Tuesday, 9 November 2010

Javascript popup appears again when back button pressed

One of the bugs I was looking at today was regarding use of the back button in our webapp. We have a screen that lets the user enter some data and click submit. The server side validates that data, and if necessary it alerts the user of errors using a alert issued with RegisterStartupScript.

If the user gets the error, corrects there input data, then resubmits they will be taken to the next page in the process. The problem comes when they click the back button. When the back button is pressed it takes them back to the first page which still has the startup script in it, causing the old validation error popup to appear again.

I did a lot of searching on this to see if there was a simple solution. I discovered the following:
  • There is no simple way to "unregister" a startup script.
  • Ideally the popup should be generated inside an onclick event, but this wasn't possible in my case due to the server side validation.
  • The first webpage can be set to expire, but this causes a "webpage expired" page to be displayed, which isn't any use.
  • The popup button can be pseudo disabled but that doesn't afford the user the behavior they expect.
  • Meta tags can be used to force a page to always refresh, rather than loading from the browser's cache. I wasn't able to get these to work correctly for me.
So after much effort trying different ways to do this, I went with this rather simple solution:

When we register the startup script with the alert, immediately follow the alert statement with one which will refresh the page. This ensures that the startup script is removed from the page as soon as the OK is clicked, and if the user returns to the page the pop up does not reappear.

Here's a code sample of the script injected with Page.RegisterStartupScript:

alert(\"Error with Data\");
__doPostBack('__Page', 'EmptyPostback');

Originally I was using "window.location = PageName.aspx" instead of "__doPostBack" to refresh my page without the popup, but that caused some ViewState that I needed to be lost. If you are developing in something other than ASP.Net then window.location may be better for you.

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