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js drag and drop function to implement code analysis

高洛峰Original: 2016-12-03 16:05:061442browse

Problems solved in this article:

1. How to implement dragging function in web pages;

2. The difference between document.documentElement and document.body.
document.documentElement.clientWidth refers to the width of the entire html document, the width of document.body.clientWidth. The two are not the same. You can test it through console.log(document.documentElement) and console.log(document.body) in the console.

3. The difference between getBoundingClientRect().left and offsetLeft.
getBoundingClientRect() is used to get the left, top, right, and bottom of the element. offset gets the left and top relative to the parent. getBoundingClientRect() gets the left, top, right, and bottom relative to the window.

4. e.clientX refers to the coordinates of the mouse point relative to the window.

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>弹窗</title>
 <style type="text/css">
  #mask {
   position: fixed;
   left:0;
   top:0;
   width:100%;
   height: 100%;
   background-color: hsla(250,100%,50%,0.6);
   display: none;
  }
  #popBox {
   position: absolute;
   background-color: #fff;
   width:200px;
   height: 200px;
   /*left:50%;
   top:50%;*/
   /*margin-top: -100px;
   margin-left: -100px;*/
  }
 </style>
</head>
<body>
 <button id="clickBtn">点击</button>
 <div id="mask">
  <div id="popBox"></div>
 </div>
 <script type="text/javascript">
  var clickBtn = document.getElementById("clickBtn");
  var popBox = document.getElementById("popBox")
  var mask = document.getElementById("mask");
  clickBtn.onclick = function() {
   mask.style.display = "block";
   popBox.style.left = (document.documentElement.clientWidth - popBox.offsetWidth)/2 + "px";
   popBox.style.top = (document.documentElement.clientHeight - popBox.offsetHeight)/2 + "px";
 
  }
  popBox.onclick = function(e) {
   var e = e || window.event;//e指所有代码的集合，通过它可以获取事件的各种属性。
   e.cancelBubble = true;//阻止事件冒泡，即点击事件不会传递到mask这一层，意味着不会触发点击mask事件代码。
  }
  mask.onclick = function() {
   mask.style.display = "none";
  }
 
  //拖拽 mousedown->mousemove->mouseup
  popBox.onmousedown = function(e) {
   var e = e || window.event;
   var pos = popBox.getBoundingClientRect();//getBoundingClientRect()用于获取元素的left、top、right、bottom。offset获取相对于父级的left和top。getBoundingClientRect()获取相对于窗口的left、top、right、bottom。
   var oX = e.clientX - pos.left;//clientX指相对于窗口的坐标。
   var oY = e.clientY - pos.top;
   document.onmousemove = function(e) {
    var e = e || window.event;
    var oLeft = e.clientX - oX;
    var oTop = e.clientY - oY;
    popBox.style.left = oLeft + "px";
    popBox.style.top = oTop + "px";
    if (oLeft<0) {
     popBox.style.left = 0 + "px";
    };
    if (oLeft>document.documentElement.clientWidth - popBox.offsetWidth) {
     popBox.style.left = document.documentElement.clientWidth - popBox.offsetWidth + "px";//document.documentElement.clientWidth指整个html文档的宽度，document.body.clientWidth的宽度。这两者是不一样的。可以在console控制台通过console.log(document.documentElement)和console.log(document.body)进行测试。
 
 
    }
    if (oTop<0) {
     popBox.style.top = 0 + "px";
    };
    if (oTop > document.documentElement.clientHeight - popBox.offsetHeight) {
     popBox.style.top = document.documentElement.clientHeight - popBox.offsetHeight + "px";
    }
 
   }
   popBox.onmouseup = function() {
    document.onmousemove = null;
   }
  }
 </script>
</body>
</html>

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