vue自定义指令之拖拽指令

定义的全局指令：
使用方式

<!-- 变成了一个可以被拖拽的元素 啦啦啦 -->
<div v-drag></div> 

import Vue from "vue"

Vue.directive("drag", {
    bind(el) {
        el.onmousedown = function (event) {
            var event = event || window.event;  //兼容IE浏览器
            //    鼠标点击物体那一刻相对于物体左侧边框的距离=点击时的位置相对于浏览器最左边的距离-物体左边框相对于浏览器最左边的距离
            var diffX = event.clientX - el.offsetLeft;
            var diffY = event.clientY - el.offsetTop;
            if (typeof el.setCapture !== 'undefined') {
                el.setCapture();
            }
            document.onmousemove = function (event) {
                var event = event || window.event;
                var moveX = event.clientX - diffX;
                var moveY = event.clientY - diffY;
                if (moveX < 0) {
                    moveX = 0
                } else if (moveX > window.innerWidth - el.offsetWidth) {
                    moveX = window.innerWidth - el.offsetWidth
                }
                if (moveY < 0) {
                    moveY = 0
                } else if (moveY > window.innerHeight - el.offsetHeight) {
                    moveY = window.innerHeight - el.offsetHeight
                }
                el.style.left = moveX + 'px';
                el.style.top = moveY + 'px'
            }
            document.onmouseup = function (event) {
                this.onmousemove = null;
                this.onmouseup = null;
                //修复低版本ie bug  
                if (typeof el.releaseCapture != 'undefined') {
                    el.releaseCapture();
                }
            }
        }
    }
})