var net=new Object();
//编写构造函数
net.AjaxRequest=function(url,onload,onerror,method,params){
  this.req=null;
  this.onload=onload;
  this.onerror=(onerror) ? onerror : this.defaultError;
  this.loadDate(url,method,params);
}
//编写用于初始化XMLHttpRequest对象并指定处理函数，最后发送HTTP请求的方法
net.AjaxRequest.prototype.loadDate=function(url,method,params){
  if (!method){
    method="GET";
  }
  if (window.XMLHttpRequest){
    this.req=new XMLHttpRequest();
  } else if (window.ActiveXObject){
    this.req=new ActiveXObject("Microsoft.XMLHTTP");
  }
  if (this.req){
    try{
      var loader=this;
      this.req.onreadystatechange=function(){
        net.AjaxRequest.onReadyState.call(loader);
      }

      this.req.open(method,url,true);
	  if(method=="POST"){
		this.req.setRequestHeader("Content-Type","application/x-www-form-urlencoded"); 
	  }
      this.req.send(params);
    }catch (err){
      this.onerror.call(this);
    }
  }
}

//重构回调函数
net.AjaxRequest.onReadyState=function(){
  var req=this.req;
  var ready=req.readyState;
  if (ready==4){
    if (req.status==200 ){
      this.onload.call(this);
    }else{
      this.onerror.call(this);
    }
  }
}
//重构默认的错误处理函籹
net.AjaxRequest.prototype.defaultError=function(){
  alert("error fetching data!"
    +"\n\nreadyState:"+this.req.readyState
    +"\nstatus: "+this.req.status
    +"\nheaders: "+this.req.getAllResponseHeaders());
}



