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基于Comet服务器推技术

    博客分类：
  • PHP
http长连接Comet服务器推技术 
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最近一直关注COMET推技术，现在已经应用很普遍了
比如renren，新浪微博，都用到了COMET技术
大部分使用java实现的，下面是用PHP实现的原理

原理：利用htmlfile这个ActiveX，往页面上放一个iframe，设置它的src为请求的地址。
comet php

1.我们需要一个持久链接的HTTP请求
2.需要一个加载javascript的html页面

这个脚本会做一个无限循环，将返回服务器的时间，只要客户端连接。

 
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<?php
   
  header("Cache-Control: no-cache, must-revalidate");
  header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
  flush();
   
  ?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
  <html xmlns="http://www.w3.org/1999/xhtml">
  <head>
   
    <title>Comet php backend</title>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
  </head>
  <body>
   
  <script type="text/javascript">
    // KHTML browser don't share javascripts between iframes
    var is_khtml = navigator.appName.match("Konqueror") || navigator.appVersion.match("KHTML");
    if (is_khtml)
    {
      var prototypejs = document.createElement('script');
      prototypejs.setAttribute('type','text/javascript');
      prototypejs.setAttribute('src','prototype.js');
      var head = document.getElementsByTagName('head');
      head[0].appendChild(prototypejs);
    }
    // load the comet object
    var comet = window.parent.comet;
   
  </script>
   
  <?php
   
  while(1) {
    echo '<script type="text/javascript">';
    echo 'comet.printServerTime('.time().');';
    echo '</script>';
    flush(); // used to send the echoed data to the client
    sleep(1); // a little break to unload the server CPU
  }
   
  ?>
   
  </body>
  </html>
  
<BR type="_moz">

创建客户端脚本

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var comet = {
    connection   : false,
    iframediv    : false,
   
    initialize: function() {
      if (navigator.appVersion.indexOf("MSIE") != -1) {
   
        // For IE browsers
        comet.connection = new ActiveXObject("htmlfile");
        comet.connection.open();
        comet.connection.write("<html>");
        comet.connection.write("<script>document.domain = '"+document.domain+"'");
        comet.connection.write("</html>");
        comet.connection.close();
        comet.iframediv = comet.connection.createElement("div");
        comet.connection.appendChild(comet.iframediv);
        comet.connection.parentWindow.comet = comet;
        comet.iframediv.innerHTML = "<iframe id='comet_iframe' src='./backend.php'></iframe>";
   
      } else if (navigator.appVersion.indexOf("KHTML") != -1) {
   
        // for KHTML browsers
        comet.connection = document.createElement('iframe');
        comet.connection.setAttribute('id',     'comet_iframe');
        comet.connection.setAttribute('src',    './backend.php');
        with (comet.connection.style) {
          position   = "absolute";
          left       = top   = "-100px";
          height     = width = "1px";
          visibility = "hidden";
        }
        document.body.appendChild(comet.connection);
   
      } else {
   
        // For other browser (Firefox...)
        comet.connection = document.createElement('iframe');
        comet.connection.setAttribute('id',     'comet_iframe');
        with (comet.connection.style) {
          left       = top   = "-100px";
          height     = width = "1px";
          visibility = "hidden";
          display    = 'none';
        }
        comet.iframediv = document.createElement('iframe');
        comet.iframediv.setAttribute('src', './backend.php');
        comet.connection.appendChild(comet.iframediv);
        document.body.appendChild(comet.connection);
   
      }
    },
   
    // this function will be called from backend.php  
    printServerTime: function (time) {
      $('content').innerHTML = time;
    },
   
    onUnload: function() {
      if (comet.connection) {
        comet.connection = false; // release the iframe to prevent problems with IE when reloading the page
      }
    }
  }
  Event.observe(window, "load",   comet.initialize);
  Event.observe(window, "unload", comet.onUnload);
 

学习资料：
Comet：基于 HTTP 长连接的“服务器推”技术
http://www.ibm.com/developerworks/cn/web/wa-lo-comet/ 

How to implement COMET with PHP
http://www.zeitoun.net/articles/comet_and_php/start

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