模拟拖拽的原理：
x1等于div.offsetLeft
y1等于div.offsetTop
x2等于ev.clientX(ev表示event事件)
y2等于ev.clientY
当我们在方块上按下鼠标的时候，x2-x1即可确定。移动鼠标之后，我们用鼠标当前的位置即x4、y4减去x2-x1、y2-y1就可以得到方块现在的位置。
效果图：点击查看
代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
#box{
width: 100px;
height: 100px;
background: red;
position: absolute;
}
</style>
</head>
<body> <div id="box"></div>
<script type="text/javascript">
var oBox = document.getElementById('box'); oBox.onmousedown = function(ev){
// 鼠标按下 var ev = ev || event; // 获取鼠标离div得距离
var mouseBoxleft = ev.clientX - this.offsetLeft;
var mouseBoxTop = ev.clientY - this.offsetTop; oBox.onmousemove = function(ev){
// 鼠标按下左键并移动 var ev = ev || event; // 设置div移动时，它的位置
oBox.style.left = ev.clientX - mouseBoxleft + 'px';
oBox.style.top = ev.clientY - mouseBoxleft + 'px'; } oBox.onmouseup = function(){
// 鼠标左键抬起 oBox.onmousemove = oBox.onmouseup = null;
}
}
</script>
</body>
</html>
优化代码：
【1】鼠标移动快的时候，鼠标会移出方块，这时方块就不会再跟随鼠标动了。
解决办法：就是将onmousemove和onmouseup加到document对象上
效果：点击查看
代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
#box{
width: 100px;
height: 100px;
background: red;
position: absolute;
}
</style>
</head>
<body> <div id="box"></div>
<script>
var oBox = document.getElementById('box'); oBox.onmousedown = function(ev){
// 鼠标按下 var ev = ev || event; // 获取鼠标离div得距离
var mouseBoxleft = ev.clientX - this.offsetLeft;
var mouseBoxTop = ev.clientY - this.offsetTop; document.onmousemove = function(ev){
// 鼠标按下左键并移动 var ev = ev || event; // 设置div移动时，它的位置
oBox.style.left = ev.clientX - mouseBoxleft + 'px';
oBox.style.top = ev.clientY - mouseBoxleft + 'px'; } document.onmouseup = function(){
// 鼠标左键抬起 document.onmousemove = document.onmouseup = null;
}
}
</script>
</body>
</html>
【2】当要拖动的方块中有文字时会触发浏览器的默认行为
解决办法：1、使用return false添加到onmousedown事件中阻止浏览器的默认行为（IE除外）
2、使用全局捕获（IE）
1、使用return false添加到onmousedown事件中阻止浏览器的默认行为（IE除外）
效果：点击查看
代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
#box{
width: 100px;
height: 100px;
background: red;
position: absolute;
top: 0;
left: 0;
}
</style>
</head>
<body> <div id="box">模拟拖拽</div>
<script>
var oBox = document.getElementById('box'); oBox.onmousedown = function(ev){
// 鼠标按下 var ev = ev || event;
// 获取鼠标离div得距离
var mouseBoxleft = ev.clientX - this.offsetLeft;
var mouseBoxTop = ev.clientY - this.offsetTop; document.onmousemove = function(ev){
// 鼠标按下左键并移动 var ev = ev || event; // 设置div移动时，它的位置
oBox.style.left = ev.clientX - mouseBoxleft + 'px';
oBox.style.top = ev.clientY - mouseBoxleft + 'px'; } document.onmouseup = function(){
// 鼠标左键抬起 document.onmousemove = document.onmouseup = null;
} // 阻止默认行为
return false;
}
</script>
</body>
</html>
2、使用全局捕获（IE）
全局捕获：当我们给一个元素这只全局捕获后，改元素会监听后续发生的所有事件，当有事件发生的时候就会触发改元素的事件
举个栗子：点击查看
代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<input type="button" id="button1" value="弹出1" />
<input type="button" id="button2" value="弹出2" />
<script type="text/javascript">
window.onload = function(){
var Btn1 = document.getElementById('button1');
var Btn2 = document.getElementById('button2'); Btn1.setCapture(); Btn1.onclick = function(){
alert(1);
}
Btn2.onclick = function(){
alert(2);
} }
</script>
</body>
</html>
给Btn1设置了全局捕获之后，即使我们点击了Btn2还是会触发Btn1的点击事件
在模拟拖拽中，给要拖拽的方块onmousedown添加全局捕获然后再onmouseup中取消全局捕获
效果：点击查看
代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
#box{
width: 100px;
height: 100px;
background: red;
position: absolute; }
</style>
</head>
<body> <div id="box">模拟拖拽</div>
<script>
var oBox = document.getElementById('box'); oBox.onmousedown = function(ev){
// 鼠标按下 var ev = ev || event; // 获取鼠标离div得距离
var mouseBoxleft = ev.clientX - this.offsetLeft;
var mouseBoxTop = ev.clientY - this.offsetTop; // IE浏览器，全局捕获
if(oBox.setCapture){
oBox.setCapture();
} document.onmousemove = function(ev){
// 鼠标按下左键并移动 var ev = ev || event; // 设置div移动时，它的位置
oBox.style.left = ev.clientX - mouseBoxleft + 'px';
oBox.style.top = ev.clientY - mouseBoxleft + 'px'; } document.onmouseup = function(){
// 鼠标左键抬起 document.onmousemove = document.onmouseup = null; //IE下，释放全局捕获 releaseCapture();
if ( oBox.releaseCapture ) {
oBox.releaseCapture();
}
} // 阻止默认行为
return false;
}
</script>
</body>
</html>
【3】封装模拟拖拽函数
效果：点击查看
代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
#box{
width: 100px;
height: 100px;
background: red;
position: absolute; }
</style>
</head>
<body> <div id="box">模拟拖拽</div>
<script>
var oBox = document.getElementById('box'); drag(oBox); function drag(obj){
obj.onmousedown = function(ev){
// 鼠标按下 var ev = ev || event; // 获取鼠标离div得距离
var mouseBoxleft = ev.clientX - this.offsetLeft;
var mouseBoxTop = ev.clientY - this.offsetTop; // IE浏览器，全局捕获
if(obj.setCapture){
obj.setCapture();
} document.onmousemove = function(ev){
// 鼠标按下左键并移动 var ev = ev || event; // 设置div移动时，它的位置
obj.style.left = ev.clientX - mouseBoxleft + 'px';
obj.style.top = ev.clientY - mouseBoxleft + 'px'; } document.onmouseup = function(){
// 鼠标左键抬起 document.onmousemove = document.onmouseup = null; //IE下，释放全局捕获 releaseCapture();
if ( obj.releaseCapture ) {
obj.releaseCapture();
}
} // 阻止默认行为
return false;
}
} </script>
</body>
</html>
