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创建xhr时异常处理Ajax之六

前面几篇一直采用最精简的方式创建Ajax的核心XMLHttpRequest对象

var xhr = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');

没有考虑其可能出现的异常，即创建失败。其实个人认为以上创建方式创建失败的几率非常之少，起码在IE6/7/8/Firefox/Safari/Chrome/Opera如此，其它浏览器就不知了。

但作为一个基础库还是完善下，如果出现创建失败，failure的第二个参数msg将会被赋值为"create xhr failed"。

如下

var xhr = function(){
	try{
		return new XMLHttpRequest();
	}catch(e){
		try{
			return new ActiveXObject('Msxml2.XMLHTTP');
		}catch(e){
			try{
				return new ActiveXObject('Microsoft.XMLHTTP');
			}catch(e){
				failure(null,'create xhr failed',e);
			}
		}
	}
}();

完整源码

Ajax =
function(){
	function request(url,opt){
		function fn(){}
		opt = opt || {};
		var async   = opt.async !== false,
			method  = opt.method 	|| 'GET',
			type    = opt.type 		|| 'text',
			encode  = opt.encode 	|| 'UTF-8',
			timeout = opt.timeout 	|| 0,
			data    = opt.data 		|| null,
			success = opt.success 	|| fn,
			failure = opt.failure 	|| fn;
			method  = method.toUpperCase();
		if(data && typeof data == 'object'){//对象转换成字符串键值对
			data = _serialize(data);
		}
		if(method == 'GET' && data){
			url += (url.indexOf('?') == -1 ? '?' : '&') + data;
			data = null;
		}
		var xhr = function(){
			try{
				return new XMLHttpRequest();
			}catch(e){
				try{
					return new ActiveXObject('Msxml2.XMLHTTP');
				}catch(e){
					try{
						return new ActiveXObject('Microsoft.XMLHTTP');
					}catch(e){
						failure(null,'create xhr failed',e);
					}
				}
			}
		}();
		if(!xhr){return;}
		var isTimeout = false, timer;
		if(async && timeout>0){
			timer = setTimeout(function(){
				xhr.abort();
				isTimeout = true;
			},timeout);
		}
		xhr.onreadystatechange = function(){
			if (xhr.readyState == 4 && !isTimeout){
				_onStateChange(xhr, type, success, failure);
				clearTimeout(timer);
			}else{}
		};
		xhr.open(method,url,async);
		if(method == 'POST'){
			xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded;charset=' + encode);
		}
		xhr.send(data);
		return xhr;	
	}
	function _serialize(obj){
		var a = [];
		for(var k in obj){
			var val = obj[k];
			if(val.constructor == Array){
				for(var i=0,len=val.length;i<len;i++){
					a.push(k + '=' + encodeURIComponent(val[i]));
				}				
			}else{
				a.push(k + '=' + encodeURIComponent(val));
			}				
		}
		return a.join('&');
	}
	function _onStateChange(xhr,type,success,failure){
		var s = xhr.status, result;
		if(s>= 200 && s < 300){
			switch(type){
				case 'text':
					result = xhr.responseText;
					break;
				case 'json':
					result = function(str){
						return (new Function('return ' + str))();
					}(xhr.responseText);
					break;
				case 'xml':
					result = xhr.responseXML;
					break;
			}
			success(result);
		}else if(s===0){
			failure(xhr,'request timeout');	
		}else{
			failure(xhr,xhr.status);
		}
		xhr = null;
	}
	return (function(){
		var Ajax = {request:request}, types = ['text','json','xml'];
		for(var i=0,len=types.length;i<len;i++){
			Ajax[types[i]] = function(i){
				return function(url,opt){
					opt = opt || {};
					opt.type = types[i];
					return request(url,opt);
				}
			}(i);
		}
		return Ajax;
	})();
}();