js實現拖動效果

實現這個功能主要是配合鼠標的mouse事件，拖動原理以下圖（以橫向x座標爲例，y軸原理是同樣的）：

html代碼：

 <div id="box"></div>

CSS代碼：

#box {
      position: absolute;
      width: 100px;
      height: 100px;
      top: 10px;
      left: 10px;
      background: red;
}

javaScript代碼：

 let box = document.getElementById('box');
    document.onmousedown = function (e) {
      let disx = e.pageX - box.offsetLeft;
      let disy = e.pageY - box.offsetTop;
      document.onmousemove = function (e) {
        box.style.left = e.pageX - disx + 'px';
        box.style.top = e.pageY - disy + 'px';
      }
    //釋放鼠標按鈕，將事件清空，不然始終會跟着鼠標移動
      document.onmouseup = function () {
        document.onmousemove = document.onmouseup = null;
      }
}