Html 简明教程

HTML - CORS

Cross-origin resource sharing (CORS) 是一种机制，它允许从网络浏览器中的另一个域加载受限的资源。

例如，假设我们单击 HTML5 演示部分中的 HTML5 视频播放器。首先，它将询问摄像头权限，如果用户允许权限，则只打开摄像头，否则不打开。

Making a CORS request

Chrome、Firefox、Opera 和 Safari 等现代浏览器都使用 XMLHttprequest2 对象，而 Internet Explorer 使用类似的 XDomainRequest 对象。

function createCORSRequest(method, url) {
   var xhr = new XMLHttpRequest();

   if ("withCredentials" in xhr) {
      // Check if the XMLHttpRequest object has a "withCredentials" property.
      // "withCredentials" only exists on XMLHTTPRequest2 objects.
      xhr.open(method, url, true);
   } else if (typeof XDomainRequest != "undefined") {
      // Otherwise, check if XDomainRequest.
      // XDomainRequest only exists in IE, and is IE's way of making CORS requests.
      xhr = new XDomainRequest();
      xhr.open(method, url);
   }
   else {
      // Otherwise, CORS is not supported by the browser.
      xhr = null;
   }
   return xhr;
}
var xhr = createCORSRequest('GET', url);
if (!xhr) {
   throw new Error('CORS not supported');
}

Event handles in CORS

Sr.No.	Event Handler & Description
1	onloadstart Starts the request
2	onprogress 加载数据并发送数据
3	onabort Abort the request
4	onerror request has failed
5	onload request load successfully
6	ontimeout 在请求完成之前超时
7	onloadend 无论请求是否成功或失败，当请求完成时

Example of onload or onerror event

xhr.onload = function() {
   var responseText = xhr.responseText;
   // process the response.
   console.log(responseText);
};
xhr.onerror = function() {
   console.log('There was an error!');
};

Example of CORS with handler

下面的示例将显示 makeCorsRequest() 和 onload 处理程序的示例

// Create the XHR object.
function createCORSRequest(method, url) {
   var xhr = new XMLHttpRequest();
   if ("withCredentials" in xhr) {

      // XHR for Chrome/Firefox/Opera/Safari.
      xhr.open(method, url, true);
   } else if (typeof XDomainRequest != "undefined") {

      // XDomainRequest for IE.
      xhr = new XDomainRequest();
      xhr.open(method, url);
   } else {

      // CORS not supported.
      xhr = null;
   }
   return xhr;
}
// Helper method to parse the title tag from the response.
function getTitle(text) {
   return text.match('<title>(.*)?</title>')[1];
}

// Make the actual CORS request.
function makeCorsRequest() {
   // All HTML5 Rocks properties support CORS.
   var url = 'http://www.tutorialspoint.com';
   var xhr = createCORSRequest('GET', url);
   if (!xhr) {
      alert('CORS not supported');
      return;
   }

   // Response handlers.
   xhr.onload = function() {
      var text = xhr.responseText;
      var title = getTitle(text);
      alert('Response from CORS request to ' + url + ': ' + title);
   };
   xhr.onerror = function() {
      alert('Woops, there was an error making the request.');
   };
   xhr.send();
}