在Titanium中使用WCF服务

Question

我已经编写了一个代码来在我的Titanium项目中使用WCF服务。

Alloy.Globals.wcfservice=function callservice()
{

        //fetching data from server.
        var xhr = Titanium.Network.createHTTPClient();
        xhr.setTimeout(1200);
        console.log(xhr.status);
        console.log(xhr.statusText);

        xhr.onerror = function(e)       /* on  error in getting data from server */
        {
            //check response status and act aaccordingly.
            if (xhr.status != 200) {
                alert("The service is currently unavailable. Please Try Again Later.");
                return;
            }
        };

        //on getting response from server.
        xhr.onload = function()
        {
            alert(this.responseText);

            var response = XML.parse(this.responseText);
        };

        //set the url for the service to the get student courses
        var request_url = getWebserviceURL() + "GetAvailableAppointmentFromWCF";
        console.log(request_url);
        xhr.open("POST", request_url);
        xhr.send();
};

但我总是收到状态为0，“该服务目前无法使用。请稍后再试”。我在这段代码中做错了什么？