Yesterday I caught myself being a little too clever for my own good with some ASP.NET code. It seems that I have forgotten some of my good old classic HTML and JavaScript skills, and become too dependent on the .NET Framework and WebControls to do the work for me. Here’s the scenario… In order to improve the User Interface and better communicate to the user when something is happening that they need to wait for, we have started to modify some of our larger (slower) pages to display messages like Processing… or Reloading… while they are cycling through a postback. (Yes, I understand this could be improved by using AJAX / Callbacks and so on, but even then, you need to let your user know that they need to wait for that section to be re-rendered, so for the moment these pages will continue to use good ol’ Postbacks.) It’s a very simple trick, really. All I want to do is when some control triggers a postback, first run a little client-side JavaScript to hide the main contents of the page (such as a GridView) and display the appropriate message. This lets the user know, “Hey, we’re doing something, don’t click another link or scroll and try to take action right now.” The first places I hooked this up were easy. Most common cause of a postback: Buttons. And when you’re writing the markup or declarative code for an ASP:Button control, there is the handy OnClientClick property which is designed for just this purpose…to run client-side JavaScript before the postback occurs. This is distinguished from the OnClick property which tells the control what Server-side code to run. Great! Done! Easy! But then there are other controls like DropDownLists and CheckBoxes that we use on our pages with the AutoPostback=True setting which cause postbacks. And these don’t have OnClientClick or OnClientSelectedIndexChanged events. So I started getting creative, using an ASP:CustomValidator control in conjunction with setting the CausesValidation and ValidationGroup settings on these controls, which basically caused the action on the control to fire the Custom Validator, which was defined with a Client Side validation function which then did the hide content/show message code (and return a meaningless IsValid setting). This also caused me to define a different ValidationGroup setting for my real data entry validator controls so that I could control them separately and only have them fire when I really wanted validation, and not just my show/hide trick. For a little while I was pretty proud of myself for coming up with this clever approach to get around what I considered to be a serious oversight on the DropDownList and CheckBox controls declarative syntax. Then, in the midst of my smugness, just as I was about to commit my changes to the source code repository, it dawned on me that there is a much simpler and much more appropriate way to accomplish this. All that I really needed to do was to put in my server-side code (I used the Page_Init section) a call to MyControl.Attributes.Add(“onClick”, “myJavaScriptFunctionName()”) for the checkboxes, and for the DropDownLists (which become select tags) use “onChange” instead of “onClick”. This is exactly the type of thing that the Attributes collection is there for…so you can add attributes to be rendered with the control that you would have otherwise stuck right into the HTML markup if you had been doing this by hand in the first place. Ugh! A few hours wasted on clever tricks that I ended up completely removing, but I did learn a lot more about custom validators and validation groups in the process. And got a good reminder that all that stuff (HTML, JavaScript, and CSS) I learned back when I wrote classic ASP pages is still valuable today. Oh, and one more thing…don’t get lulled into too much reliance on the the whiz-bang tool to do it for you. After all, WebControls are just another layer of abstraction, and sometimes you need to dig down through the layers and get a little closer to the native language.
