createModal (id) {
 let modal = document.getElementById(id)
 if (!modal) { // 在没有遮罩的时候创建遮罩
  modal = document.createElement('div')
  modal.id = id
  modal.style.cssText = `position: fixed; left: 0; top: 0; right: 0; bottom: 0; z-index: 999;`
  document.body.appendChild(modal)
 }
},

handleTouchstart (e) { // 开始拖拽
 // 创建遮罩层
 this.createModal(this.modalID) // modalID 遮罩层的id，由外部定义
 let element = e.targetTouches[0]
 let target = e.target.cloneNode(true) // 拷贝目标元素
 target.id = this.copyID // copyID 拷贝元素的id，由外部定义
 // 记录初始点击位置 client，用于计算移动距离
 this.source.client = {
  x: element.clientX,
  y: element.clientY
 }
 // 算出目标元素的固定位置
 let disX = this.source.start.left = element.target.getBoundingClientRect().left
 let disY = this.source.start.top = element.target.getBoundingClientRect().top
 target.style.cssText = `position: fixed; left: ${disX}px; top: ${disY}px;`
 // 将拷贝的元素放到遮罩中
 document.getElementById(this.modalID).appendChild(target)
},

handleTouchmove (e) { // 拖拽中
 let element = e.targetTouches[0]
 let target = document.getElementById(this.copyID)
 // 根据初始点击位置 client 计算移动距离
 let left = this.source.start.left + element.clientX - this.source.client.x
 let top = this.source.start.top + element.clientY - this.source.client.y
 // 移动当前元素
 target.style.left = `${left}px`
 target.style.top = `${top}px`
},

handleTouchend (e) { // 拖拽结束
 let end = {
  x: e.changedTouches[0].clientX,
  y: e.changedTouches[0].clientY
 }
 // 删除遮罩层
 let modal = document.getElementById(this.modalID)
 document.body.removeChild(modal)
 // 处理结果
 this.doingSth(end)
},

handleTouchstart (e) { // 开始拖拽
 let element = e.targetTouches[0]// 记录初始 client 位置，用于计算移动距离
 this.source.client = {
  x: element.clientX,
  y: element.clientY
 }
},

handleTouchmove (e) { // 拖拽中
 let element = e.targetTouches[0]
 // 根据初始 client 位置计算移动距离
 let x = element.clientX - this.source.client.x
 let y = element.clientY - this.source.client.y
 // 移动当前元素
 element.target.style.cssText = `transform: translate(${x}px, ${y}px);`
},

handleTouchend (e) { // 拖拽结束
 // 清除拖拽样式
 e.target.style.cssText = `transform: none;`
},