Jquery Deferred objects

以前写的，后来想不起来了。写在这里以便查找

Deferred objects对我来说就是如果页面里面有两个ajax call对数据库进行操作。比如说两个grid 读取，Ajax执行异步操作的时候，页面显示loading...; 当两个操作都完成时，loading 隐藏

以下是代码片断

  $(document).ready(function(e) {

                        
                        var _loadingImageContainer = $('#IA5010_2_loadingImageContainer_outer');
                        _loadingImageContainer.removeClass("hidden");
                        var _outer_groupcategorysection =$('#outer_groupcategorysection');
                        var _outer_attr=$('#outer_attr');
                        _outer_groupcategorysection.addClass("hidden");
                        _outer_attr.addClass("hidden");

                        var get_para=JSON.stringify({ EnvironmentId: <%=Me.EnvironmentId %>, Company: <%=Me.CompanyNumber%>, 
                            Department: '<%=IA5010_2_Department.Text%>', SizeratioId: <%=CInt(HiddenSizeRatioID.Value)%>});

                        var get_groupcategorysection=AjaxWrapper( '<%= ResolveUrl("~/IA_Allocation_Vb/IA5010D.aspx/GetGroupCategorySection")%>',get_para);
                        var get_attr=AjaxWrapper( '<%= ResolveUrl("~/IA_Allocation_Vb/IA5010D.aspx/GetProductAttributes")%>',
                                                     get_para);

                        var loadingdata=$.when (get_groupcategorysection,get_attr);

                        //success
                        loadingdata.done(function(data_groupcategorysection,data_attr){  //当两个异步操作成功的时候被调用
                        
                            IA5010_1_populatedata(data_groupcategorysection,data_attr);
                        

                           

                        });  //loading data done

                        loadingdata.always(function(){   //无论成功与否，这个函数都会被调用。把loading隐藏写在这里
                            _loadingImageContainer.addClass("hidden");
                            _outer_groupcategorysection.removeClass("hidden");
                            _outer_attr.removeClass("hidden");                  
                    
                        });

                        //deal with fail
                        loadingdata.fail(function(){
                    
                        })
                    
                });  //document ready


                function AjaxWrapper(webMethodrul, parameterListInJsonFormat) {
                    var promiseObject = $.ajax({
                        url: webMethodrul,
                        contentType: "application/json; charset=utf-8",
                        type: "POST",
                        data: parameterListInJsonFormat,
                        dataType: "json",
                        cache: false,
                    });

                    return promiseObject;
                }