描述器 排序（根据属性）

NSSortDescriptor *classWithSort=[[NSSortDescriptor alloc]initWithKey:@"class" ascending:YES];

        NSSortDescriptor *ageWithSort=[[NSSortDescriptor alloc]initWithKey:@"age" ascending:YES];

        NSArray *dic = [NSArray arrayWithObjects:classWithSort,ageWithSort,nil];

        NSArray *array = [arr sortedArrayUsingDescriptors:dic];