手写可拖曳 div

参考代码：https://jsbin.com/munuzureya/edit?html,js,output

要点：

• 注意监听范围，不能只监听 div
• 不要使用 drag 事件，很难用。
• 使用 transform 会比 top / left 性能更好，因为可以避免 reflow 和 repaint

<div id="xxx"></div>

div{
  border: 1px solid red;
  position: absolute;
  top: 0;
  left: 0;
  width: 100px;
  height: 100px;
}

*{margin:0; padding: 0;}

var dragging = false
var position = null

xxx.addEventListener('mousedown',function(e){
  dragging = true
  position = [e.clientX, e.clientY]
})


document.addEventListener('mousemove', function(e){
  if(dragging === false){return}
  const x = e.clientX
  const y = e.clientY
  const deltaX = x - position[0]
  const deltaY = y - position[1]
  const left = parseInt(xxx.style.left || 0)
  const top = parseInt(xxx.style.top || 0)
  xxx.style.left = left + deltaX + 'px'
  xxx.style.top = top + deltaY + 'px'
  position = [x, y]
})
document.addEventListener('mouseup', function(e){
  dragging = false
})