How to tell if a page unload in ASP is a PostBack

Question

This seems like a common question but search is not returning anything.

I have the following code that executes before the page unloads.The problem is if the unload is a postback i do not want to fire my warning to the user but i can't figure out how to differentiate between a postback and a user navigating to another page for example.

// This is executed before the page actually unloads
        $(window).bind("beforeunload", function () {

            if (prompt) {

                //prompt
                return true;
            }
            else {

                //reset our prompt variable
                prompt = true;
            }
        })

Running script in the code behind i.e. if Page.IsPostBack then set prompt is not an option.

Any ideas?

EDIT:

Here is the solution I ended up with:

 function DoNotPrompt() {
              prompt = false;
        }

I then added this to all the controls where the user could do something that result in a post back.

OnClientClick="DoNotPrompt()

Then checked this flag and only returned a string in "beforeunload" if the user was really moving away from the page i.e. not a postback.

I also had to use this code: var magicInput = document.getElementById('__EVENTTARGET');

    if (magicInput && magicInput.value) {
        // the page is being posted back by an ASP control 
        prompt = false;
    }

The reason being i had a custom user control that was a list box and I could not add the above method. So used this to catch that event and set the flag to false.

Not the most elegent solution.

Thanks, Michael

Answer 1

You can capture the submit and reset the onbeforeunload as:

jQuery(function($) {
    var form = $('form'), oldSubmit = form[0].onsubmit;
    form[0].onsubmit = null;

    $('form').submit(function() {       
        // reset the onbeforeunload
        window.onbeforeunload = null;

        // run what actually was on
        if(oldSubmit)           
            oldSubmit.call(this);           
    });
});

This is a tested code from my pages :)

Answer 2

This may not cover all of the postback situations, but you can tell if the page was posted back by an ASP control by interrogating the __EVENTTARGET hidden input.

This input is set by ASP when the page is posted back by an ASP control.

var magicInput = document.getElementById('__EVENTTARGET');

if (magicInput && magicInput.value) {
   // the page is being posted back by an ASP control
}