2019-04-08 记录一下用 onMouseDown 模拟 ondrag ,ondrag 有bug

这个是模拟 ondarg 的万能用法

const onMouseDown = ( event ) = > {

const target = event.target;
document.onmousemove = (element) => {
  element.preventDefault();
  console.log('element', element);
};

target.onmouseup = () => {
  document.onmousemove = null;
  target.onmouseup = null;
};

}