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Published: Wednesday, April 20, 2005

An Update on Prompting a User to Save When Leaving an ASP.NET Page

By Scott Mitchell


Several months ago I wrote an article here on 4Guys titled Using ASP.NET to Prompt a User to Save When Leaving a Page, which, as its name implies, provides a technique for alerting a user when they're about to leave a Web page that has "unsaved" data. Specifically, the article looked at using the onbeforeunload client-side event, which fires whenever a Web page is being exited, be it through the user closing the browser window, clicking on a bookmark, clicking on a link in the Web page, or any other task that would cause the Web page to unload.

With some clever client-side programming you can use onbeforeunload to display a modal dialog box whenever the user is about to leave a page whose content they have changed but have yet to save the changes. This is an especially handy technique for applications that will be used by people who are not Web-savvy users, and might not realize that they need to click some sort of Save button in order to save the changes they made.

Over the months since the original article's publication I have received a number of questions from interested readers. In this article I plan on addressing the two most popular questions: dealing with auto-postback Web controls (such as DropDownLists or CheckBoxes with their AutoPostBack property set to True); and how to prevent "Unspecified error" script errors that can creep up depending on how, exactly, the page was unloaded. Read on to learn more!

- continued -

A Quick Summary of the Prompting Technique

If you've not yet read the article Using ASP.NET to Prompt a User to Save When Leaving a Page I'd encourage you to do so now, along with the article Prompting a User to Save When Leaving a Page. The Prompting a User to Save When Leaving a Page article introduces the onbeforeunload event and examines how to utilize it to prompt the user when exiting a page. The article Using ASP.NET to Prompt a User to Save When Leaving a Page illustrates how to apply this technique to an ASP.NET page and covers issues and workarounds that certain ASP.NET controls, such as LinkButtons, raise.

The basic premise for the two previous articles was that we had a Web page with various data-entry fields that contained existing data the user had entered. In addition to the data-entry fields, there was some sort of "Save" button at the bottom of the page. The workflow for this page would be as follows: if the user wanted to change some data, they'd visit this page, update the appropriate data-entry fields, and click the Save button. The aim of the previous articles was to examine a means to prevent the user from visiting the page, making some changes to the data-entry fields, then accidentally leaving the page prior to saving those changes. In order to help prevent mistakenly leaving a page before saving, the previous articles illustrate how to have the user prompted with a modal dialog box upon leaving a page with changed data-entry values.

To quickly summarize, the prompting technique works via the client-side onbeforeunload event, which, in modern browsers, fires immediately prior to the document unloading. To actually prompt upon exit you need to wire up this event to an event handler. If the event handler returns a string, that string is displayed in a modal dialog box, like the one shown to the right. This dialog box informs the user that they are about to leave the page and presents two buttons, OK and Cancel. The user can click Cancel to stay on the current page (thereby giving them an opportunity to save the unsaved data, if needed), or can click OK to continue exiting the page.

Of course we don't want this dialog box to popup every time the user leaves a page. Rather, we only want the user to be prompted if they are leaving the page after they had changed the values of one or more input fields on the page without clicking the Save button to commit their changes. To accommodate this, client-side script is used to record the initial values of those input fields that need to be monitored. In the onbeforeunload event handler these initial values are compared against the current values. If there are any discrepancies, a string is returned, thereby displaying the prompt to the end user. The article Using ASP.NET to Prompt a User to Save When Leaving a Page introduced a base class for ASP.NET code-behind classes that contained methods to mark particular input Web controls for being monitored, as well as indicating that particular Web controls should not cause a prompt when they induce a postback (such as the Save button itself).

Suppressing the Prompt for Button Web Controls

The onbeforeunload event fires every time a page is unloaded, including when it is posted back. For example, imagine that a user comes to the page, makes some changes to the data-entry fields, and then clicks the Save button. Clicking Save will cause a postback, thereby causing onbeforeunload event to fire, which causes the associated client-side event handler to execute. This client-side event handler will notice that the data-entry values have changed since the page was initially loaded and will therefore return a string, causing the user to see the prompt. However, if the user is clicking Save we need to suppress this prompt, since even though the page is being unloaded, it's being unloaded to save the changes.

In Prompting a User to Save When Leaving a Page we saw how to accomplish this. Namely, we added a global JavaScript variable that served as a flag in the onbeforeunload event handler: if the flag had a value of true then the comparison of the input values proceeded and the prompt was displayed if there was a discrepancy; if the flag had a value of false, this check was circumvented, and no prompt was displayed, regardless of whether or not there were any changes in the data-entry input values since the page's load. To suppress the prompt from potentially appearing when clicking the Save button, we simply added a bit of JavaScript in the button's client-side onclick event that set this flag to false.

This concept was taken one step further in Using ASP.NET to Prompt a User to Save When Leaving a Page through the introduction of two server-side methods: BypassModifiedMethod(WebControl) and GetBypassModifiedMethodScript(). The GetBypassModifiedMethodScript() method simply returned the JavaScript code needed to set suppress the prompt (by setting the flag to false). The BypassModifiedMethod(WebControl) accepts a Web control instance as input and sets its client-side onclick event to the JavaScript returned by GetBypassModifiedMethodScript(). Typically this method will be called passing in a Button Web control instance for those buttons that, when clicked, should not cause a prompt to be displayed, even if the data-entry form fields' data had been modified. (As mentioned earlier, typically you'll want to suppress the prompt for the Save and Cancel buttons on the page.)

Suppressing the Prompt for AutoPostback Web Controls

Since publication of Using ASP.NET to Prompt a User to Save When Leaving a Page one of the most common questions I've received has been, "I have a DropDownList Web control on my page that has its AutoPostback property set to True. Whenever a user makes a change in this DropDownList the user sees the 'Data on this page has changed' prompt. How can I prevent this?" This question applies not only to DropDownLists but to any Web control whose AutoPostback property is set to True, such as a CheckBox or RadioButton. As with suppressing the prompt on a Button Web control, the workaround is to set the JavaScript flag to false.

For the CheckBox and RadioButton the client-side onclick event can be used to set this flag to false, meaning that you can simply call the BypassModifiedMethod(WebControl) method, passing in the CheckBox or RadioButton instance. For the DropDownList you want to set the flag to false on the client-side onchange event. To accomplish this you could create an additional server-side method similar to the BypassModifiedMethod(WebControl) method, but that sets the onchange event instead, or you could just use the following single line of code:

DropDownListInstance.Attributes("onchange") = GetBypassModifiedMethodScript()

That's all there is to it!

Suppressing "Unspecified error" Client-Side Script Errors When Using eval()

Another common question I have received has been from people encountering "Unspecified error" script errors in Internet Explorer when having a page unloaded through a JavaScript eval() statement and clicking the Cancel button in the prompt. If a page is exited through client-side script - like eval("window.location.href='http://www.google.com/';"); - the onbeforeunload event will fire, as expected. The associated event handler will run, checking to see if the data-entry values have changed. If they have, a string will be returned, causing the prompt to be display. Now here's where the script error creeps up: if the user clicks OK, indicating that they want to page to continue unloading, everything works fine. But if the user clicks Cancel, indicating that they want to stop unloading the page, Internet Explorer throws up an "Unspecified error" script error.

This script error only occurs when attempting to unload a page through an eval() statement, and only in Internet Explorer (things work smoothly in FireFox). The workaround I've found is to place the eval() statement in a try...catch block, like so:

catch (blah)

By using a try...catch block the "Unspecified error" script error will be suppressed. Not the most graceful workaround, but it's one that works and the only one that I could find.

Happy Programming!

  • By Scott Mitchell

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