我tm终于成功了！！！大仇得报！文末有问题总结。

文件目录：

index.vue 文件

<template>
  <a-modal
    :class="[modalClass, simpleClass]"
    :visible="visible"
    v-bind="$props"
    :footer="null"
    :bodyStyle="{padding:0}"
    @ok="handleOk"
    @cancel="handleCancel">
    <div class="ant-modal-body" :style="bodyStyle">
      <slot></slot>
    </div>
    <div class="ant-modal-footer relative">
      <slot name="footer">
        <a-button @click="handleCancel">取消</a-button>
        <a-button type="primary" @click="handleOk">确定</a-button>
      </slot>
    </div>
    <div v-if="!title && title !== ''" slot="title">
      <slot name="title"></slot>
    </div>
  </a-modal>
</template>

<script>
  import props from './props.js'
  var mouseDownX = 0
  var mouseDownY = 0
  var deltaX = 0
  var deltaY = 0
  var sumX = 0
  var sumY = 0

  var header = null
  var contain = null
  var modalContent = null

  var onmousedown = false
  export default {
    name: 'DragModal',
    mixins: [props],
    props: {
      // 容器的类名
      modalClass: {
        type: String,
        default: () => {
          return 'modal-box'
        }
      },
      visible: {
        type: Boolean,
        default: () => {
          return false
        }
      },
      title: {
        type: String,
        default: () => {
          return undefined
        }
      },
      width: {
        type: Number,
        default: () => {
          return '70%'
        }
      },
      footer: {
        type: Boolean,
        default: () => {
          return true
        }
      }
    },
    data () {
      return {
      }
    },
    computed: {
      simpleClass () {
        return Math.random().toString(36).substring(2)
      }
    },
    watch: {
      visible () {
        this.$nextTick(() => {
          this.initialEvent(this.visible)
        })
      }
    },
    mounted () {
      this.$nextTick(() => {
        this.initialEvent(this.visible)
      })
    },
    created () {},
    beforeDestroy () {
      this.removeMove()
      window.removeEventListener('mouseup', this.removeUp, false)
    },
    methods: {
      handleOk (e) {
        this.resetNum()
        this.$emit('ok', e)
      },
      handleCancel (e) {
        this.resetNum()
        this.$emit('cancel', e)
      },
      resetNum () {
        mouseDownX = 0
        mouseDownY = 0
        deltaX = 0
        deltaY = 0
        sumX = 0
        sumY = 0
      },
      handleMove (event) {
        const delta1X = event.pageX - mouseDownX
        const delta1Y = event.pageY - mouseDownY

        deltaX = delta1X
        deltaY = delta1Y
        // console.log('delta1X:' + delta1X, 'sumX:' + sumX, 'delta1Y:' + delta1Y, 'sumY:' + sumY)
        modalContent.style.transform = `translate(${delta1X + sumX}px, ${delta1Y + sumY}px)`
      },
      initialEvent (visible) {
        // console.log('--------- 初始化')
        // console.log('simpleClass===>', this.simpleClass)
        // console.log('document===>', document)
        if (visible) {
          setTimeout(() => {
            window.removeEventListener('mouseup', this.removeUp, false)
            contain = document.getElementsByClassName(this.simpleClass)[0]
            header = contain.getElementsByClassName('ant-modal-header')[0]
            modalContent = contain.getElementsByClassName('ant-modal-content')[0]

            modalContent.style.left = 0
            modalContent.style.transform = 'translate(0px,0px)'

            // console.log('初始化-header:', header)
            // console.log('初始化-contain:', contain)
            // console.log('初始化-modalContent:', modalContent)

            header.style.cursor = 'all-scroll'

            // contain.onmousedown = (e) => {
            header.onmousedown = (e) => {
              onmousedown = true
              mouseDownX = e.pageX
              mouseDownY = e.pageY
              document.body.onselectstart = () => false
              window.addEventListener('mousemove', this.handleMove, false)
            }

            window.addEventListener('mouseup', this.removeUp, false)
          }, 0)
        }
      },
      removeMove () {
        window.removeEventListener('mousemove', this.handleMove, false)
      },
      removeUp (e) {
        // console.log('removeUp')
        document.body.onselectstart = () => true

        if (onmousedown && !(e.pageX === mouseDownX && e.pageY === mouseDownY)) {
          onmousedown = false
          sumX = sumX + deltaX
          sumY = sumY + deltaY
          // console.log('sumX:' + sumX, 'sumY:' + sumY)
        }

        this.removeMove()
        // this.checkMove()
      }
    }
  }
</script>

props.js 文件

export default {
  props: [
    'afterClose', //     Modal 完全关闭后的回调    function    无
    'bodyStyle', //     Modal body 样式    object    {}
    'cancelText', //     取消按钮文字    string| slot    取消
    'centered', //     垂直居中展示 Modal    Boolean    false
    'closable', //     是否显示右上角的关闭按钮    boolean    true
    'closeIcon', //     自定义关闭图标    VNode | slot    -    1.5.0
    'confirmLoading', //     确定按钮 loading    boolean    无
    'destroyOnClose', //     关闭时销毁 Modal 里的子元素    boolean    false
    // 'footer', //     底部内容，当不需要默认底部按钮时，可以设为 :footer="null"    string|slot    确定取消按钮
    'forceRender', //     强制渲染 Modal    boolean    false
    'getContainer', //     指定 Modal 挂载的 HTML 节点    (instance): HTMLElement    () => document.body
    'keyboard', //     是否支持键盘 esc 关闭    boolean    true
    'mask', //     是否展示遮罩    Boolean    true
    'maskClosable', //     点击蒙层是否允许关闭    boolean    true
    'maskStyle', //     遮罩样式    object    {}
    'okText', //     确认按钮文字    string|slot    确定
    'okType', //     确认按钮类型    string    primary
    'okButtonProps', //     ok 按钮 props, 遵循 jsx规范    {props: ButtonProps, on: {}}    -
    'cancelButtonProps', //     cancel 按钮 props, 遵循 jsx规范    {props: ButtonProps, on: {}}    -
    'title', //     标题    string|slot    无
    'visible', // (v-model)    对话框是否可见    boolean    无
    'width', //     宽度    string|number    520
    'wrapClassName', //     对话框外层容器的类名    string    -
    'zIndex', //     设置 Modal 的 z-index    Number    1000
    'dialogStyle', //     可用于设置浮层的样式，调整浮层位置等    object    -    1.6.1
    'dialogClass' //     可用于设置浮层的类名    string
  ]
}

在main.js中全局引入

import DragModal from '@/components/DragModal'
Vue.component('DragModal', DragModal)

可直接调用：

<!-- 图片预览弹窗 -->
<drag-modal :visible="visible" @cancel="()=>visible=false">
  内容
</drag-modal>

问题总结：
1. 原Po大神的代码是没有底部按钮的，需要自己按需添加。默认的确定和取消按钮方法已经写好了，只需要传入事件，具体事件按需求重写。

2. width属性分为两种，String|Number，默认为String类型，本程序需要使用Number。

最后再次要感谢原Po大神的代码！！