vue指令实现拖动的高级写法

不熟悉vue自定义指令看这里： https://cn.vuejs.org/v2/guide/custom-directive.html

vue指令实现拖动方法很方便也挺简单，但是网上大部分的教程代码，一般都是把代码全部写一个方法里面，代码不够美观，代码逻辑也不太清晰，不推荐这种写法，比如下面这样：

Vue.directives: {
  drag: {
   // 使用bind会有可能没有渲染完成
   inserted: function(el, binding, vnode) {
    const _el = el; //获取当前元素
    const ref = vnode.context.$refs[binding.value]; // 判断基于移动的是哪一个盒子
    const masterNode = ref ? ref : document; // 用于绑定事件
    const masterBody = ref ? ref : document.body; // 用于获取高和宽
    const mgl = _el.offsetLeft;
    const mgt = _el.offsetTop;
    const maxWidth = masterBody.clientWidth;
    const maxHeight = masterBody.clientHeight;
    const elWidth = _el.clientWidth;
    const elHeight = _el.clientHeight;
    let positionX = 0,
     positionY = 0;
    _el.onmousedown = e => {
     //算出鼠标相对元素的位置，加上的值是margin的值
     let disX = e.clientX - _el.offsetLeft + mgl; 
     let disY = e.clientY - _el.offsetTop + mgt;
     masterNode.onmousemove = e => {
      //用鼠标的位置减去鼠标相对元素的位置，得到元素的位置
      let left = e.clientX - disX;
      let top = e.clientY - disY;
      // 绑定的值不能滑出基于盒子的范围
      left < 0 && (left = 0);
      left > (maxWidth - elWidth - mgl) && (left = maxWidth - elWidth - mgl);
      top < 0 && (top = 0);
      top > (maxHeight - elHeight - mgt) && (top = maxHeight - elHeight - mgt);
      //绑定元素位置到positionX和positionY上面
      positionX = top;
      positionY = left;
      
      //移动当前元素
      _el.style.left = left + "px";
      _el.style.top = top + "px";
     };
     // 这里是鼠标超出基于盒子范围之后再松开，会监听不到
     document.onmouseup = e => {
      masterNode.onmousemove = null;
      document.onmouseup = null;
     };
    };
   }
  }
 }

这里介绍一种比较方美观，逻辑清晰的写法，代码如下：

Vue.directive('drag', {
  bind (el, binding) {
    el.style.cursor = 'move'
    el.style.position = 'fixed'
    el.mousedownPoint = {
      x: 0,
      y: 0
    }
    // bind 改变函数内部 this 指向，让 this 指向 el
    // el.handleMouseup, el.handleMousemove, el.handleMousedown 这三个可以是其他的全局变量
    el.handleMouseup = handleMouseup.bind(el)
    el.handleMousemove = handleMousemove.bind(el)
    el.handleMousedown = handleMousedown.bind(el)
    el.addEventListener('mousedown', el.handleMousedown)
    document.body.addEventListener('mouseup', el.handleMouseup)
    document.body.addEventListener('mousemove', el.handleMousemove)
  },
  unbind (el) {
    document.body.removeEventListener('mouseup', el.handleMouseup)
    document.body.removeEventListener('mousemove', el.handleMousemove)
  }
});

const handleMousedown = function (e) {
   // 这里的this被bind改变了，是el
   // 这里的e是 MouseEvent 对象
  const initialPosition = this.getBoundingClientRect()
  this.style.width = initialPosition.width + 'px'
  this.position = {
    left: initialPosition.left,
    top: initialPosition.top
  }
  this.readyToMove = true
  this.mousedownPoint.x = e.screenX
  this.mousedownPoint.y = e.screenY
}
const handleMousemove = function (e) {
  if (!this.readyToMove) return false
  const position = this.position
  position.left = position.left + e.screenX - this.mousedownPoint.x
  position.top = position.top + e.screenY - this.mousedownPoint.y
  this.mousedownPoint.x = e.screenX
  this.mousedownPoint.y = e.screenY
  this.style.left = position.left + 'px'
  this.style.transform = 'none'
  this.style.marginLeft = 0
  this.style.marginTop = 0
  this.style.top = position.top + 'px'
  this.style.bottom = 'auto'
  this.style.right = 'auto'
}
const handleMouseup = function (e) {
  this.readyToMove = false
}

这种写法主要利用了bind的特性，回一个新的函数，并且这个函数的 this 已经被改成我们想要的 this, 推荐这种写法。