vue指令實現拖動的高級寫法

不熟悉vue自定義指令看這里： https://cn.vuejs.org/v2/guide/custom-directive.html

vue指令實現拖動方法很方便也挺簡單，但是網上大部分的教程代碼，一般都是把代碼全部寫一個方法里面，代碼不夠美觀，代碼邏輯也不太清晰，不推薦這種寫法，比如下面這樣：

Vue.directives: {
  drag: {
   // 使用bind會有可能沒有渲染完成
   inserted: function(el, binding, vnode) {
    const _el = el; //獲取當前元素
    const ref = vnode.context.$refs[binding.value]; // 判斷基於移動的是哪一個盒子
    const masterNode = ref ? ref : document; // 用於綁定事件
    const masterBody = ref ? ref : document.body; // 用於獲取高和寬
    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 => {
     //算出鼠標相對元素的位置，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 => {
      //用鼠標的位置減去鼠標相對元素的位置，得到元素的位置
      let left = e.clientX - disX;
      let top = e.clientY - disY;
      // 綁定的值不能滑出基於盒子的范圍
      left < 0 && (left = 0);
      left > (maxWidth - elWidth - mgl) && (left = maxWidth - elWidth - mgl);
      top < 0 && (top = 0);
      top > (maxHeight - elHeight - mgt) && (top = maxHeight - elHeight - mgt);
      //綁定元素位置到positionX和positionY上面
      positionX = top;
      positionY = left;
      
      //移動當前元素
      _el.style.left = left + "px";
      _el.style.top = top + "px";
     };
     // 這里是鼠標超出基於盒子范圍之后再松開，會監聽不到
     document.onmouseup = e => {
      masterNode.onmousemove = null;
      document.onmouseup = null;
     };
    };
   }
  }
 }

這里介紹一種比較方美觀，邏輯清晰的寫法，代碼如下：

Vue.directive('drag', {
  bind (el, binding) {
    el.style.cursor = 'move'
    el.style.position = 'fixed'
    el.mousedownPoint = {
      x: 0,
      y: 0
    }
    // bind 改變函數內部 this 指向，讓 this 指向 el
    // el.handleMouseup, el.handleMousemove, el.handleMousedown 這三個可以是其他的全局變量
    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) {
   // 這里的this被bind改變了，是el
   // 這里的e是 MouseEvent 對象
  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 }

這種寫法主要利用了bind的特性，回一個新的函數，並且這個函數的 this 已經被改成我們想要的 this, 推薦這種寫法。