本文的内容是关于在HTML5/">html5中如何实现html元素拖拽功能。在html5之前要实现拖拽，需要借助js，现在html5内部就支持了拖拽的功能，但是要实现稍微复杂的功能还是少不了js的帮忙。下面我们看几个例子。
1.创建拖拽对象我们可以通过draggable属性告诉浏览器，哪些元素需要实现拖拽功能。draggable有三个值：true：元素可以被拖拽；false：元素不能被拖拽；auto：浏览器自己判断元素是否能被拖拽。
系统默认值是auto，但auto情况下浏览器对不同元素拖拽功能的支持是不一样，如：支持img对象，不支持div对象。所以，如果需要拖拽一个元素，最好还是把draggale设置为true。下面我们看一个例子：
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<style>
#src > *
{
float: left;
}
#target, #src > img
{
border: thin solid black;
padding: 2px;
margin: 4px;
}
#target
{
height: 123px;
width: 220px;
text-align: center;
display: table;
}
#target > p
{
display: table-cell;
vertical-align: middle;
}
#target > img
{
margin: 1px;
}
</style>
</head>
<body>
<div id="src">
<img draggable="true" id="car1" src="img/1.jpg" />
<img draggable="true" id="car2" src="img/2.jpg" />
<img draggable="true" id="car3" src="img/3.jpg" />
<div id="target">
<p id="msg">
drop here</p>
</div>
</div>
<script>
var src = document.getElementById("src");
var target = document.getElementById("target");
</script>
</body>
</html>
运行效果：
2.处理拖拽事件现在我们来了解拖拽相关的事件，有两种类型的事件，一种是拖拽对象的事件，一种是投放区的事件。拖拽事件包括：dragstart：当元素拖拽开始触发；drag：在元素拖拽过程中触发；dragend：元素拖拽结束时触发。下面我们就看一个例子：
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<style>
#src > *
{
float: left;
}
#target, #src > img
{
border: thin solid black;
padding: 2px;
margin: 4px;
}
#target
{
height: 123px;
width: 220px;
text-align: center;
display: table;
}
#target > p
{
display: table-cell;
vertical-align: middle;
}
#target > img
{
margin: 1px;
}
img.dragged
{
background-color: Orange;
}
</style>
</head>
<body>
<div id="src">
<img draggable="true" id="car1" src="img/1.jpg" />
<img draggable="true" id="car2" src="img/2.jpg" />
<img draggable="true" id="car3" src="img/3.jpg" />
<div id="target">
<p id="msg">
drop here</p>
</div>
</div>
<script>
var src = document.getElementById("src");
var target = document.getElementById("target");
var msg = document.getElementById("msg");
src.ondragstart = function (e) {
e.target.classList.add("dragged");
}
src.ondragend = function (e) {
e.target.classList.remove("dragged");
msg.innerHTML = "drop here";
}
src.ondrag = function (e) {
msg.innerHTML = e.target.id;
}
</script>
</body>
</html>
运行效果：
3.创建投放区我们来看投放区相关的事件：dragenter：当拖拽对象进入投放区时触发；dragover：拖拽对象在投放区内移动时触发；dragleave：拖拽对象没有投放到投放区，离开投放区的时候触发；drop：拖拽对象投放在投放区时触发。
我们来看一个例子：
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<style>
#src > *
{
float: left;
}
#target, #src > img
{
border: thin solid black;
padding: 2px;
margin: 4px;
}
#target
{
height: 123px;
width: 220px;
text-align: center;
display: table;
}
#target > p
{
display: table-cell;
vertical-align: middle;
}
#target > img
{
margin: 1px;
}
img.dragged
{
background-color: lightgrey;
}
</style>
</head>
<body>
<div id="src">
<img draggable="true" id="car1" src="img/1.jpg" />
<img draggable="true" id="car2" src="img/2.jpg" />
<img draggable="true" id="car3" src="img/3.jpg" />
<div id="target">
<p id="msg">
drop here</p>
</div>
</div>
<script>
var src = document.getElementById("src");
var target = document.getElementById("target");
var msg = document.getElementById("msg");
var draggedID;
target.ondragenter = handleDrag;
target.ondragover = handleDrag;
function handleDrag(e) {
e.preventDefault();
}
target.ondrop = function (e) {
var newElem = document.getElementById(draggedID).cloneNode(false);
target.innerHTML = "";
target.appendChild(newElem);
e.preventDefault();
}
src.ondragstart = function (e) {
draggedID = e.target.id;
e.target.classList.add("dragged");
}
src.ondragend = function (e) {
var elems = document.querySelectorAll(".dragged");
for (var i = 0; i < elems.length; i++) {
elems[i].classList.remove("dragged");
}
}
</script>
</body>
</html>
运行结果：
4.使用DataTransfer我们使用DataTransfer从拖拽对象向投放区传递数据。DataTransfer有下面的属性和方法：types：返回数据的格式；getData(<format>)：返回指定格式数据；setData(<format>, <data>)：设置指定格式数据；clearData(<format>)：移除指定格式数据；files：返回已经投放的文件数组。
我们来看下面的例子，他实现的效果和例3一样：
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<style>
#src > *
{
float: left;
}
#src > img
{
border: thin solid black;
padding: 2px;
margin: 4px;
}
#target
{
border: thin solid black;
margin: 4px;
}
#target
{
height: 123px;
width: 220px;
text-align: center;
display: table;
}
#target > p
{
display: table-cell;
vertical-align: middle;
}
img.dragged
{
background-color: Orange;
}
</style>
</head>
<body>
<div id="src">
<img draggable="true" id="car1" src="img/1.jpg" />
<img draggable="true" id="car2" src="img/2.jpg" />
<img draggable="true" id="car3" src="img/3.jpg" />
<div id="target">
<p id="msg">
drop here</p>
</div>
</div>
<script>
var src = document.getElementById("src");
var target = document.getElementById("target");
target.ondragenter = handleDrag;
target.ondragover = handleDrag;
function handleDrag(e) {
e.preventDefault();
}
target.ondrop = function (e) {
var droppedID = e.dataTransfer.getData("Text");
var newElem = document.getElementById(droppedID).cloneNode(false);
target.innerHTML = "";
target.appendChild(newElem);
e.preventDefault();
}
src.ondragstart = function (e) {
e.dataTransfer.setData("Text", e.target.id);
e.target.classList.add("dragged");
}
src.ondragend = function (e) {
var elems = document.querySelectorAll(".dragged");
for (var i = 0; i < elems.length; i++) {
elems[i].classList.remove("dragged");
}
}
</script>
</body>
</html>
5.拖拽文件html5支持file api，可以让我们操作本地文件。一般我们不直接使用file api，我们可以结合其他特性一起使用，比如结合拖拽特效，如下例：
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<style>
body > *
{
float: left;
}
#target
{
border: medium double black;
margin: 4px;
height: 75px;
width: 200px;
text-align: center;
display: table;
}
#target > p
{
display: table-cell;
vertical-align: middle;
}
table
{
margin: 4px;
border-collapse: collapse;
}
th, td
{
padding: 4px;
}
</style>
</head>
<body>
<div id="target">
<p id="msg">
Drop Files Here</p>
</div>
<table id="data">
</table>
<script>
var target = document.getElementById("target");
target.ondragenter = handleDrag;
target.ondragover = handleDrag;
function handleDrag(e) {
e.preventDefault();
}
target.ondrop = function (e) {
var files = e.dataTransfer.files;
var tableElem = document.getElementById("data");
tableElem.innerHTML = "<tr><th>Name</th><th>Type</th><th>Size</th></tr>";
for (var i = 0; i < files.length; i++) {
var row = "<tr><td>" + files[i].name + "</td><td>" + files[i].type + "</td><td>" + files[i].size + "</td></tr>";
tableElem.innerHTML += row;
}
e.preventDefault();
}
</script>
</body>
</html>
DataTransfer返回FileList对象，我们可以把他当做file数组对象，file包含下面属性：name：文件名字；type：文件类型（MIME类型）；size：文件大小。
运行效果：
6.上传文件下面介绍一个通过拖拽ajax上传文件的实例。
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<style>
.table
{
display: table;
}
.row
{
display: table-row;
}
.cell
{
display: table-cell;
padding: 5px;
}
.label
{
text-align: right;
}
#target
{
border: medium double black;
margin: 4px;
height: 50px;
width: 200px;
text-align: center;
display: table;
}
#target > p
{
display: table-cell;
vertical-align: middle;
}
</style>
</head>
<body>
<form id="fruitform" method="post" action="/UploadHandler.ashx">
<div>
<div>
<div>
Bananas:</div>
<div>
<input name="bananas" value="2" /></div>
</div>
<div>
<div>
Apples:</div>
<div>
<input name="apples" value="5" /></div>
</div>
<div>
<div>
Cherries:</div>
<div>
<input name="cherries" value="20" /></div>
</div>
<div>
<div>
File:</div>
<div>
<input type="file" name="file" /></div>
</div>
<div>
<div>
Total:</div>
<div id="results">
items</div>
</div>
</div>
<div id="target">
<p id="msg">
Drop Files Here</p>
</div>
<button id="submit" type="submit">
Submit Form</button>
</form>
<script type="text/javascript">
var target = document.getElementById("target");
var httpRequest;
var fileList;
target.ondragenter = handleDrag;
target.ondragover = handleDrag;
function handleDrag(e) {
e.preventDefault();
}
target.ondrop = function (e) {
fileList = e.dataTransfer.files;
e.preventDefault();
}
document.getElementById("submit").onclick = function handleButtonPress(e) {
e.preventDefault();
var form = document.getElementById("fruitform");
var formData = new FormData(form);
if (fileList) {
for (var i = 0; i < fileList.length; i++) {
formData.append("file" + i, fileList[i]);
}
}
httpRequest = new XMLHttpRequest();
httpRequest.onreadystatechange = handleResponse;
httpRequest.open("POST", form.action);
httpRequest.send(formData);
}
function handleResponse() {
if (httpRequest.readyState == 4 && httpRequest.status == 200) {
var data = JSON.parse(httpRequest.responseText);
document.getElementById("results").innerHTML = "You ordered " + data.total + " items";
}
}
</script>
</body>
</html>
效果：
上面的一些例子不同浏览器运行效果可能不同，我用的是chrome浏览器，除了例5和6不支持多文件外，其他例子运行正常。大家可以下载demo。
demo下载地址：Html5Guide.draggable.rar