child":null},{"name":"金湾区","id":"440404","child":null}]
    }
   ]
  },
  {
   "name":"南沙群岛","id":"900001","child":null
  }
 ];

 var eText = document.getElementById('input');
 new CascadeMenu(eText,json);

一个封装好的js级联下拉功能就完成了，可以根据图片自己编写css样式以达到需要的效果。

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