kv.rules['ajaxData'] = {
        validator: function (val, ajaxMethod) {
            var result = true;
            $.when(ajaxMethod(val)).then(function (data) {
                result = kv.utils.isEmptyVal(val) || data; //val為空走reqired邏輯,不為空再走ajax邏輯
            });
            return result;
        },
        message: 'Please enter info is no corrent.'
    };

function existUser(name) {
            return $.ajax({ //必須(xu)加return才可以(yi)將它(ta)返回(hui)
                url: "/ef/IsNotExist?name=" + name,
                type: "get",
                dataType: "json",
                async: false,
                success: function (data) {
                    return data;
                }
            });
        }

            self.UserName = ko.observable().extend({
                minLength: 2,
                maxLength: { params: 30, message: "名稱最大長度(du)為30個字符" },
                required: {
                    params: true,
                    message: "請輸(shu)入名(ming)稱"
                },
                ajaxData: {
                    params: existUser,
                    message: "用戶已(yi)經存在"
                }
            });