XHR2：让使用纯JS实现上传进度条变成了可能

早期我们想要实现上传进度条的话一般要借助插件如flash/silverlight，不过现在我们用纯JS来实现了，下面是XMLHttpRequest Level 2的一些新特性：

• 设置过期时间
• 使用FormData更好的数据托管
• 传送二进制流
• 添加了发送进度改变事件，可以侦听发送了多少字节。
  
• 允许安全跨域，类似于加载其他域的script.
• 可复写加载内容的MIME type和编码

XHR的事件例表：

attribute	type	Explanation
onloadstart	loadstart	When the request starts.
onprogress	progress	While loading and sending data.
onabort	abort	When the request has been aborted, either by invoking the abort() method or navigating away from the page.
onerror	error	When the request has failed.
onload	load	When the request has successfully completed.
ontimeout	timeout	When the author specified timeout has passed before the request could complete.
onloadend	loadend	When the request has completed, regardless of whether or not it was successful.

示例代码：

var xhr = new XMLHttpRequest();

var onProgressHandler = function(event) {
  if(event.lengthComputable) {
    var howmuch = (event.loaded / event.total) * 100;
    document.querySelector('progress').value = Math.ceil(howmuch);
  } else {
    console.log("Can't determine the size of the file.");
  }
}

var onLoadHandler = function() {
  displayLoadedMessage();
}

var onErrorHandler = function() {
  displayErrorMesssage();
}

xhr.upload.addEventListener('progress', onProgressHandler, false);
xhr.upload.addEventListener('load', onLoadHandler, false);
xhr.upload.addEventListener('error', onErrorHandler, false);

var onReadyStateHandler = function(event) {
  if( event.target.readyState == 4 && event.target.status == 200){
   
  }
}

xhr.open('POST','/path_to_data');
xhr.onreadystatechange = onReadyStateHandler;
xhr.send(dataToSend);

更详细的可查看Opera上面的一篇技术文章 ：
http://dev.opera.com/articles/view/xhr2/